ENGINEERING

SEISMOLOGY
 Engineering Seismology is the study of Seismology as related to
Engineering. This involves understanding the source, the size and the
mechanisms of earthquakes, how the ground motion propagates from
the source to the site of engineering importance, the characteristics of
ground motion at the site and how the ground motion is evaluated for
engineering design. This subject is therefore related to the hazard of
earthquakes. The seismic hazard at a site cannot be controlled. It can
only be assessed.
 In the same context, Earthquake Engineering is the subject of
analysis and design of structures to resist stresses caused by
the earthquake ground motion. Resisting the stresses imply
either resisting without failure or yielding to the stresses
gracefully without collapse. This subject is related to the
vulnerability of built structures to seismic ground motion. The
vulnerability is controlled by design. The decision to control
the vulnerability of a structure is based on the economics of
the situation and on the judgement about the acceptable risk
to the community.
 "Seismic Hazard" is defined as the probability of
occurrence of a ground motion of a given size
within a given period of time at the site of interest.
This will depend on the possible sources of
earthquakes within a reasonable distance of the site
and the seismic activity of these sources in relation
to size and time.
 The "Vulnerability" is a measure of the probability
of damage (loss) to the structure to a ground
motion of a given size. Different structures have
different vulnerability curves.
 Whereas the focus in the past was on what to do after a catastrophic
earthquake, nowadays attention is shifting more and more to prevention,
comprising the implementation of measures designed to mitigate risks. To
this end, engineering seismology aims to lay down and develop
corresponding foundations and methods used to enable the assessment of
site-specific seismic hazards. It combines the disciplines of 
historical seismology, strong motion seismology and location effects, 
numerical modelling and earthquake-induced phenomena, and then
crystallises that knowledge into basic practical guidelines for civil
engineering, spatial planning and setting technical standards for construction.
MAGNITUDE AND INTENSITY
OF EARTHQUAKE
 Magnitude and Intensity measure different characteristics of
earthquakes. Magnitude measures the energy released at the source
of the earthquake. Magnitude is determined from measurements on
seismographs. Intensity measures the strength of shaking produced
by the earthquake at a certain location. Intensity is determined from
effects on people, human structures, and the natural environment.
 Magnitude Intensity

1.0 - 3.0 I

3.0 - 3.9 II - III

4.0 - 4.9 IV - V

5.0 - 5.9 VI - VII

6.0 - 6.9 VII - IX

7.0 and higher VIII or higher

Intensity 1
• Delicately balanced objects are disturbed slightly.
• Still water in containers oscillates slowly.

Intensity 2
• Felt by few individuals at rest indoors.
• Hanging objects swing slightly.
• Still water in containers oscillates noticeably.

Intensity 3
• Felt by many people indoors, especially in upper floors.
• Hanging objects swing moderately.
• Still water in containers oscillates moderately.
• Feels like a light truck passing by.
Intensity 4
• Felt generally by people indoors and by some people outdoors.
• Hanging objects swing considerably.
• Motorcars may rock slightly.
• Liquids in containers are slightly disturbed.
• Water in containers oscillates strongly.
• A rumbling sound may sometimes be heard.
• Feels like a heavy truck passing by.
Intensity 5
• Felt by most people indoors and outdoors.
• A strong shaking and rocking is felt throughout building.
• Hanging objects swing violently.
• Small, light, and unstable objects may fall or overturn.
• Liquids spill from filled open containers.
• Vehicles rock noticeably.
• Leaves and twigs of trees shake.
Intensity 6
• Furniture and other heavy objects are displaced.
• Wall plaster may crack.
• People may lose balance.
• Small church bells may ring.
• If on the road, it may feel like driving with flat tires.
• Very old or poorly built houses and man-made structures are slightly damaged though well-built structures are not
affected.
• Limited rockfalls and rolling boulders occur in hilly to mountainous areas and escarpments.
• Trees shake
Intensity 7
• Heavy objects and furniture overturn or topple.
• Difficulty standing on upper floors.
• Old or poorly-built structures suffer considerable damage.
• Some well-built structures are slightly damaged.
• Some cracks may appear on dikes, fishponds, road surface, or concrete hollow block walls.
• Limited liquefaction, lateral spreading and landslides are observed.
• Trees shake noticeably.
Intensity 8
• People will find it difficult to even stand outdoors.
• Many well-built buildings are considerably damaged.
• Concrete dikes and the foundations of bridges are destroyed by ground settling or toppling.
• Railway tracks are bent or broken.
• Utility posts, towers, and monuments may tilt or topple.
• Water and sewer pipes may be bent, twisted, or broken.
• Liquefaction and lateral spreading cause manmade structures to sink, tilt, or topple.
• Numerous landslides and rockfalls occur in mountainous and hilly areas.
• Boulders are thrown out from their positions particularly near the epicenter.
• Fissures and faults rupture.
• Trees shake violently.
• Water splashes over dikes or the banks of rivers.
Intensity 9
• People are forced to the ground.
• Most buildings are totally damaged.
• Bridges and elevated concrete structures are toppled or destroyed.
• Numerous utility posts, towers, and monuments are tilted, toppled, or broken.
• Sewer pipes are bent, twisted or broken.
• Landslides and liquefaction with lateral spreadings and sandboils are widespread.
• The ground is distorted into undulations.
• Trees are shaken very violently with some toppled or broken.
• Boulders are commonly thrown out.
• River water splashes violently on slops over dikes and banks.
Intensity 10
• Many trees are toppled, broken, and uprooted.
• Practically all man-made structures are destroyed.
• Massive landslides and liquefaction, large-scale subsidence and the uplifting of land forms and many ground
fissures are observed.
• Changes in river courses and destructive seiches in large lakes occur.
LIST OF E A R T H Q U A K E S
IN THE PHILIPPINES (21 CENTURY) st

 The Philippines lies along the Pacific Ring of Fire, which causes the

country to have frequent seismic and volcanic activity. Much larger
numbers of earthquakes of smaller magnitude occur very regularly
due to the meeting of major tectonic plates in the region.
 A magnitude of 7.5 quake struck Mindanao on January 1, 2001 at a depth of 33 km.
 A magnitude of 7.5 quake struck Central and Southern Mindanao on March 5, 2002 at a depth of
31 km. At least 15 people were killed, 100 injured and 800 buildings were damaged or destroyed.
 A magnitude of 6.1 quake struck Sultan Kudarat on March 6, 2002. Office of Civil Defense (OCD)
records show that 8 people had died and 41 were injured due to the earthquake. It affected 7,684
families in the provinces of Sultan Kudarat, Sarangani, North Cotabato and South Cotabato
 including four cities and 17 municipalities.
 A magnitude of 6.2 quake struck Masbate on February 15, 2003 at a depth of 22 km. The quake
damaged major infrastructures in Masbate.
 A magnitude of 6.5 quake struck Samar on November 18, 2003. One person was reported dead.
 A magnitude of 6.5 quake struck Mindoro on October 8, 2004.
 A magnitude of 6.6 quake struck Moro Gulf on October 4, 2009.
 A series of quakes with the main quake's magnitude of 7.3 struck Moro Gulf on July 23–24, 2010.
 A magnitude of 6.4 quake struck Ilocos Region on March 20, 2011.
 A magnitude of 5.2 quake struck Valencia City, Bukidnon on November 8, 2011 at a depth of 1 km. 39
people were injured, and several establishments were damaged.
 A magnitude of 6.9 quake struck Negros, the rest of Central Visayas
, and some parts of Mindanao on February 6, 2012, at a depth of 20 km. The quake killed people, caused
major damage on infrastructures, and buildings. A tsunami alert level 2 was raised due to the quake. The
quake also caused a landslide, burying a barangay. More than a thousand of aftershocks were recorded by
PHIVOLCS within 2 days since the quake occurred. According to National Disaster Risk Reduction and
Management Council, as of February 18, 2012, the death toll have risen to 51 with 62 people still missing,
and injuring 112 people. Most deaths came from the city of Guihulngan and La Libertad where landslides
occurred. 63, 697 from provinces in Region VII were affected by the quake. 15, 483 houses were partially or
totally damaged, and a total of ₱383-million worth of damage to buildings, roads and bridges, and other
infrastructures were recorded.
 A magnitude of 5.9 quake struck Surigao City on March 16, 2012. Many were injured in the city for that
certain day was the grand opening of Gaisano Capital Surigao. An estimated 6,000 people were in
Gaisano when the earthquake happened. The earthquake caused a stampede which injured people.
 An earthquake with magnitude 7.6 struck 106 km near Guiuan, Eastern Samar on August 31, 2012. It was
felt in certain areas of Visayas and Mindanao. One person died, and another one was injured in 
Cagayan de Oro City after being trapped in a collapsed house due to a landslide. Minutes after the quake,
power interruptions occurred in the affected areas. The 
National Disaster Risk Reduction and Management Council reported that a house in Agusan del Sur
 province caught fire sparked by a gas lamp that was toppled during the earthquake. It also reported that
two bridges in Eastern Samar, particularly the Buyayawan Bridge in Mercedes town and the Barangay
Casuroy Bridge in San Julian town, were partially damaged. The Abreeza Mall in Bajada, Davao City
suffered minor cracks on the floor due to the earthquake. In General MacArthur, Eastern Samar, 77
homes were damaged. There were also 6 houses damaged in Barangay Casoroy, San Julian. In Balangiga
, Eastern Samar, a hospital sustained serious damage. A wall from an old building collapsed in Butuan
City. The NDRRMC reported on Saturday noon there were cracks on some roads and bridges and other
establishments in areas where the quake was felt. Most of the homes destroyed were those made of light
materials, while overall damage to infrastructure remained minimal. A tsunami warning of Level 3 was
raised by the Philippine Institute of Volcanology and Seismology, but was lifted 5 hours after the quake
only caused tiny waves.
 A series of earthquakes, struck cities of Malaybalay and Valencia, and the sitio of Musuan, 
Maramag in Bukidnon last September 3–4, 2012. The first quake has a magnitude of 3.4 and was
felt at 06:48 PM, and was followed by magnitude 4.0 at 07:45 PM, and 4.7 at 09:21 PM. Hours after
the first three, a series of quakes occurred in 03:44 AM and in 03:52 AM. The first quake had a
magnitude of 5.6 with a depth of focus of 3 km; while the second one was recorded at a magnitude
of 4.9 with a depth focus of 3 km. The quake was felt as far as Cagayan de Oro, Kidapawan, Butuan
, and Cotabato cities. A nun was injured in Barangay Lourdes in Valencia City after the incident.
Valencia City Disaster Risk Reduction and Management Council reported that 144 houses and
structures were destroyed. Mayor Ignacio Zubiri of Malaybalay City reported no casualties nor
damage in his city. The quakes were of tectonic in origin. A total of 131 aftershocks were recorded
after the 5.6 quake in September 4, 2012.
 A 6.2 earthquake struck off the coast of Southern Mindanao on February 16, 2013, 28 kilometers
southeast of Caburan, Davao del Sur. According to the United States Geological Survey, the quake
struck at a depth of 98.2 km.
 A 5.7 earthquake struck the island of Mindanao on June 1, 2013. The quake's epicenter was
located in Carmen, Cotabato and struck with a depth of 5 kilometers. The said quake
injured six people, 4 of them were children, and fully or partly destroyed several houses,
and some school buildings. It also damaged a bridge at Barangay Kimadzil, and another
one at Barangay Kibudtungan. The quake was followed by 15 aftershocks, the last one was
followed by a 4.3 quake on June 2, 2013. The quake caused ₱71-million worth of damage.
Another quake jolted the said town after 4:00 AM on June 3, 2013. The quake was
recorded at 5.7 and struck at a depth of 3 kilometers. The newest quake further injured 8
more people, and damaged more houses. Classes which was slated to open on June 3,
2013, were cancelled due to a series of quakes that hit the town since June 1, 2013.
 A magnitude of 7.2 earthquake struck Bohol on October 15, 2013 at 8:12 a.m. (PST). Its
epicenter was located 6 kilometres (3.7 mi) SW of Sagbayan at a depth of 12 kilometres
(7.5 mi). According to the official report by the 
National Disaster Risk Reduction and Management Council, 222 were reported dead while
796 people were injured. Tens of thousands of structures were damaged by the earthquake.
Most notable were the national historical churches in Bohol and Cebu.
Loon Church before the earthquake

Loon Church after the earthquake

 A magnitude 5.8 earthquake struck Occidental Mindoro on June 25, 2014, at 7:52 p.m. at a depth of
53 km.
 A magnitude 5.9 quake struck San Antonio, Zambales before dawn of Sunday, January 11, 2015, at
3:32 am at a depth of 48 km.
 A magnitude 6.1 quake struck 47 km N 47°E of Burgos (Surigao del Norte) on July 3, 2015, 2:43 pm at
a depth of 26 km.
 A magnitude 6.5 earthquake struck 10 km SW of Basilisa, Dinagat Islands on February 10, 2017, 10:03
pm at a depth of 15 km. Eight people were killed and 202 injured.
 Batangas experiences an earthquake swarm in April 2017. A magnitude 5.5 earthquake struck 5 km W
of Batangas on April 4, 2017, 8:58 pm at a depth of 82 km. Twin earthquakes with magnitudes of 5.6
and 6.0 hit the barangays of Bagalangit and Tanauan, both in Mabini, Batangas respectively on April 8,
2017, 3:07 and 3:09 pm at a depth of 40.4 km. Damages on some buildings and houses have been
reported. Several aftershocks followed.
 A magnitude 5.4 earthquake struck 74 km N 33°E of Palapag, Northern Samar on April 10, 2017, at 8:43
am at a depth of 17 km.
 A magnitude 5.7 earthquake struck 199 km S 54°E of Governor Generoso, Davao Oriental on April
10, 2017 at 6:38 pm at a depth of 43 km.
 A magnitude 6.9 earthquake struck 47 km S 26° W of Sarangani and Davao Occidental provinces on
April 29, 2017 at 4:23 am at a depth of 57 km. Five people were injured
 A magnitude 5.4 earthquake struck 2 km W 15° SW of San Narciso, Zambales on May 26, 2017 at
10:27 pm at a depth of 10 km. The quake was felt in nearby provinces including Metro Manila and
some parts of Northern, Central and Southern Luzon.
 A magnitude 6.5 earthquake struck 3 km S 16° E of Jaro, Leyte on July 6, 2017 at 4:03 pm at a depth
of 8 km. It caused island-wide blackouts in the provinces of Samar, Bohol, Leyte, and parts of 
Southern Leyte In Kananga, a commercial building collapsed killing two persons and injuring 20
others. In Ormoc City, a landslide was triggered. Four people died and 100 others were injured. [60]
 A magnitude 5.4 earthquake struck 3 km S 87° SE of Ormoc City, Leyte on July 10, 2017 at 9:41 am
at a depth of 3 km. Dozen of high school students lost consciousness and fainted after the quake.
Classes suspended in Ormoc City. This was an aftershock of the M6.5 that hit Jaro on July 6, 2017.
 A magnitude 6.2 earthquake struck 18 km S 36° W of Nasugbu, Batangas on August 11, 2017 at 1:28
pm at a depth of 177 km.
 TEN DEADLIEST RECORDED
EARTHQUAKES IN THE
PHILIPPINES SINCE THE 1600S
 Ten deadliest recorded earthquakes in the Philippines since the 1600s

Magnitude Origin Location Date Mortality Missing Injured Damage Source

1 7.9 Tectonic Moro Gulf August 16, 1976 4,791 2,288 9,928

2 7.8 Tectonic Luzon Island July 16, 1990 1,666 1,000 3,000 < ₱ 10 billion

3 7.5 Tectonic Luzon Island November 30, 1645 600 < 3,000 < Unknown

4 7.6 Tectonic Mindanao March 31, 1955 400 < Unknown US$5 million

5 7.6 Tectonic Casiguran, Aurora August 2, 1968 271 261

₱ 4 billion
6 7.2 Tectonic Bohol and Cebu October 15, 2013 222 8 796
(est.)

7 7.1 Tectonic Mindoro November 15, 1994 78 430 ₱ 5.15 million

8 6.7 Tectonic Negros Oriental February 6, 2012 51 62 112 ₱ 383 million

9 7.8 Tectonic Panay (Lady Caycay) January 25, 1948 50 (est.) ₱ 7 million

10 5.5 Tectonic Manila June 19, 1665 19 Unknown

An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the
same area of the main shock. If an aftershock is larger than the main shock, the aftershock
is redesignated as the main shock and the original main shock is redesignated as a 
foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to
the effects of the main shock.
The epicenter, or epicentrum in seismology is the point on the Earth's surface directly
above a hypocenter or focus, the point where an earthquake or an underground explosion
originates.