Vol 1 - Iloilo City Earthquake Model - Event-Based Probabilistic Seismic Risk Assessment (ICEM-EBPSRA) 2023 (ISBN - 978-621-8345!08!9)
Vol 1 - Iloilo City Earthquake Model - Event-Based Probabilistic Seismic Risk Assessment (ICEM-EBPSRA) 2023 (ISBN - 978-621-8345!08!9)
Vol 1 - Iloilo City Earthquake Model - Event-Based Probabilistic Seismic Risk Assessment (ICEM-EBPSRA) 2023 (ISBN - 978-621-8345!08!9)
An image courtesy of the City Planning and Development Office, Iloilo City Government showing
the figure-ground map of Iloilo City and the contrast between built-up areas and extent of open
spaces.
ISBN: 978-621-8345-08-9
January 2023
Publisher:
Department of Science and Technology
Philippine Institute of Volcanology and Seismology
PHIVOLCS Building, C. P. Garcia Ave., U.P. Campus
Diliman, Quezon City 1101
(+632) 8426 1468 to 79 local 216
https://www.phivolcs.dost.gov.ph/
phivolcs_mail@phivolcs.dost.gov.ph
/PHIVOLCS
@phivolcs_dost
DOST-PHIVOLCS
Citation
DOST-PHIVOLCS (2023). Iloilo City Earthquake Model: Event-Based Probabilistic Seismic Risk
Assessment. Technical Report. Volume 1. ISBN: 978-621-8345-08-9
Probabilistic Seismic Hazard and Risk Assessment (PSHRA)
of the Philippines Project
1Peñarubia, H.C., Ysulan, A.K.S., Enriquez, M.C., Antioquia, R.A., Sabar, J.A.L., Grutas, R.N.,
Narag, I.C., Bacolcol, T.C.
2Gustilo,
M.J., Labramonte, R.F.R., Lataquin Jr., M., Ober, E.C., Endriga, C.J.R., Diño III, L.R.,
Malimban, A.J.M., Villanueva, M.A.G., Chua, A.P., Tabocalde, R.
3Cameña, K.D, Peñalosa, J.R., Acapulco, H.D., Alloso, M.I.
4Sevilla, J.J., Magno, M., Chan, C.G.
5Magno, D.P., Papa, D.J.G., Deocampo, A.S., Baquiano, R.L., Agudo, F.A.., Ledesma, S.P.,
Jusden, A.M., Belmis, S.J.C.
6Carnaje, L.M.P, Sinay, M.L.M., Fernandez, J.V.
Disclaimer
The Iloilo City Earthquake Model: Event-Based Probabilistic Seismic Risk Assessment is produced
and distributed free of charge in the interest of local stakeholders but without any warranty. While
every precaution has been taken in the development of this document, under no circumstance shall
the authors, the DOST-PHIVOLCS, and the Iloilo City Government be liable to any party for direct,
indirect, special, incidental, or consequential damages, including lost profits, arising out of the use
of the information contained in this document or from the use of programs and source code that
may accompany it even if the authors, the DOST-PHIVOLCS, and the Iloilo City Government have
been advised of the possibility of such damage. The results provided hereunder are on an “as is”
basis, and the authors, the DOST-PHIVOLCS, and the Iloilo City Government have no obligations
to provide support, updates, or modifications.
Messages
Republic of the Philippines
CITY GOVERNMENT OF ILOILO
OFFICE OF THE MAYOR
Iloilo City
This joint product is a pilot study for the event-based risk assessment in
the Philippines that includes a city-scale probabilistic Peak Ground
Acceleration (PGA) hazard map, spectral response acceleration hazard
maps at 0.2 seconds (Ss) and at 1.0 second (S1) structure periods,
probable seismic events, and estimated annualized structural losses as
well as other useful information.
TERESITO C. BACOLCOL
Director, PHIVOLCS
Contents
Executive Summary.......................................................................................................i
Glossary ....................................................................................................................... ii
List of Acronyms..................................................................................................... ii
List of Terms .......................................................................................................... iii
1 Introduction .........................................................................................................1
1.1 Objectives.............................................................................................................. 1
1.2 Methods ................................................................................................................ 2
2 Probabilistic Seismic Hazard Analysis ..............................................................4
2.1 Regional Geologic and Tectonic Setting ................................................................ 4
2.1.1 Philippine Fault ............................................................................................. 5
2.1.2 West Panay Fault .......................................................................................... 5
2.1.3 Negros Trench ............................................................................................... 5
2.2 Local Geology ....................................................................................................... 5
2.3 Earthquake Sources .............................................................................................. 6
2.4 Source Model ........................................................................................................ 6
2.5 Site Model ............................................................................................................. 8
2.6 Source-to-Site Attenuation Model .......................................................................... 9
2.7 Calculation Software .............................................................................................. 9
3 Classical PSHA Model for Iloilo City ................................................................10
3.1 Hazard Curve ...................................................................................................... 10
3.2 Uniform Hazard Spectra ...................................................................................... 11
3.3 Hazard Map ......................................................................................................... 12
3.4 Disaggregation .................................................................................................... 13
4 Iloilo City Buildings Portfolio ...........................................................................16
4.1 Iloilo City Districts ................................................................................................ 16
4.1.1 Arevalo District............................................................................................ 17
4.1.2 City Proper................................................................................................... 17
4.1.3 Jaro District ................................................................................................. 17
4.1.4 La Paz District ............................................................................................. 17
4.1.5 Lapuz District .............................................................................................. 17
4.1.6 Mandurriao District ..................................................................................... 17
4.1.7 Molo District ................................................................................................ 17
4.2 Building Exposure Data ....................................................................................... 18
4.3 Building Types ..................................................................................................... 19
5 Vulnerability Models ......................................................................................... 22
6 Event-Based Risk Assessment........................................................................ 24
6.1 Stochastic Event Sets .......................................................................................... 24
6.2 Synthetic Catalog................................................................................................. 29
6.3 Loss Curves ......................................................................................................... 31
6.4 Aggregated Structural Loss per Event ................................................................. 33
6.5 Top High-Risk Probable Earthquake Events ........................................................ 33
6.6 Average Annual Loss Estimates for Individual Assets in Iloilo City ....................... 36
7 Limitations and Future Developments ............................................................ 42
8 Final Remarks ................................................................................................... 43
9 Acknowledgments ............................................................................................ 44
10 Bibliography...................................................................................................... 45
Books/Atlas................................................................................................................ 45
Articles....................................................................................................................... 45
Other Sources ........................................................................................................... 46
11 Appendices ....................................................................................................... 47
Appendix A: Maps ................................................................................................ 47
Appendix B: Tables .............................................................................................. 58
Appendix C: Photos ........................................................................................... 317
List of Figures
Figure 1.1 - Probabilistic Seismic Hazard Analysis (PSHA) Flow Chart ..........................................2
Figure 1.2 - Probabilistic Seismic Risk Assessment (PSRA) Flow Chart ........................................3
Figure 2.1 - Regional tectonic setting of the Philippines (Queaño et al., 2020) ...............................4
Figure 2.2 - The tectonostratigraphic columns of Panay Island (Peña, 2008). ................................6
Figure 2.3 - Active Faults and Trenches in Iloilo City and vicinity ....................................................7
Figure 2.4 - Declustered Seismicity in the vicinity of Iloilo City ........................................................8
Figure 2.5 - Probabilistic Seismic Hazard Analysis Outputs ............................................................9
List of Tables
Table 4.1 - Buildings Portfolio Summary (Number of Units per Building Type for each District) ...21
Table 4.2 - Buildings Portfolio Summary (Structure Cost per Building Type for each District) ......21
Iloilo City was selected as the pilot city for the first-ever city-wide event-based probabilistic seismic
risk assessment in the Philippines. Being a dynamic first-class highly-urbanized city in Western
Visayas, with structures and houses over 100,000, including modern high-rise buildings and
centuries-old heritage buildings, and an estimated current population of almost 500,000, the city is
poised for an investment influx. With the aggressive cooperation and technical collaboration of the
City Government and the immediate availability of workable exposure data, this study came to
fruition. A 2-volume report on the event-based probabilistic seismic risk assessment of Iloilo City
was produced.
The first volume contains the technical details involved in the development and completion of the
study, including full-page ground motion hazard and risk maps, and their corresponding tables. The
second volume on the other hand, contains the building exposure data utilized in the risk
assessment. Both volumes may be updated and enhanced independently in scope and extent.
The earthquake model for the city showed a probable mean Peak Ground Acceleration (PGA) of
0.38g to 0.42g on rock site at 475-year mean recurrence interval (MRI). Seismic source, magnitude,
distance, and tectonic region type disaggregation revealed that the highest contributor to a rock site
PGA intensity measure level of 0.43g which Iloilo City may experience once in 475 years, on
average, is an Mw6.25 to Mw6.75 active shallow crustal earthquake that may take place within 70
km epicentral distance from the city.
The study forecasts 6,822 earthquake events that may cause structural economic losses to
buildings in Iloilo City. Out of these events, 85.91% may likely originate from active faults and point
or area sources, while the rest may be attributed to subduction interface events from the Negros
Trench. The seismic event sizes in moment magnitude ranges from Mw5.05 to Mw8.15 at depths
down to 60 km as reckoned from the ground surface.
The estimated average annualized aggregated loss to the entire portfolio is enumerated per
probable destructive earthquake event, ranging from PHP1 Million to a little greater than PHP10.30
billion. The building stock, consisting of 4,735 assets with building permits from 2015 to 2022, is
approximately 5% of the actual number of buildings and informal structures in Iloilo City. Owing to
currently incomplete information, the rest of the structures were not included but may be considered
in subsequent reassessment when necessary data becomes available.
The mean aggregated structural loss values to the entire portfolio ranges from PHP3.84 million and
a little less than PHP3.50 billion. Their respective ratios at specified return periods between 1 year
and 100 years, are shown for reference.
The average annualized losses for every building or structure considered are listed at mean values
ranging from less than PHP500.00 to a little more than PHP14 Million. The 0.15 and 0.85 quantile
loss values per structure are also provided considering uncertainties.
The estimated building losses were also intersected with the administrative boundaries to
comparatively illustrate the aggregated loss maps attributable to each District and Barangay of the
City of Iloilo.
P a g e |i
Glossary
List of Acronyms
Mw Moment Magnitude
RC Reinforced Concrete
SA
Spectral Acceleration
ii | P a g e
SES Stochastic Event Sets
Vs30 Ave. Shear Wave Velocity of the upper 30 meters of soil layer
List of Terms
acceleration, g
Unit of ground motion to denote the rate of change of velocity reckoned in this study as a fraction or
multiple of the acceleration due to gravity equal to 9.815 m/sec2.
active fault
A fault that has moved in the past 10,000 years.
active shallow crust
The seismic region on the upper crust close to the surface.
aftershock
A smaller earthquake that follows the mainshock in an earthquake sequence.
aggregated structural economic loss
The estimated structural economic loss to the entire portfolio.
area source
Refers to a seismic activity occurring uniformly over a geographical region.
aseismic
A term to describe a fault (or geographical region) in which no earthquakes have been recorded or
observed.
Benioff zone
A planar zone of seismicity corresponding with the down-going slab in a subduction zone.
building type
Refers to the lateral load resisting system of the structure in this study. It may also refer to the
categorization of buildings based on materials, structure, height, and vintage to indicate code
compliance.
buildings portfolio
A set of adequately-described buildings used as exposure database in this study.
centroid
The center of energy release in an earthquake, usually near the center of the rupture area.
clastic sediment
Consists of fragments of rocks transported elsewhere, and redeposited before forming into another
rock.
P a g e | iii
concrete hollow block
A typical low-cost concrete block made from a mixture of cement and sand and extensively used as
masonry wall in the Philippines.
declustering
The procedure by which independent earthquakes (mainshocks) in a catalog are identified and
separated from other dependent occurrences (foreshocks and aftershocks) for further statistical
treatment.
deterministic seismic hazard analysis
A method used in determining ground motion hazard using discrete single-valued earthquake source
parameters, ground motion attenuation equation, and site condition.
disaggregation
An analysis on the contribution of the different earthquake ruptures defined in a source model to the
probability of exceeding a certain ground motion level at a given location.
earthquake
A weak to violent shaking of the ground produced by sudden movement of rock materials below the
earth’s surface.
earthquake catalogue
A database of earthquakes, wherein each entry includes the basic information about an earthquake
on record, at minimum including the earthquake date and time of occurrence, magnitude, and
hypocentral location.
earthquake event forecast
One of the probable earthquake events in the stochastic event set or synthetic catalog.
epicenter
The point on the surface of the Earth directly above the focus of an earthquake.
event
In seismology, a term used to refer to an earthquake occurrence.
event-based probabilistic seismic risk assessment
An analysis of the potential impact to an exposed asset from a randomly simulated seismicity in a
region as described by a source model, and the resulting ground shaking on a set of locations in
accordance with a ground motion model.
faulting style
The rock deformation orientation that reflects the type of forces pulling or pushing in the region.
forearc basin
A sedimentary basin formed in the arc-trench gap between a volcanic arc and a plate subduction
zone.
foreshock
A smaller earthquake that precedes the mainshock in an earthquake sequence.
geology
The science that examines Earth, its form and composition, and the changes it has undergone and is
undergoing.
ground motion attenuation
The decrease in ground motion severity as seismic waves propagate from the source to the site.
ground motion model
A relationship defining the distribution of ground motion at a site during an earthquake.
iv | P a g e
ground-motion prediction equation model
The estimated ground motion level and its associated uncertainties on a particular site, based on the
magnitude of an earthquake, source-to-site distance, and local site conditions.
hazard curve
A graphical relation of the ground motion severity with the annual probability of exceedance or mean
recurrence interval.
Holocene
The current epoch in the geologic time-scale.
logic tree
Is a decision flow path consisting of nodes and branches, each branch represents a discrete choice
of a parameter with assigned likelihood of being correct.
mainshock
The largest earthquake in a sequence, sometimes preceded by a foreshock/s and usually followed
by aftershocks.
mean recurrence interval
An average repeated instances of specific ground motion severity over time.
moment magnitude
A measure of the size of an earthquake based on the area of the fault rupture, the average
displacement, and the force required to overcome the friction sticking the rocks together that were
offset by faulting.
OpenQuake engine
An open-source hazard and risk calculation software developed by the Global Earthquake Model
(GEM) initiative.
peak ground acceleration
The ground acceleration at the short period end of the hazard or response spectrum.
period
The time interval for one full cycle of a wave in seconds.
Philippine Mobile Belt
A seismically active, rapidly deforming plate boundary zone situated along the convergent Philippine
Sea/Eurasian plate.
PHIVOLCS-ISC GEM combined declustered catalogue
The earthquake database jointly produced by the DOST-PHIVOLCS and the GEM Hazard Team and
used in the PSHA modelling of the Philippines.
physical vulnerability model
An estimation describing the ability of the built environment to withstand impacts of earthquake ground
shaking.
point source
Refers to a seismic activity nucleating in a single geographical location.
probabilistic seismic hazard analysis
An evaluation of the probability of occurrence of a ground motion hazard on a particular site due to all
known possible earthquake sources using the Total Probability Theorem.
probabilistic seismic risk assessment
An earthquake risk analysis based on probable seismic hazard models.
P a g e |v
probability of exceedance
The chance that the predicted site-specific ground motion hazard may be greater in severity.
quantile
Derived from the word quantity where a sample is divided into equal-sized, adjacent subgroups. A 0.5
quantile corresponds to the median value of the data set.
response spectrum
A plot of the peak or amplitude of response of a series of damped oscillators of varying natural
frequency (used to approximate the elastic response of structures with similar fundamental
frequencies and damping) that are forced into motion by the same base vibration or shock.
rupture area
The dimension of the rupture plane that moved during the earthquake.
SS, SA (0.2 seconds)
The mapped spectral response acceleration at 0.2 seconds or short period.
S1, SA (1.0-second)
The mapped spectral response acceleration at 1.0 second period.
sedimentary strata
Parallel layers of sedimentary rocks.
seismic source zone
A region where earthquake activities are recorded and observed.
shear wave velocity, Vs
The transversal speed of the seismic wave, in meters or feet per second.
short period
The point along the hazard or response spectrum corresponding to 0.2 seconds
sinistral
Denotes a left lateral strike-slip fault in which the block across the fault moves to the left. Also called
a sinistral strike-slip fault.
site model
The site classification corresponding to the properties and configuration of the upper material of
a particular site which may generally increase or decrease the amplitude of earthquake ground motion
as it propagates through it.
soil profile type
Corresponds to SA for hard rock; SB, for rock; SC, for very dense soil and soft rock; SD. for stiff soil;
SE, for soft soil; and SF, for soil requiring site-specific evaluation, in Table 208-2 of the NSCP 2015.
seismic source model
The collection of seismic sources characterizing the areal or regional seismic activity.
stochastic event set
Simulated seismic events that conform to the temporal, spatial, and ground motion distribution of
actual seismic events.
subduction interface events
Earthquake occurrences within the boundary of the lower crust and the subducting plate.
subduction intraslab events
Earthquake occurrences within the down-going section of the subducted tectonic plate.
subduction zone
A long, narrow zone where one lithospheric plate descends beneath another.
vi | P a g e
synthetic catalog
The probable earthquakes used in this study
tectonic plates
Refers to numerous segments of the outermost rigid layer of the Earth that are in motion relative to
one another.
tectonic region type
The earthquake source region, whether coming from the geologically shallow upper part of the crust,
from the down-going slab of the subducting plate, or from the interface section.
tectonostratigraphic
A vertical and lateral lithological variation resulting from tectonic, magmatic, and/or sedimentary
processes prior to or during subduction/exhumation.
terrane
A crustal block bounded by faults whose geologic history is distinct from the histories of adjoining
crustal blocks.
trench
An elongated depression on the seafloor produced by bending of oceanic crust during subduction.
uniform hazard spectrum
A graphical representation of the probability of exceedance estimates of ground motion hazard at a
particular site due to all possible future earthquake.
vulnerability functions
A function that describes the probability distribution of loss ratio, conditioned on an intensity measure
level.
vulnerability model
A set of vulnerability functions used to model the physical vulnerability of all the assets in the exposure
model.
Vs30
A seismic site condition index adopted by the earthquake engineering community typically based on
invasive and non-invasive geophysical measurements, describing the average shear wave velocity of
the upper 30 meters of soil layer.
P a g e | vii
1 Introduction
The economic loss estimation in a probabilistic seismic risk assessment of buildings or structures
requires a ground motion hazard model, an exposed building stock, the representative physical
vulnerability functions of each respective building, and a robust calculation software.
The publication of the global seismic hazard map (Pagani et al., 2018), and the subsequent
development of the probabilistic seismic hazard analysis (PSHA) model for the Philippines
(Peñarubia et al., 2020), a PHIVOLCS – Global Earthquake Model (GEM) Foundation technical
collaboration, paved the way for the classical PSHA modeling of cities and municipalities in the
Philippines. Consequently, the development of a global seismic risk model (Silva et al., 2020) and
the availability of both the building vulnerability curves and the equally important calculation
software (Silva et al., 2014) (https://www.globalquakemodel.org/), led to the prospect of conducting
a probabilistic seismic risk assessment of high-hazard areas anywhere in the country.
The City of Iloilo was selected as the pilot City in this first-ever city-wide event-based probabilistic
seismic risk assessment (EBPSRA) in the Philippines. A deterministic seismic hazard analysis
(DSHA) was initiated in the same city in 2012 (Bautista et al., 2012). The Office of the Building
Official (OBO) provided the necessary information on buildings exposed to ground motion hazard
(Volume 2: Iloilo City Buildings Portfolio Exposure Database 2015–2022). These are the buildings
in the city that were granted building permits from 2015 to 2022. The database is the currently
complete dataset suitable for use in the risk assessment.
Foreknowledge and insight of these probable events and their likely effect on the building stock
may engender proactive building-specific preparedness measures and city-wide disaster mitigation
efforts by concerned and mandated stakeholders in the region.
1.1 Objectives
The technical collaboration between the DOST-PHIVOLCS and the Iloilo City Government aims to
jointly produce and freely provide to current and future local and regional stakeholders in both the
public and private sectors the following relevant deliverables:
3. A list of Iloilo City buildings or structures and the corresponding structural economic loss
estimates.
The classical probabilistic seismic hazard analysis for Iloilo City is leveraged upon the “Probabilistic
seismic hazard analysis model for the Philippines”, on which the modeling methods and inputs were
adopted.
Using the freely downloadable and open-source probabilistic seismic hazard analysis and risk
assessment software, OpenQuake Engine, the ground motion hazard maps in Figure 1.1, for Peak
Ground Acceleration or PGA, at 0.2 second or short period, Ss and at the 1-second period, S1 were
calculated and the results of which were processed and plotted as full-page maps in Appendix A
for quick reference. A hazard source disaggregation was undertaken to determine the relative
hazard contribution of each particular earthquake magnitude, epicentral distance, and tectonic
region source combination.
The PSHA model was also used to create an earthquake rupture forecast which is then further used
to generate random earthquake occurrences or stochastic event sets (SES) (Silva, et al., 2020).
Because of its random nature, a large number of SES was utilized to attain statistical convergence
and used logic trees to accommodate epistemic uncertainties in both hazard and risk assessment
(Pagani et al., 2014). Each event in the SES generates a ground motion field considering the
tectonic region type associated ground motion prediction equations (GMPEs). The aleatory
variabilities from the GMPEs are propagated using a Monte Carlo approach, while the USGS Global
Vs30 (https://earthquake.usgs.gov/data/vs30/) was used for the site condition.
All probable events that may likely cause economic loss to the entire building stock were included
to comprise the synthetic catalog.
Each individual event in the synthetic catalog/probable earthquakes catalog or earthquake event
forecast may produce economic losses to structures in Iloilo City dependent on many factors such
as magnitude, proximity to Iloilo City, depth from the surface, and faulting style at the seismic
source.
The aggregated structural economic loss estimates that may be calculated from Figure 1.2 to the
entire buildings portfolio likely to be caused by each of these probable earthquake events, and the
average annualized structural loss in quantiles to each building are discussed in the foregoing
sections and listed completely in table forms in Appendix B.
Finally, the administrative barangay and district boundaries (DENR Land Management Bureau,
2013) were intersected with the buildings portfolio to generate the estimated loss maps.
The Philippine archipelago has a complicated geologic history since it has been the convergence
zone of three (3) tectonic plates: Eurasian Plate, the Philippine Sea Plate, and the Indo-Australian
Plate. With the possibility of arc-arc collision, subduction zone flipping, and superposition of younger
arcs over the older dormant arcs, reconstructing the geologic history of the area becomes more
challenging.
To the west of the archipelago, the largely aseismic Palawan-Mindoro microcontinent represents a
portion rifted from continental Eurasia in the mid-Cenozoic (Pastoriza et al., 2018) and to the east,
the seismically active Philippine Mobile Belt which comprises most of the country as vividly shown
in Figure 2.1 (Queano, et al., 2020). The latter is an actively deforming zone made up of terranes
with varied affinities (e.g., from the ancient Philippine Sea Plate and the Indo-Australian margin)
(Yumul et al., 2005) that are bounded by subduction zones: the west-dipping Philippine Trench,
and East Luzon Trough on the east and the east-dipping Manila, Negros, Sulu, and Cotabato
trenches on the west (Gervasio, 1966; MGB, 2010).
Figure 2.1 - Regional tectonic setting of the Philippines (Queaño et al., 2020)
The Philippine Fault is ~1250 km long, but only around 700-km stretch of it is subaerially exposed.
In general, the Philippine Fault is convex to the east (Tsutsumi & Perez, 2013). It traverses
practically the whole length of the country, from northwest Luzon to southeast Mindanao and may
have been formed as a result of the Philippine Sea Plate’s oblique convergence with the Philippine
Mobile Belt (DeMets et al., 2010). Shallow earthquakes are scattered across the Philippine Fault,
showing that active deformation due to the plate tectonic processes are still active (Cardwell et al.,
1980).
The West Panay Fault is a north-south trending fault that transects the Panay Island. It is
responsible for the 25 January 1948 event known as the Lady Caycay Earthquake. During this
earthquake, “55 Spanish-era churches in Panay Island were damaged, 17 of which totally collapsed
and 20 cracked beyond repair” (Bautista et al., 2011). This event toppled the belfry of Jaro Cathedral
and the historical church at Oton, Iloilo Province was demolished due to severe damage.
The Negros Trench is an oceanic trench located west of Panay Island in the Visayas. It consumes
the Sulu Sea basin’s oceanic crust which runs parallel to the western coast of the Panay and Negros
islands. The presence of an arc, which has been active since its commencement at 10 Ma, is
indicative of subduction. (Mitchel et al., 1986; Rangin, 1989). The related Benioff zone is not well
defined, but it appears to extend to a depth of about 100 km (Cardwell et al., 1980).
The Negros Trench intersects the Sulu Trench towards the southwest. The Sulu Trench, similar to
the Negros Trench, lacks a well-defined seismic zone. However, a belt of Pliocene to Quaternary
volcanoes (Solidum et al., 2003; Pastoriza et al., 2018), a well-developed accretionary prism, and
a forearc basin system characterize its activity (Casulla et al., 2022).
Panay Island is situated in the western Visayas, Central Philippines. It is made up and divided into
four (4) tectonostratigraphic groups namely, the Buruanga Peninsula, the Western Panay Antique
Range, the Eastern Panay Magmatic Arc, and the Central Panay – Iloilo Basin (Figure 2.2).
The Central Panay Iloilo Basin, where the City of Iloilo is located, appears to open to the north to
the Sibuyan Sea and to the south to the Guimaras Strait. It has a thick succession of sedimentary
strata dating from the Oligocene to the Holocene. Coraline limestones, coarse clastic sediments,
and interbedded augite olivine basalt flows make up the upper Oligocene-lower Miocene strata in
the west (Peña, 2008).
Up section, the Miocene to Pliocene strata give way to fine-grained sandstone and shale phases.
There are no volcanic flows in this younger section.
Panay Island is bounded to its west by an east-dipping subduction zone defined by the active
Negros Trench, to its east by the central segment of the Philippine Fault Zone (PFZ), and traversing
the island, is the West Panay Fault.
Earthquakes nucleate at active faults, point and area sources, and subduction regions, which can
be typically categorized in active shallow crust, subduction interface, or subduction Intraslab
tectonic regions.
Active fault, point, or area source earthquakes are generated in the active shallow crust region while
subduction intraslab events are generated in the down-going slab of subducting trenches at depths
greater than 70 km in this study. Subduction interface events occur in the interface region of the
crust and of the subducting slab. The seismicity of the region that is not attributable to active faults
or subduction zones is considered as distributed seismicity, combining area sources with smoothed
seismicity (Frankel, 1995) onto a 10 km x 10 km grid in this study.
The PSHA for Iloilo City in itself requires in combination a model for all known earthquake sources
in the vicinity, a model for the source-to-site ground motion attenuation, and a model for the local
site condition.
The source model used in this study was adopted from the PHIVOLCS-GEM joint study. A cropped
map version of the known active faults and trenches in the Philippines
(https://gisweb.phivolcs.dost.gov.ph/gisweb/earthquake-volcano-related-hazard-gis-information) is
depicted in Figure 2.3, while the full-page map may be found as Map 1 in Appendix A. The active
faults’ parameters and faulting styles are listed in Table 2.1.
The seismicity of the region (250 km radius from Iloilo City) was also adopted from the PHIVOLCS-
GEM joint study. Destructive ground motion may be expected from strong to very large earthquakes
(Mw5.0 – Mw7.9) originating from about 200 km distant from the site. Probable mega earthquakes
(Mw8.0 and above) at distances beyond 250 km away in this study are shown to be relatively of
lesser ground motion hazard contribution than nearby strong-to-very large earthquakes. However,
those events may cause structural damage and economic loss to extremely high-rise buildings and
very long-span structures due to resonance effect.
Both historical accounts of earthquakes from 1600 and instrumental records from 1900 until 2015
were included in the catalog (DOST-PHIVOLCS and ISC-GEM Combined Declustered Catalogue,
2019).
The completeness test by Stepp (1971) was utilized while foreshocks and aftershocks were
declustered using Uhrhammer (1985), retaining only the main shock earthquakes with moment
magnitudes (Mw) equal to or greater than Mw5.0. This is because earthquake events less than Mw5.0
in most cases do not cause significant structural damage and economic loss to engineered or
properly-built structures.
Figure 2.4 shows the locations of the declustered seismicities on a cropped map in the vicinity of
Iloilo City with their differentiated magnitude sizes, and labeled with the year of occurrence for
epicenters with magnitudes greater than Mw7.0. The same catalog was used in the development of
the Spectral Acceleration Maps of the Philippines (SAM PH) Atlas (PHIVOLCS, 2021).
The full-page declustered seismicity map may be found in Map 1 in Appendix A and the parameters
for each earthquake event may be found in Table B1 in Appendix B.
For the classical PSHA modeling, the site condition used for the entire city was rock site or Soil
Profile Type SB (Table 208-2, NSCP 2015) with shear wave velocity Vs of 780 m/sec assumed at
the upper 30 meters of the soil layer. The depth to Vs = 1000 m/sec and Vs = 2500 m/sec were
assumed to be at 626 m and 2.0 km from the ground surface, respectively.
These assumptions are consistent with those used in the SAM PH Atlas, which may be accessed
or downloaded from the DOST-PHIVOLCS website at
https://www.phivolcs.dost.gov.ph/index.php/publications/books/earthquake-models.
The source-to-site seismic wave attenuation model, which characterizes the seismic wave
dissipation as it travels from the earthquake source to the site primarily due to scattering and
absorption, utilized four (4) Ground Motion Prediction Equations (GMPEs) for active shallow crust
tectonic region type (TRT) earthquake events, four (4) GMPEs for subduction interface events, and
three (3) GMPEs for subduction intraslab events. These combinatorial GMPEs reflect variabilities
leading to various realizations.
Equal uncertainties were assigned to each TRT, as shown in Table 2.2. The equal weighting for
the ground motion prediction model (GMPM) logic tree for each TRT was meant for the more likely
hazard results rather than the extreme. The selected GMPEs were lifted from the PHIVOLCS-GEM
technical collaboration.
The open-source platform, OpenQuake Engine, developed by the Global Earthquake Model (GEM)
Foundation, was utilized as the calculation program for both the classical probabilistic seismic
hazard analysis and the event-based risk assessment.
The classical PSHA calculation produced 1) hazard curves, 2) uniform hazard spectra, and 3)
hazard maps as results (Figure 2.5), while the event-based risk calculation generated results in the
form of 1) the earthquake event forecast or the probable earthquake catalog; 2) the aggregated
economic losses to the entire buildings portfolio from each forecast event, and 3) the annualized
average loss estimates to each asset or building.
Hazard
Curves
Uniform
Hazard
Spectra
Hazard
Maps
The hazard curves in Figure 3.1 characterize the resulting probabilistic ground motion hazard on
rock site at 475-year mean recurrence interval specifically on the ground surface between Iloilo City
Hall and the Plaza Libertad area. In this location, the PGA value of 0.0001g has a 98.0% chance
of occurrence in a year (blue curve on black crosshairs), while the PGA value of 0.20g has an
estimated average recurrence of 95 years or that a PGA value of 0.20g (approximately felt on the
ground at intensity VII) may occur at least once in 95 years, on average.
The curve demonstrates that the longer the recurrence period is, the larger the acceleration value
may be expected. The hazard curves indicate the annual probability of the estimated mean
acceleration values which is of fundamental importance in the seismic evaluation of existing
structures or seismic design of structures to be built in the area.
The actual site condition, presumed to be less dense than rock site or soil profile type SB at Iloilo
City Hall – Plaza Libertad area, when incorporated into the modeling, may likely amplify the
estimated ground motion values, especially at longer structure periods.
The uniform hazard spectra (UHS) shown in Figure 3.2 at every site within the boundaries of Iloilo
City, also varies, though not significantly evident as graphically presented. This is mainly due to the
proximity of the sites to one another.
The UHS reveals the maximum ground motion hazard (mean) on rock site that may be
experienced in any particular location with a 90% chance of non-exceedance in 50 years from all
possible sources, or an average recurrence of 475 years.
Stakeholders and users of this product are reminded that the PSHA modeling results shown in
Figure 3.1 and Figure 3.2 are mean values of the forty-eight (48) realizations produced from the
combination of source-to-site attenuation models for each tectonic region type as listed in Table
2.2.
In some realizations, the mean values are grossly underestimated while in others, overestimated.
This implies that scenario earthquake or deterministic seismic hazard analysis (DSHA) or actual
destructive earthquake events that may occur in the near future may result in a significantly greater
or lesser ground motion hazard on-site than the mean values shown in this study.
To err on the side of caution, one may opt to be too conservative by designing engineering
structures too sturdy rather than economical. Others may elect to design for the least overall
expense hoping that the occurrence of larger seismic coefficients or ground motion values may not
happen in the lifetime of the structure. The decision lies with the stakeholder/s.
This study however will show and discuss the mean results.
The hazard map of Iloilo City (Figure 3.3) is produced by combining the mean ground motion hazard
at all the individual sites. Notice that the hazard estimates are calculated at every hundred meters
as shown by the gridded locations of each site.
Acceleration, g
The full-page maps are shown in Appendix A as Map A3 for the mean PGA, Map A4 for the mean
ground motion hazard at 0.2 second or short period (Ss), and Map A5 for the mean ground motion
hazard at 1-second period (S1).
These hazard maps may be referenced, with prudence, in the seismic design of structures, in
retrofitting existing buildings and facilities, for siting considerations of high-risk and critical facilities,
for disaster mitigation, for business continuity exigencies, for insurance premium rates, and for the
short to the long-term development plan of the City.
Local and foreign investors may also consider the availability of these hazard maps along with the
accompanying risk assessment in establishing their foothold, business viability, and contingency
plans.
For the stakeholders’ better appreciation, a hazard source disaggregation or deaggregation for
magnitude size, distance, and tectonic region type source was added in this study. It was
undertaken for a rock site PGA value of 0.43g at 10% Probability of Exceedance (PoE) in 50 years
or an average recurrence period of 475 years. A 0.43g PGA value is approximately equivalent to
an intensity VIII ground shaking and is considered very destructive
(https://www.phivolcs.dost.gov.ph/index.php/earthquake/earthquake-intensity-scale).
Figure 3.4 illustrates that a possible destructive earthquake event with a moment magnitude
between Mw6.25 and Mw6.75 (ave = Mw6.5) has the highest contribution, closely followed by an
event with a moment magnitude between Mw7.25 and Mw7.75 (ave = Mw7.5).
An earthquake event that may cause damage to structures in Iloilo City with a magnitude greater
than Mw8.0 has the least probability of occurrence and therefore, has the least contribution to the
hazard.
On the other hand, Figure 3.5 signifies that the more likely epicentral distance of the event that
could generate the ground motion hazard described in Appendix A (Map A3, Map A4, and Map A5)
is within 70 km distance from the city.
The probability of a damaging earthquake beyond 70 km, albeit possible, is relatively less as shown
in Figures 3.5 and Figure 3.6.
Stakeholders may opt to prepare for the more likely occurrence of a nearby moderately-sized to
very large active shallow crustal earthquakes from known active faults such as the West Panay
Fault and from point or area sources, or from very strong subduction interface earthquake events
that may originate from the Negros Trench as these could impact significant damage and economic
loss to buildings and structures, rather than from subduction intraslab or from subduction interface
mega earthquakes (magnitude ≥ Mw8.0) that may be generated by the Philippine Trench hundreds
of kilometers away.
Shown in Figure 4.1 is the map of Iloilo City where the district boundaries are overlain in black lines.
Bounding to the west of the city is the municipality of Oton, Iloilo Province; to the north is the
municipality of Pavia; to the north-east is the municipality of Leganes; and across the Iloilo Strait to
the south through the East is the Island of Guimaras. The static map data used in Figure 4.1 (and
Map A10 in the Appendix Section) is from OpenStreetMap available under the Open Database
License, while the appropriately labeled administrative boundaries (black lines) were from DENR-
LMB, 2023.
Leganes
Pavia
Jaro
La Paz
Oton
Mandurriao
Lapuz
City
Proper
Molo
Arevalo Guimaras
Island
The building exposure database used in this study was provided by the Office of the Building Official
(OBO) of the Iloilo City Government (Iloilo City Buildings Portfolio Exposure Database 2015 – 2022,
Volume 2 to this Technical Report). The OBO is the mandated office of the Iloilo City Government
to require, approve, oversee and archive building permit applications and structural design
information for the new construction, extension, rehabilitation, and repair of all residential,
commercial, lifeline, and industrial buildings and facilities.
The buildings comprising the buildings portfolio are those that have approved building permits
covering the years from 2015 to June 2022. Other buildings and structures in the repository of the
OBO and from other LGU Offices such as the City Planning and Development Office (CPDO), the
Assessor’s Office, and the City Engineering Office may be subsequently appended in a later
update, when complete information for loss estimation becomes available.
For this study, the current buildings portfolio is geographically plotted as points on the map of Iloilo
City in Figure 4.2. The dataset was reviewed and verified for consistency and data completeness.
The road network is from DPWH, 2013.
Color-coded legend indicates the range of the construction value in peso of each asset as declared
in the building permit applications. Hot-colored sites on the map pinpoint high-value commercial
The database is composed of 4,735 buildings or structures located within the one hundred eighty
(180) barangays of the seven (7) districts of Iloilo City. Information on the building type and cost of
the structure are summarized per District in Table 4.1 and Table 4.2.
The complete information contained in the portfolio used in this study may be found in Volume 2
and may also be personally requested from the OBO or may be digitally accessed through the
official website of the Iloilo City Government (https://iloilocity.gov.ph/main/).
A series of field validations were jointly conducted by DOST-PHIVOLCS project proponents and by
the technical staff of the OBO from November 22 through December 8, 2021 (see Collage C1 and
Collage C2 in Appendix C).
The locations, building types and usage, cost estimates of the structure, and the current condition
of the buildings were jointly assessed. A few of the plotted structures found to be mislocated and
therefore needing further validation, are nevertheless included in the assessment as they most
certainly do exist.
One of the important pieces of information required for structural loss estimation is the
determination of the lateral load resisting system (LLRS) or the structural frame of each building.
This information was matched with the appropriate building type vulnerability models available
(https://www.globalquakemodel.org/) to determine its likely performance against actual
earthquake ground motions.
The buildings portfolio of Iloilo City provided this information along with the corresponding number
of storeys, floor areas, and total costs of each of the structures as stipulated in the building owners’
or property managers’ building permit applications.
The greatest number of structures with approved building permits can be found in the District of
Mandurriao, followed by the Districts of Jaro, La Paz, Arevalo, City Proper, Molo, and Lapuz, in
decreasing order. Moreover, the total worth of building assets as declared in the building permit
applications ranked first again the District of Mandurriao, followed by the Districts of La Paz, City
Proper, Jaro, Arevalo, Molo, and Lapuz.
The total worth of the Iloilo City Buildings Portfolio exceeded PHP26.3 billion.
Shown in Collage C1 in Appendix C are the photos of building types verified in Iloilo City such as
steel-framed structures, reinforced concrete warehouses, a 2-storey single-detached reinforced
concrete residential houses, multi-story reinforced concrete commercial buildings, and two-storey
row house residential buildings, to name a few.
Confined masonry structures are reinforced concrete structures wherein the masonry walls are
built-up prior to the pouring of concrete columns and beams. This type of construction system is
typical for low-rise and low-cost residential houses. In this study, all structures with project
construction costs equal to or less than PHP 200,000.00 are categorized in this building type, unless
otherwise specified in the portfolio.
Other concrete structures not categorized as confined masonry or CHB structures in this study are
considered reinforced concrete structures with infill walls.
The steel structures in the portfolio refer to a few multi-level steel-framed parking buildings,
commercial buildings, cell site towers, warehouses, and coal handling facilities.
Mandurriao district topped all building types in the total number of units considered. The same
district ranked first in total economic value per structure type, except for steel structure building type
which La Paz district, though having fewer in number than the District of Mandurriao, ranked first in
terms of the total worth.
Table 4.1 - Buildings Portfolio Summary (Number of Units per Building Type for each District)
Reinforced Reinforced Reinforced Reinforced Reinforced
Confined Concrete Grand
District Concrete Concrete Concrete Concrete Concrete Steel Structure
Masonry Hollow Blocks Total
1 Floor 2 Floors 3-5 Floors 6-10 Floors 10+ Floors
Arevalo 8 128 314 17 1 468
City Proper 6 172 36 31 4 1 3 253
Jaro 13 456 270 24 3 2 768
Lapaz 1 204 298 56 1 5 565
Lapuz 1 33 34 2 1 71
Mandurriao 58 3 1082 1133 112 11 10 10 2419
Molo 4 73 93 18 1 2 191
Grand Total 91 3 2148 2178 260 19 12 24 4735
Table 4.2 - Buildings Portfolio Summary (Structure Cost per Building Type for each District)
Concrete Reinforced Reinforced Reinforced Reinforced Reinforced
Confined Grand
District Hollow Concrete Concrete Concrete Concrete Concrete Steel Structure
Masonry Total
Blocks 1 Floor 2 Floors 3-5 Floors 6-10 Floors 10+ Floors
Arevalo 1,052,715.72 185,935,983.11 602,006,126.69 107,957,090.45 1,125,292.00 898,077,207.97
City Proper 907,949.53 682,816,304.92 127,285,598.19 486,358,219.98 1,195,446,178.60 503,651,251.00 51,330,581.74 3,047,796,083.96
Jaro 1,716,090.00 1,089,855,383.13 672,874,399.24 325,349,311.09 348,416,553.29 4,203,901.36 2,442,415,638.11
Lapaz 183,123.30 692,776,661.79 956,162,567.04 689,932,110.39 542,867,441.40 688,694,741.30 3,570,616,645.22
Lapuz 108,460.00 104,576,896.68 92,547,986.49 13,146,335.63 58,249,964.00 268,629,642.80
Mandurriao 6,821,990.92 698,449.79 2,167,823,227.81 2,135,217,748.08 3,141,687,821.95 2,941,725,738.32 4,542,132,778.00 618,652,921.75 15,554,760,676.62
Molo 459,539.76 113,751,589.69 238,951,298.37 153,711,385.23 66,999,681.46 3,155,367.97 577,028,862.48
Grand Total 11,249,869.23 698,449.79 5,037,536,047.13 4,825,045,724.10 4,918,142,274.72 4,552,588,151.67 5,588,651,470.40 1,425,412,770.12 26,359,324,757.16
The graphs below show the vulnerability curves used in this study as a function of intensity measure
at specific structure periods against the loss ratio in percentage (Figures 5.1 – a to h).
a. Reinforced Concrete Infill Walls 1-2 Storeys b. Reinforced Concrete Infill Walls 2 Storeys
c. Reinforced Concrete Infill Walls 3-5 Storeys d. Reinforced Concrete Infill Walls 6-10 Storeys
Although this set of risk assessment physical vulnerability models for specific building types are
being used globally, other vulnerability studies specifically suited for Philippine building types when
available may be implemented to further enhance the results.
As per field validation, the actual buildings included in the Iloilo City Buildings Portfolio in most
cases cannot be simply arbitrated with a hundred percent certainty as described above. The
physically vulnerability function for reinforced concrete structures with 15 storeys to 30 storeys, due
to non-availability for instance, utilized the vulnerability curves for 10 to 14 storeys. This implies
that the estimated results for this set of building stock at best approximate the likely actual loss
values.
Currently, of the eight models enumerated, only the vulnerability model for C1 low-rise confined
masonry and for CHB low-rise concrete hollow blocks were a result of a building vulnerability study
made in Bohol, Philippines after the Mw7.2 Bohol Earthquake on February 15, 2013 (Naguit et al.,
2017).
Using all the foregoing models as inputs in the event-based risk assessment, ground motion fields
are generated for Iloilo City. The details for the event-based risk assessment methodology may be
referred to Silva et al., 2020. For risk assessment purposes, one may not restrict the need for a
recurrence of the same specific earthquake size on the same location, and with the same faulting
style as in the actual earthquakes in the declustered catalog of Figure 2.4 and Table B1 in Appendix
B.
As in the Classical PSHA Model, the similar ground motion field may also be produced from a
random synthetic earthquake catalog. The same result may be randomly generated using the
Monte Carlo sampling procedure imbedded in the OpenQuake Engine risk calculator, as Stochastic
Event Set (SES) of earthquakes or earthquake event forecast. An SES is a set of probable events
as defined by a seismic source model (corresponding ground motion field) within the time window
of investigation.
Figure 6.1a, Figure 6.1b, and Figure 6.1c are some of the SES generated for Iloilo City where each
event within the SES is geographically plotted, magnitude size-differentiated, and appropriately
labeled. Each SES or a random set of earthquakes, composed of 70 to 110 individual earthquake
events in this study, on a combination of events with varying magnitudes, centroidal locations, and
faulting style, conversely produces the same ground motion fields in Iloilo City (red-shaded area),
as with the source model.
Figure 6.2a is an SES wherein a few more large-magnitude event forecasts are included. It features
one of the largest inland probable earthquakes that may be generated by the West Panay Fault.
Figure 6.2b prominently features the largest probable earthquake located close by the City of Iloilo.
Figure 6.2c includes another large probable offshore event WSW of the City. At these distance and
magnitude combination, moderate to severe damage to the City’s building stock may be expected.
Meanwhile, some event forecasts are produced in the same approximate location in more than one
(1) SES. This is called multiplicity. A large-magnitude event forecast in an unlikely location, but with
multiplicity, is included in the catalog. This is the case for Figure 6.3a and Figure 6.3b wherein an
Mw7.15 (green circle) is forecast even though there is no known active fault in the immediate vicinity.
Moreover, in almost the same location, another SES forecast an M w6.55 earthquake event (see
Figure 6.4).
For this study, 100 SESs were implemented in the risk modeling on a risk investigation time of 1
year. A risk investigation time of 100 years for the same number of SES produced essentially the
same results.
The 100 SESs used in this study equate to 13,736 earthquake rupture forecast. Of these, a total of
2,476 events had aggregated losses to the entire portfolio of less than PHP1 Million which reckons
to an average loss of less than PHP220.00 per building. These events translates to essentially
insignificant damage, hence excluded. Meanwhile, 6,094 events rendered no structural losses,
including large subduction intraslab events with magnitudes exceeding Mw8.0. This may primarily
be due to great distances from Iloilo City, thus also excluded from the catalog.
The final synthetic catalog in this study is therefore composed of 4,346 individual earthquake events
with moment magnitudes ranging from Mw5.05 to Mw8.15. The average centroidal depths vary from
5 km to 55 km as reckoned from the ground surface. Of these 4,346 events, 3,771 or 86.77% are
from active shallow faults, distributed seismicity, and point sources, while 575 events or 13.23%
are categorized as subduction interface events.
Figure 6.4 displays on the map the 4,346 forecasted earthquake events contained in the synthetic
catalog with moment magnitudes greater than Mw5.0. The Iloilo City boundaries are in red and
Philippine Islands in white.
The largest probable offshore subduction interface event forecast is an M w8.15 from the Negros
Trench – Sulu Trench approximately SW of the Island of Negros while the largest probable inland
active shallow crust event forecast is an Mw8.05 event from the West Panay Fault, approximately
located in the vicinity of the administrative boundaries of the provinces of Iloilo, Capiz, and Antique.
Mw (units)
Figure 6.4 - Earthquake Event Forecast with Moment Magnitude > Mw5.0
For clarity and easy reference, Map A6 is labeled for event IDs, Map A7 is labeled for moment
magnitudes, and Map A8 is differentiated by tectonic region types of earthquake source.
Table 6.1 shows the list of the first few earthquakes contained in the synthetic catalog arranged
according to Event ID in ascending order, while the entire list may be found in Table B2 in Appendix
B.
The hazard modeling utilized 4 GMPEs for the active shallow crust tectonic region types, 4 GMPEs
for the subduction interface, and 3 GMPEs for subduction intraslab. This led to a risk calculation
combination of 48 realizations.
Figure 6.5 shows that the 48 loss curves generated significantly vary from the least to the largest
aggregated estimates by as much as a factor of 20 in the 100-year return period. Risk assessment
loss estimates, therefore, are largely dependent on the choice of which GMPEs are deemed
appropriate for the Philippine tectonic setting with respect to tectonic region types and uncertainty
weights.
These combinations may graphically be represented by the mean values obtained from all
realizations. However, it should be noted that substantial underestimation may be observed for the
realizations with loss curves steeper than the mean, while an overestimation may be observed in
realizations with smoother curves.
To accommodate these uncertainties, the 0.15 and 0.85 quantiles were also calculated and plotted
altogether with the mean loss curve as shown in Figure 6.6.
Shown in Table 6.2 is the aggregated loss curves in quantiles and means with respect to return
periods of 1, 2, 5, 10, 20, 50, and 100 years. The longer the recurrence period the larger the
estimated structural loss values expected.
Listed in Table 6.3 are the centroidal parameters and faulting styles of the top 55 earthquake events
contained in the synthetic catalog starting from the largest moment magnitude. Five (5) subduction
interface events from the Negros Trench were included in the list but dominated by crustal events.,
the foremost of which is from the West Panay Fault. The largest probable crustal events are an
Mw8.15 offshore earthquake located West of the Island of Panay, an Mw8.15 offshore earthquake
West of the Island of Negros, and an Mw8.15 offshore earthquake Southwest of the Island of
Siquijor.
For clarity, note that the estimated moment magnitude is binned at every 0.1 magnitude scale,
hence an Mw8.05 event in this study means that the estimated magnitude is in the range between
Mw8.0 to Mw8.1. This applies to all estimated magnitudes in this study.
In addition, seismic ground motion hazard and consequent structural losses are not entirely
dependent on the size of the earthquake. Seismic source-to-site distance, depth from the surface,
actual soil condition onsite, and strike, rake and dip angles of faulting significantly contribute to the
overall hazard and loss estimates. An Mw8.15 earthquake does not necessarily mean to be the
most destructive earthquake event to buildings in Iloilo City.
Also shown are the aggregated structural loss estimates in Philippine Pesos per event and the
percentage loss in reference to the construction value of the entire portfolio. The largest loss
estimates amounting to PHP 10,374,700,000.00 which is equivalent to a percentage loss of
39.38%, may be sustained from Event ID 13079. See the complete list in Table B3 in Appendix B.
Shown in Table 6.4 are the probable earthquake events that potentially pose the largest structural
losses to the entire portfolio. On top of the list is the Mw8.05 subduction interface earthquake that
may be generated by the Negros Trench although disaggregation discussed previously shows this
is of the least probability of occurrence among the top 20 high-risk probable events. Secondly, by
an Mw7.65 shallow-seated offshore earthquake located approximately 64 km to the southwest, and
followed by the Mw6.55 shallow-seated earthquake approximately 16km to the south-southeast of
the City Hall.
An Mw 5.35 crustal earthquake event may also have the potential to wreak havoc and cause
structural losses to the portfolio exceeding PHP 2.3 billion. One (1) of such possible event that may
be generated nearby the City made it to the top high-risk events in Table 6.4 – Event ID 14114 at
35 km depth. Two (2) smaller events within the top 100 high-risk earthquake events forecast,
namely Event ID 7042 (Mw 5.15) at 15km depth, and Event ID 14153 (Mw 5.05) at 45 km depth also
resulted to aggregated losses to the portfolio exceeding PHP 1 billion.
A full-page map of the top high-risk events in terms of aggregated structural loss estimates greater
than PHP 1 billion may be found in Map A9 in Appendix A.
Table 6.5 lists the first few structures in Iloilo City Proper with average annualized structural losses as calculated from all destructive probable events. Notice
that the 0.15 quantile, the mean, and the 0.85 quantile estimated loss values are also shown to accommodate uncertainties in the assessment. The
respective structural economic loss for each of the buildings in the entire Iloilo City Buildings Portfolio may be found in Table B3 in Appendix B. Table 6.6
lists a few of the high-risk building assets in terms of average structural loss from all possible destructive earthquake event in Iloilo City Proper, while Table
6.7 lists the top building assets with the highest average losses percentage. The stakeholders such as building owners, property managers, insurers,
reinsurers, and risk financers are prudenty advised to jointly consider the range of probable structural losses in this study towards the most amicable and
and equitable appraisal.
Table 6.5 - List of Few Building Assets with Estimated Average Structural Losses
Cost of Structural Loss Structural Structural Loss
% Loss
ID OBO Code City District Structure / Number of Storeys / Usage Structure 0.15 Quantile Loss Mean 0.85 Quantile
Mean
(PHP) (PHP) (PHP) (PHP)
469 06-002-2018-0469 Iloilo City Proper Commercial 1,664,507.73 2,097.15 5,871.00 10,362.40 0.35%
470 06-002-2021-0470 Iloilo City Proper Two-storey Boarding House 8,355,416.20 8,902.20 30,097.50 61,208.20 0.36%
471 06-002-2021-0471 Iloilo City Proper Interior Renovation of JB Iloilo Atrium 3,051,675.31 3,620.83 12,118.70 22,104.00 0.40%
472 06-002-2021-0472 Iloilo City Proper Three-storey Residential-commercial 2,067,532.00 13,314.70 28,684.10 48,368.00 1.39%
473 06-002-2021-0473 Iloilo City Proper Booking Office 283,487.96 318.64 1,054.80 2,160.04 0.37%
474 06-002-2021-0474 Iloilo City Proper Interior Renovation Nailscesitty Salon & Spa 162,523.70 204.77 573.25 1,011.80 0.35%
475 06-002-2021-0475 Iloilo City Proper Two-storey Boarding House 1,447,751.90 1,542.49 5,215.02 10,605.60 0.36%
476 06-002-2021-0476 Iloilo City Proper Bongbong Pasalubong Center 253,590.13 157.89 473.74 891.73 0.19%
477 06-002-2021-0477 Iloilo City Proper Interior Renovation Popoy's Batchoy 600,000.00 755.95 2,116.30 3,735.32 0.35%
478 06-002-2021-0478 Iloilo City Proper ACE Hardware Renovation & Downsizing 2,180,220.00 2,586.85 8,658.00 15,791.80 0.40%
479 06-002-2021-0479 Iloilo City Proper Interior 894,005.20 1,126.38 3,153.30 5,565.66 0.35%
480 06-002-2021-0480 Iloilo City Proper Two-storey Residential 1,602,888.00 1,880.26 5,381.13 10,068.10 0.34%
481 06-002-2021-0481 Iloilo City Proper Two-storey w/ Roof Deck 4,516,856.07 4,812.44 16,270.40 33,088.60 0.36%
482 06-002-2021-0482 Iloilo City Proper Residential 916,755.07 1,358.85 3,411.48 5,620.21 0.37%
483 06-002-2021-0483 Iloilo City Proper Commercial 4,000,000.00 6,678.16 16,982.00 27,767.00 0.42%
484 06-002-2021-0484 Iloilo City Proper Three-storey Residential w/ Roof Deck 4,390,296.22 20,230.70 52,527.20 100,657.00 1.20%
485 06-002-2021-0485 Iloilo City Proper 12-M Bipod Antenna 1,662,058.00 2,070.29 5,784.19 10,738.40 0.35%
486 06-002-2021-0486 Iloilo City Proper M & K Cocolicious Chocolate Store 500,000.00 593.25 1,985.58 3,621.62 0.40%
487 06-002-2021-0487 Iloilo City Proper Bldg Extension & Facade Rehabilitation 3,370,867.75 5,627.80 14,311.00 23,399.70 0.42%
488 06-002-2021-0488 Iloilo City Proper Three-storey Commercial-residential 5,637,477.05 36,304.80 78,212.00 131,884.00 1.39%
489 06-002-2022-0489 Iloilo City Proper 1 Storey Commercial 1,453,000.00 2,425.84 6,168.71 10,086.40 0.42%
490 06-002-2022-0490 Iloilo City Proper 2 Storeys Commercial 7,579,756.00 12,210.30 31,107.60 50,602.90 0.41%
491 06-002-2022-0491 Iloilo City Proper 2 Storeys w/deck Commercial 6,369,751.65 8,916.94 22,905.90 37,580.50 0.36%
Given that the buildings portfolio had all building permits, meaning that each of these buildings is of
standard construction to which the structural performance during a destructive ground motion may
generally not differ significantly from one another (especially if of the same building type), the larger
relative losses may be found in the districts and barangays with higher construction cost of structures
and facilities.
Notable on the map is that the majority of structures have average annualized structural losses not
exceeding PHP 20,000.00. Buildings with relatively higher structural losses (orange to red circles) are
the more expensive and newer buildings located in the Mandurriao District.
Aggregated Loss,
PHP
The largest economic loss is estimated in the district of Mandurriao. Being the new business district of
the city, high-rise buildings, posh condominiums, and more recent and classy malls and facilities are
concentrated in this area, in addition to having the greatest number of structures in the portfolio. The
district of La Paz, where the diesel and coal power plants are located, ranked second, while City Proper,
the older business and commercial district ranked third.
The district of Lapuz, being an essentially residential enclave of mostly 1 to 2-storey houses, apartments,
shops, and stores cum residences, and has the fewest in the number of buildings considered in this
study, is estimated to have the least aggregated economic loss in the event of a destructive earthquake.
Aggregated Loss,
PHP
Blue shaded barangays such as Brgys. Santo Rosario-Duran, Rizal Palapala I, Rizal Palapala II, Flores,
Gloria, and Tanza-Esperanza in City Proper District; Brgys. Simon Ledesma, Libertad Santa Isabel,
Taytay Zone II, and Luna in Jaro District; Brgy. Aguinaldo in La Paz; Brgy. Punong-Lapuz in Lapuz
District; and Brgy. South Baluarte in Molo District had the least estimated annual average economic
losses at less than PHP 10,000.00. This is mainly due to the fact that only a single or a couple of
structures were considered in these barangays as shown in the portfolio.
Unshaded barangays on the map had no buildings included in the portfolio, hence no estimates were
made.
On the other hand, any significant update on the portfolio and the physical vulnerability models used for
each respective building type will certainly enhance the accuracy of the loss estimates per asset and its
economic impact to every political Barangay and District, and in the entire City as a whole.
The assessed buildings portfolio is only a fraction (~5%) of the existing structures on the ground, hence
the Iloilo City Government and local stakeholders are encouraged to include the rest of the exposed
buildings and structures (with complete information) within the city in the buildings portfolio and conduct
risk reassessment to amplify the quality and reliability of the results to each existing residential house,
low-rise to high-rise commercial buildings, government buildings, industrial facilities, and to the entire
buildings portfolio.
This pilot study did not cover the determination of damage levels to structures, injuries, and fatalities due
to collapsed or partially collapsed structures, but envisions the Iloilo City Government and local partners
such as the business and insurance sector, construction and real estate industry, as well as the
communication, transportation and energy providers, being the primary stakeholders, to further pursue
this enterprise and with prudence venture into a more comprehensive and encompassing seismic risk
reassessment of Iloilo City.
The hazard model developed in this study recommends that the currently enforced minimum building
design requirements and construction standards should be strictly implemented, if not enhanced.
The Office of the Building Official and the City Planning and Development Office may appropriately adopt
the study results prior to the approval of building permits, locational clearances, zoning, site
development, and certificate of occupancies. The current comprehensive land use plan (CLUP) of the
city may be updated in the succeeding revisions considering the delivered results.
The 4,346 earthquake events listed in the probable earthquakes catalog, as an output of this joint study
convey urgent attention to the likelihood of these destructive earthquakes forecast to occur during the
lifetime of every building and structure in the city.
The aggregated economic loss estimates to the buildings portfolio per earthquake event forecast may
serve as a trigger for district or city-wide disaster mitigation efforts to curb potentially alarming structural
losses to a minimum.
The average annual loss estimates for individual building assets, identifiable through the coded building
permit number or OBO code, may stir the groundwork for building owners' and property managers’
proactive solutions and initiatives toward disaster mitigation, risk reduction, quick recovery, and business
continuity in the event of a destructive earthquake occurrence.
The various offices of the Iloilo City Government may altogether build up and enrich the building
exposure database and extend the probabilistic seismic risk reassessment to the determination of
building damage levels, casualties and fatalities, benefit-cost ratios, and with optimism, expand further
the partnership and assessment coverage beyond the confines of Iloilo City.
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Appendix A: Maps
Legend:
Legend:
7924 5.05 122.4237 11.3313 35.0 ASC 44.99 45.00 90 81,284,200.00 0.3084%
7926 5.15 122.4237 11.3313 25.0 ASC 44.99 45.01 90 1,488,220.00 0.0056%
7927 5.15 122.4237 11.3313 25.0 ASC 30.01 69.99 0 1,102,520.00 0.0042%
7928 5.45 122.4237 11.3313 25.0 ASC 45.00 44.99 90 75,176,100.00 0.2852%
7930 5.45 122.4237 11.3313 35.0 ASC 45.00 44.99 90 214,822,000.00 0.8150%
4 3