Earthquakes and Faults(1)(2)

8/12/2019 Earthquakes and Faults(1)(2)

http://slidepdf.com/reader/full/earthquakes-and-faults12 1/16

Earthquakes and Faults

By:Joeseph Co And Johann Liam Ang



15.1 Active and Inactive FaultsYou have learned that the surface or the outermost layer of Earth is called the crust. Every

part of the earth’s crust is made of rocks. Earth’s crust is made of two sections: the

continental crust and the oceanic crust . Continental crust. Continental crust makes up

Earth’s landmasses. Hence there are continents such as Asia, North America , and Europe.

Oceanic crust is found in the ocean floor and is usually thinner than the continental crust.

Why are some parts of the crust thicker than others? How does the crust change its shape

over time? Some force is at work pushing and pulling on the crust causing changes on the

surface and often well beneath the surface. This push-pull is called stress. As Rocks

undergo stress, they slowly change shape and volume.



Sometimes stress causes a rock to become more compressed or squeezed togother. Thus, it

becomes smaller in volume. At other times, it may stretch out over a larger area. In this

case, the volume increases.

Thus, rocks are very slowly, but continuously moving and changing shape under stress.

The stress may cause the rocks to twist or tear apart. This twisting or tearing apart causes

the formation of faults.

Faults are fractures in rocks along which movement has occurred. Occasionally,small

faults can be recognized in road custs while large faults usually have displacement of

hundreds of kilometers long.In addition to variation in size and orientation, different faults

can accommodate different styles of rock deformation,such as compression and extension.

Some faults may cause the ground to crack,rise or lower.Study the fault formed during the

earthquake in 1954 at Nevada, USA shown in figure 15-1.Notice how the rock layer was

raised from the surface.





A cross section of a faulted rock has two blocks of rock,one on top of the other.The block of rock

above th efault is called the hanging wall.The block bellow is called the foot wall.

Stress can cause either the hanging wall or foot wall to move up or down along a fault.If a stress

pulls the rocks apart due to tension,the hanging wall will move down relative to the foot wall.This

fault is called normal fault.If stress squeezed the rocks due to compression,the hanging wall will

move up relative to the foot wall.This type of fault is called a reverse

fault.Sometimes,compression causes the hanging wall to slide over the foot wall.In this case,thrust

faults are formed.Thrust faults are almost horizontal and usually carry rocks many kilometersfrom their original position.Rocks are severely bent at the same time when thrust faulting occurs.

Faults may either be active or inactive depending on how they were formed.Active faults are

structures where there is a displacement.Usually active faults produce a shallow

earthquake.Inactive faults are structures that ca be identified but which do not cause

earthquakes.If a fault has been inactive for millions of years,it is safe to call it

inactive.However,some faults only have large earthquakes once in thousands of years,and

therefore there is a need to evaluate their potential hazards.Notice the difference between an active

and inactive fault in figure 15-2.





Most faults are not perfectly straight ; instead they consist of numerous branches and

small fractures. In general, faults are locked , except for brief abrupt movement that

accompany an earthquake rupture. This is due to the pressure exerted by the overlaying

crust. Thus even faults that have been inactive for thousands of years can rupture again if

tresses increase continously.

The motion along faults can be explained by the plate tectonics theory,which states that

large slabs of Earth’s lithosphere are continuously moving slowly.These plates interact

with other plates deforming the nearby rocks.

The Philippines has several fault lines and the following map(figure 15-3)from the

Phillipine Institute of Volcanology and Seismology (PHILVOCS) shows the faults and

trenches found in the entire country.Trenches are the deepest part of the oceans and are

usually V-shaped.

They are found close to countries or near strings of islands such as the Pacific Ocean.The

Pacific Ocean has many trenches along its edges of which is the Philippine Trench.Near

these trenches,the crust is ative causing stress,creating faults.





Scientists believe that the ocean floor is being pushed down deep into the earth along the

trenches.When these rocks are pushed deep enough,they are melted by the heat ofEarth.Some of the molten rocks will rise up through the crust and produce volcanoes but

most of it will bacome part of Earth’s mantle.



15.2 Earthquake Focus and Epicenters

When you throw a pebble into a pond,waves move outward in all directions.In a similar

manner,when rocks in the earth’s crust break,earthquakes occur and earthquake waves

travel through the earth in all directions.An earthquake is the shaking and trembling of the

earth that results from the sudden movement of part of Earth’s crust.Most often,erthquakes

are caused by the slippage along a fault in Earth’s crust.During a severe earthquake,theground can rise and fall like waves in the ocean.The motion of the ground causes

buildings,trees,and telephone poles to sway and fall.Sometimes loud noises can be heard

coming from the earth.What causes such sudden movements of the earth’s crust?

The most common cause of earthquake is faulting.As you have learned,a fault is a break in

the earth’s crust.During faulting,energy is released as the rocks break and move.As they

move,they cause nearby rocks to move also.The rocks continue to move this way until the

energy is used up.







When an earthquake occurs only a part of a fault is involved in the rupture. That area is usually

outlined by the distribution of aftershocks in the sequence. Some faults are deep inside the

earth. Others are close to or at the earth’s surface. Most faults occur between the surface and a

depth of 74 kilometers.



The ‚point‛(or region) where an earthquake ruptures and the rocks break is the focus or the

hypocenter. The focus is the underground point of origin of an earthquake. The point on

Earth’s surface directly above the focus is called the epicenter. Earthquake waves reach the

epicenter first. This is where the most violent shaking of the ground occurs. To plot earthquake

locations on a map , epicenter must be located first.

Earthquake waves are known as seismic waves. Seismic waves are the waves of energy caused

by the sudden breaking of rock within Earth or an explosion. They are the energy that travelsthrough Earth and is recorded on seismographs. Scientist learn much about earthquakes and

the interior of Earth by studying the propagation of seismic waves. Seismic waves can be

distinguished by a number of properties including the speed the waves travel, the direction that

the waves move particles as the pass by, and where they do not propagate.

There are three main types of seismic waves, and they all move in different ways. These are

the primary waves, secondary waves , and surface waves. Each type of wave has a

characteristic speed and manner of travel.



Primary wave or p wave is the fastest of seismic wave,and,consequently,the first to ‘arrive’ at

a seismic station before any type of seismic wave.The P wave can move through solid rock

and fluids,like water or the liquid layers of the earth.It pushes and pulls the rocks it moves

through just like sound waves push and pull the air.Have you ever heard a big clap of thunderand heard the windows rattle at the same time?The windows rattle because the sound waves

pushing and pulling on the window glass much like P waves push and pull on

rocks.Sometimes animals can hear the P waves of an earthquake.Dogs,for instance,commonly

begin barking hysterically just before an earthquake ‘hits’.Usually people can only feel the

bump and rattle of these waves.

The secondary wave or S wave,is the second wave you feel in an earthquake.An S wave is

slower than a Pwave and can only move through solid rock, not through liquid or gases.It isthis property of S waves that led seismologists to conclude that Earth’s outer core is

liqiud.Beacause S waves do not travel through liquids,they are not always recorded at all

lovations during an earthquake.What happens to S waves when they reach liquid part of the

earth?Here the S waves stop.



S waves move rock particles up and down to,or side - to-side — perpendicular to the directionthe wave is travelling in.

Earthquake radiates P and S waves in all directions and the interaction of the P and S waves

with Earth’s surface and shaallow structure produces surface waves or L waves.L wavws

arrive at a given point after primary and secondary waves.L waves travel from the focus

directly upward to the epicenter.Then they move along the earth’s surface the way waves

travel in the ocean.Just as the surface of water risesnand falls with each wave,the earth’s

surface moves up and down with each L wave that passes.L waves cause most of thedamage during an earthquake because they bend and twist the earth’s surface.

Documents

Earthquakes and Faults(1)(2)