Different Types of Faults

Types of movement of crustal blocks that can occur along faults during an earthquake:

1. Where the crust is being pulled

apart, normal faulting occurs, in
which the overlying (hanging-wall)
block moves down with respect to the
lower (foot wall) block.

2. Where the crust is being

compressed, reverse faulting occurs,
in which the hanging-wall block moves
up and over the footwall block –
reverse slip on a gently inclined plane
is referred to as thrust faulting.

3. Crustal blocks may also move

sideways past each other, usually
along nearly-vertical faults.
This ‘strike-slip’ movement is
described as sinistral when the far
side moves to the left, and dextral,
when the far side moves to the right.

4. An oblique slip involves various combinations of these basic movements, as in the

1855 Wairarapa Fault rupture, which included both reverse and dextral movement.

Faults can be as short as a few metres and as long as 1000km. The fault rupture from
an earthquake isn’t always a straight or continuous line. Sometimes there can be short
offsets between parts of the fault, and even major faults can have large bends in them.
 Earthquake Distribution

In 1954 French seismologist J. P. Rothé published this map showing the

worldwide distribution of earthquakes. The volcanoes, represented by dots, are
clearly concentrated at or near the plate boundaries.

Image courtesy of the US Geological Survey, from its publication This Dynamic
Earth: The Story of Plate Tectonics.
Destructive Earthquakes in the Philippines

Bohol Earthquake (○2013 Ms 7.2 ○1996 Ms 5.6 ○1990 Ms 6.8)

2013 October 15 Ms7.2 Bohol Earthquake
1996 May 27 Ms5.6 Bohol Earthquake
1990 February 08 Ms6.8 Bohol Earthquake

Negros Oriental Earthquake (2012 Ms 6.7)

2012 February 6 Ms6.7 Negros Oriental Earthquake

Masbate Earthquake (2003 Ms 6.2)

2003 February 15 Ms6.2 Masbate Earthquake

Palimbang Earthquake (2002 Ms 6.8)

2002 March 06 Ms6.8 Palimbang Earthquake

Bayugan Earthquake (1999 Ms 5.1)

1999 June 07 Ms5.1 Bayugan Earthquake

Mindoro Earthquake (1994 Ms 7.1)

1994 November 15 Ms7.1 Mindoro Earthquake

Luzon Earthquake (1990 Ms 7.8)

1990 July 16 Ms7.8 Luzon Earthquake

Panay Earthquake (1990 Ms. 7.1)

1990 June 14 Ms7.1 Panay Earthquake

Laoag Earthquake (1983 Ms 6.5)

1983 August 17 Ms6.5 Laoag Earthquake

Casiguran Earthquake (1968 Ms 7.3)

1968 August 02 Ms 7.3 Casiguran Earthquake

Moro Gulf Earthquake (1976 Ms 7.9)

1976 August 17 Ms7.9 Moro Gulf Earthquake

Ragay Gulf Earthquake (1973 Ms 7.0)

1973 March 17 Ms7.0 Ragay Gulf Earthquake
Seismic waves

Diagram of a seismic wave

When an earthquake happens deep underground a crack will start to open on a pre-
existing line of weakness in the Earth's brittle crust. This crack will then grow larger and
larger, relieving built-up stress as it goes.

The speed at which the crack propagates or grows is 2–3 km/sec. Eventually the rupture
will cease to grow and will slow down and stop. The size or magnitude of the earthquake
depends upon how much the fault has ruptured (the slip) and also the area over which
the rupture has occurred.

This rupturing process creates elastic waves in the Earth that propagate away from the
rupture front at a much faster speed than the rupture propagates, the exact speed
depends upon the nature of the wave (a longitudinal or P-wave is faster than a
transverse or S-wave), and on the elastic properties of the Earth. As you go deeper into
the Earth, the density and pressure increases and so do the velocities of seismic waves.

Types of wave
Seismic waves are fundamentally of two types, compressional, longitudinal waves or
shear, transverse waves.

Through the body of the Earth these are called P-waves (for primary because they are
fastest) and S-waves (for secondary since they are slower). However, where a free
surface is present (like the Earth–air interface) these two types of motion can combine to
form complex surface waves.

Although often ignored in introductory texts, surface waves are very important since they
propagate along the surface of the Earth (where all the buildings and people are) and
usually have much higher amplitudes than the P-waves and S-waves. It is usually
surface waves which knock down buildings.

Seismic waves, like all waves, transfer energy from one place to another without moving
material.
Summary of seismic wave types and properties
names) Particle moon Typical velocity Other characteristics

P Alternating VP ~ 5 – 7 km/s in P motion travels fastest in

Compressional compressions typical Earth's crust materials, so the P-wave is the
Primary ('pushes') and : first-arriving energy on a
Longitudinal dilations ('pulls') in >~ 8 km/s in seismogram. Generally smaller
the same direction Earth's mantle and and higher frequency than the S
as the wave is core; 1.5 km/s in and surface waves. P-waves in a
propagating water; 0.3 km/s in liquid or gas are pressure waves,
air including sound waves.

S Alternating VS ~ 3 – 4 km/s in S-waves do not travel through

Shear transverse motions typical Earth's crust fluids, so do not exist in Earth's
Secondary perpendicular to : liquid outer core or in air or water
Transverse the direction of >~ 4.5 km/s in or molten rock (magma). S-waves
propagation. Earth's travel slower than P-waves in a
mantle; ~ 2.5-3.0 solid and, therefore, arrive after
km/s in (solid) inner the P-wave.
core

L Transverse VL ~ 2.0 - 4.5 km/s Love waves exist because of the

Love horizontal motion, in the Earth Earth's surface. They are largest
Surface waves perpendicular to depending on at the surface and decrease in
the direction of frequency of the amplitude with depth. Love waves
propagation and propagating wave are dispersive, that is, the wave
generally parallel to velocity is dependent on
the Earth's surface frequency, with low frequencies
normally propagating at higher
velocity. Depth of penetration of
the Love waves is also dependent
on frequency, with lower
frequencies penetrating to greater
depth.

R Motion is both in VR ~ 2.0 - 4.5 km/s Rayleigh waves are also

Rayleigh the direction of in the Earth dispersive and the amplitudes
Surface waves propagation and depending on generally decrease with depth in
perpendicular (in a frequency of the the Earth. Appearance and
vertical plane) propagating wave particle motion are similar to water
waves.
P-wave and S-wave propagation through a 3D grid

Wave

propagation through a grid representing a volume of material. The directions X and Y are
parallel to the Earth's surface and the Z direction is depth. T = 0 through T = 3 indicate
successive times. The material returns to its original shape after the wave has passed.

P-wave and S-wave propagation animations

To better understand the

particle motion and
characteristics of the both
types of wave, notice the
deformation of the black
rectangle as the wave
propagates through it.
Surface wave, Rayleigh and Love, propagation through a 3D grid
Rayleigh Wave

Love wave

Surface waves have a complex motion that decreases in amplitude with depth, the
material returns to its original shape after the wave has passed.
Surface wave, Rayleigh and Love, propagation animations

To better understand the particle motion and characteristics of the Rayleigh

wave, notice the deformation of the black rectangle as the wave propagates through it.
Also, if you focus on the top edge of the end of the 3-D grid while the wave is passing
through that region, the retrograde elliptical particle motion is very clearly seen.

To better understand the particle motion and characteristics of the Love wave,
notice the deformation of the black rectangle as the wave propagates through it. While
viewing the Love wave animation, remember that the particle motion is purely horizontal
(in the plus and minus Y direction in the diagram) and perpendicular to the direction of
motion (the X direction in the diagram). Although it may appear in the animation (due to
the perspective view of the 3-D grid) that the top surface of the grid is moving vertically
(parallel to the Z axis), the particle motion is purely horizontal. To aid in seeing that the
particle motion is horizontal and perpendicular to the direction of propagation, focus on
the Y axis (red line) while the wave in the animations is propagating through the grid.
TECTONIC PLATE
From the deepest ocean trench to the tallest
mountain, plate tectonics explains the features
and movement of Earth's surface in the present
and the past.

Plate tectonics is the theory that Earth's outer

shell is divided into several plates that glide
over the mantle, the rocky inner layer above the
core. The plates act like a hard and rigid shell
compared to Earth's mantle. This strong outer
layer is called the lithosphere, which is 100 km
(60 miles) thick, according to Encyclopedia
Britannica.

The lithosphere includes the crust and outer

part of the mantle. Below the lithosphere is the
asthenosphere, which is malleable or partially
malleable, allowing the lithosphere to move
around. How it moves around is an evolving
idea.
Top 10 Strongest Earthquakes to hit the Philippines
What’s left of Maribojoc Church
after the magnitude 7.2
earthquake in Bohol.

We all know that the

Philippines is located along the
Pacific Ring of Fire, which is
the reason why our country is
prone to seismic and volcanic
activity.

Now, the country is preparing

for “The Big One”, a magnitude
7.2 magnitude earthquake from
the West Valley Fault which is
expected to hit Metro Manila and Quezon City. Thus, the Office of Civil Defense (OCD)
in coordination with the Bureau of Fire Protection (BFP) and other government agencies
has been conducting series of earthquake drills. This is to prepare the people for the
estimated impacts projected in the 2004 Metro Manila Earthquake Impact Reduction
Study (MMEIRS). According to MMEIRS, “The Big One” could destroy about 40% of
residential buildings and damage 35 percent of public buildings. It could also result to
34,000 casualties, 114,000 individuals will be seriously injured, and the possible fire
incidents that would follow could add another 18,000 deaths. With the drills and
information campaigns conducted, the authorities are hoping that those numbers would
be lessened.

Actually, there had been many stronger earthquakes which hit the Philippines since
1600s but they caused lesser damage to properties. Casualties were also fewer
because the cities then weren’t so densely populated unlike today. And as we
know, Manila is the most densely populated city in the Philippines, if a magnitude 7.2
earthquake hits, there would surely be thousands of casualties and billions of damages
on properties and infrastructures.

The following are top 10 strongest earthquakes in the Philippines that caused major
destructions and casualties:

1. Magnitude 8.0 earthquake in Mindanao (August 17, 1976)

A magnitude 8.0 earthquake took place near Mindanao and Sulu a little past midnight of
August 17, 1976 that was felt as far as Visayas. It was then followed by a massive 4 to 5
meters high tsunami covering 700 kilometers of coastline bordering the island. Because
it was dark, the people were caught by the raging water which claimed 8,000 lives,
injuring 10,000, and leaving 90,000 more, homeless.

2. Magnitude 7.8 earthquake in Northern and Central Luzon (July 16, 1990)
A total of 2,412 people died and at least ₱10-billion worth of damages to public and
private properties was reported after a magnitude 7.8 earthquake struck Northern and
Central Luzon at around 4:00 p.m. of July 16, 1990.
Hyatt Terraces Plaza, Nevada Hotel, Baguio Hilltop Hotel, Baguio Park Hotel, and FRB
Hotel, all in Baguio collapsed trapping and burying people alive.
Although the epicenter was recorded in Nueva Ecija, it caused more damage in the City
of Pines. And the quake that just lasted for about a minute was one of the tragedies in
the country that would never be forgotten.

3. Magnitude 7.5 earthquake in Luzon (November 30, 1645)

The magnitude 7.5 earthquake that crushed Luzon on November 30, 1645 at about 8:00
pm was called the “most terrible earthquake” in Philippines history. The Epicenter of the
said quake was in Nueva Ecija caused by the San Manuel and Gabaldon Faults.
The extent of the tremor was felt as far as Cagayan Valley. It has caused many
landslides which buried many people alive and destroyed many buildings and churches
including Manila Cathedral.

That time, only Spanish are counted so the recorded number of casualties was only 600
while the injured was 3,000.

4. Magnitude 7.3 earthquake in Casiguran (August 2, 1968)

Most of the people in Casiguran, Aurora was still fast asleep when a magnitude 7.3
earthquake struck at 4:19 a.m. of August 2, 1968.

It was another deadly and shocking seismic activity in the country. And the City of Manila
got the most severe damage. Many buildings were either damaged or destroyed totally.
The said event was also called the Ruby Tower earthquake after the said six-story
building located in Binondo collapsed, and caused the death of 260 people. A total of
268 people died that day and 261 more were injured.

5. Magnitude 7.2 earthquake in Bohol (October 15, 2013)

I can still remember how people panicked in the morning of October 15, 2013. It was
around 8:12 a.m. when a strong earthquake was felt here in Tacloban City. Only to find
out after the lights came back that what we have experienced was nothing compared to
the damage it caused in Bohol which was the epicenter of the magnitude 7.2
earthquake.

The quake affected most of Central Visayas, particularly Bohol and Cebu. It was felt in
the whole area of Visayas and reached as far as Masbate Island in the north and
Cotabato in Southern Mindanao.
According to the National Disaster Risk Reduction and Management Council
(NDRRMC), a total of 222 people died, 8 went missing and 976 others were injured. An
estimated 73,000 structures were damaged wherein more than 14,500 of which were
destroyed totally.

6. Magnitude 7.1 earthquake in Mindoro (November 15, 1994)

November 15, 1994, at around 3:15 a.m., a magnitude 7.1 earthquake rocked Mindoro.
A gigantic 8.5 meters (28 ft) tsunami then followed which devastated the islands of Baco
and Calapan, Mindoro.

A total of 7,566 houses were washed out and some 78 people died because of that
tragedy.
7. Magnitude 6.9 earthquake in Central Visayas (February 6, 2012)
A total of 51 people died, 62 still missing and 112 were injured when a 6.9 earthquake
Central Visayas, particularly Negros and parts of Mindanao on February 6, 2012.
It caused a landslide which buried a barangay, damaged 15,483 houses, and a total
damage of ₱383-million on infrastructures and buildings was recorded.

8. Magnitude 7.5 earthquake in Central and Southern Mindanao (March 5, 2002)

A magnitude 7.5 earthquake resulted to the death of 15 people and injuring around a
hundred more in Central and Southern Mindanao on March 5, 2002.

The said quake originated near the Cotabato Trench that was followed by a tsunami. But
it was the flood that was generated by landslides and falling debris that caused damage
to an estimated 800 buildings.

9. Magnitude 6.5 quake in Ilocos Norte (August 17, 1983)

The magnitude 6.5 quake in Ilocos Norte on August 17, 1983 happened around 8:18
p.m. and resulted to 16 casualties and 47 people got injured.
It caused damages on various establishments such as schools, buildings, malls,
residences, and etc. There were also landslides and sand boils that followed the event.

10. Magnitude 7.6 earthquake happened near Guiuan, Eastern Samar (August 31,
2012)
A very strong earthquake with a magnitude of 7.6 happened near Guiuan, Eastern
Samar on August 31, 2012 that was felt as far as Mindanao.

The Philippine Institute of Volcanology and Seismology (PHIVOLCS) issued a tsunami

warning Level 3, but it was lifted 5 hours later.

The quake caused damage on homes, bridges, and other infrastructures. There were
also power interruptions in the affected areas. But despite the intensity only one person
was reported dead and one injured because of the landslide in Cagayan de Oro City.
The bad thing about an earthquake is that we cannot prevent it and we cannot predict it.
We do not know when exactly it would happen. Because of that, it would be better for us
to be prepared at all times. Participate in Earthquake drills conducted by authorities,
keep an emergency kit in the house, don’t panic when it happens and most importantly
don’t forget to pray.