LABORATORY 8

Earthquakes

MAIN IDEAS

 

INTRODUCTION

In this lab we will study the three types of faults that can form.  Next, we will look at how and earthquake forms along a fault.  Then we will see how a seismometer records an earthquake and how the location and magnitude are determined. Finally we will look at the relationship between earthquakes and plate tectonics.

 

FAULTS

A fault is a fracture or zone of fractures slong which there has been displacement of the rock on either side of the fracture.   Faulting is a basic mechanism by which rocks deform.  Faults are generally planar and are classified according to the nature of the movements as observed perpendicular to the plane of the fault.  Four common types of faults are shown below.  Tensional forces cause normal faulting, whereas compressional forces cause reverse and thrust faulting.  Notice how the relative movements along the faults differ and are caused from the different forces.

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SEISMOLOGY

Seismology is the study of earthquakes.   The principal tool to measure earthquakes is a seismometer which measures the arrival of earthquake waves.  Sudden displacement along a fault (earthquake) will generate different types of waves that travel through the earth and along its surface.   These waves are defined by the type of motion of a particle in the path of the wave.

The study of seismic waves is an effective means of interpreting the nature of the earth's interior.  The velocity of P and S waves generally increases with depth in the earth which causes the waves to bend.   The waves also reflect and refract off abrupt discontinuities such as the crust/mantle boundary (Moho).   Fluid layers in the earth block S waves and create "shadow zones".  

TYPES OF SEISMIC WAVES

Waves in air, water, and rock transfer energy long distances without moving the constituent particles of these substances very far.   For example, an ocean wave can travel across an ocean but each individual water molecule only moves a few meters back and forth.   Similarly, a sound wave in air can go tens and hundreds of kilometers but the air molecules themselves only shift a fraction of a millimeter.   Equivalent types of waves occur in solid rock as well.  


 

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P waves (or "longitudinal waves") travel through fluids, and solids.   They are compression waves and rely on the compressional strength and elasticity of the materials to propagate.  They are known as body waves because they travel though the body of a material in all directions and not just at the surface, as water waves do.   For P waves, the motion of the meterial particles that transmit the energy move parallel to the direction of propagation.  P waves travel the same way as sound waves in air.   The transmission of compressional waves is due to the strong electronic between atoms that get squeezed together too tightly.   P waves are the fastest seismic waves ^M and travel at roughly 6.0 km/s in the crust (more than seven times the speed of sound).  




 

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S waves depends on the shear strength of the material.  Imagine a very long and narrow block of Jello, and then imagine shaking the end of it and then imagine shaking the end of it from side to side.  A shear wave will propagate down the long length of it.  You shake it from side to side but the wave travels forward and perpendicular to the direction of shaking.  You can try this with a long spring or a Slinky suspended from strings also. If you give it a sudden sideways deflection and a transverse or shear wave will travel both lengths of the spring.  Now try to imagine doing the same thing with water in a tank.  No shear wave will propagate because gases and fluids have no shear strength.  They give too easily.  However, the strength of atomic bonds in solids allows them to transmit tranverse motions.  S waves do not travel as fast as P waves and have a velocity of about 3.5 km/s in the crust.  


 

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Surface waves are very similar to ocean waves as they only occur at the surface of the earth and do not penetrate into the interior deeply.   There are two types of surface waves: Love waves and Rayleigh waves.   Typically, it the surface waves that do the most damage during an earthquake, especially at distances far from the epicenter.   Most of the damage in the 1985 Mexico City earthquake was from surface waves that had traveled over 200 kilometers from the epicenter located near the west coast of Mexico.   The velocity of surface waves varies with their wavelength but always travel slower than P and S waves.  

 


An earthquake will generate all of these types of waves and they will propagate over the surface of the earth and through the body of the earth.  The waves can be distinguished by the differing velocities and particle motions.  Seismometers measure the particle motion produced by these waves. 

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Table 1.  Main types of seismic waves.
wave type particle motion name
body waves longitudinal P wave
transverse S wave
surface waves horizontal transverse Love wave
vertical elliptical Rayleigh wave

 

LOCATING EARTHQUAKES WITH SEISMIC WAVES

As we have seen above, earthquakes produce all three types of seismic waves: P waves, S waves, and surface waves.   Because the different waves travel at different velocities, the time it takes each wave to arrive depends on the distance to the earthquake.   (just like thunder and lightning; the farther away the lightning is, the longer it takes the thunder to arrive).   If we have a recording of the seismic waves made by a seismometer, we can measure the time between the P and S waves.   From that time we can calculate the distance to the earthquake.  

Above we see two maps showing the location of a small (magnitude 3.8) earthquake that occurred along the San Jacinto fault northeast of San Diego in 1997.   The yellow triangles mark the location of some of the seismic stations that recorded the earthquake.   Active faults are marked by red lines. The red dot marks the official earthquake location as calculated by the United States Geological Survey.   Below we see the seismograms recorded at different stations for a this earthquake.   The seismograms are the yellow sqiggles and show the vertical movement of the earth as measured by a seismometer located at the stations TRO, LVA2, FRD, and RDM.   The horizontal axis is time and is marked in hours, minutes, and seconds (1997207 refers to day 207 of 1997).   The vertical lines are 5 seconds apart.   At TRO, the nearest station, the P wave arrived at just before 3:15.   At station LVA2, which is slightly farther from the earthquake than TRO, the P wave arrived a little later, at almost exactly 3:15.   The last station to record the earthquake was RDM, which recorded the P wave at 3:15:05.   Notice that the gap between the P and S increases with the distance to the earthquake also.

We can measure the time separation between the S and P times to determine the location of the earthquake.   (in the case, we already know where the earthquake is, but we can test the method).   Below is a table showing the P and S times as measured from the seismograms above.   From the P and S times we can calculate the S minus P time (in seconds).   By multiplying the S minus P time by a factor of 8, we can get the approximate distance in km between the seismic station and the earthquake.   For example, the S minus P time at RDM is 6.2 seconds so the distance to the earthquake is 6.2 times 8, which equals 49.6.   If we draw a circle around RDM at a distance of 49.6 km on a map, we can find all possible locations of the earthquake.   If we do this for all four stations, we can determine the location of the earthquake (epicenter).   The map below shows circles corresponding to the distances in Table 2.   All four circles intersect on the red dot.   Our location has a slight error because the earthquake actually occurred at a depth of 14 km and not at the surface of the earth.   Seismologists use computers to locate earthquakes but the computer programs still use the same method.  

Table 2.  S and P wave arrival times and distance
station P time S time S -P time distance (km)
TRO 3:14:59.2 3:15:01.9 2.7 21.8
LVA2 3:14:59.9 3:15:02.9 3.0 24.1
FRD 3:15:00.7 3:15:04.4 3.0 29.8
RDM 3:15:05.0 3:15:11.2 6.2 49.6

MAGNITUDE AND INTENSITY

The size, or magnitude, of an earthquake depends mainly on how large the original fault break is. For example, in the 1906 San Francisco earthquake, the fault rupture was about 200 miles long. In the biggest earthquake ever recorded (in 1960 in Chile), the broken fault was over 800 miles long. For small earthquakes, however, the size of the fault rupture might only be a few hundred feet. Because it is not always easy to measure the size of the fault directly (it might be under the ocean, or very deep), the size of the earthquake is estimated by the amplitude of the seismic waves. This can be done by measuring the P and S waves or the surface waves. One way of doing this was is called the Richter magnitude, after the person who invented it. Magnitude is expressed as number scaled to the size of the earthquake. The intensity of an earthquake measures the amount of shaking that is produced. This depends both on the distance to an earthquake and the magnitude of the earthquake. A nearby small earthquake can produce the same amount of shaking as a more distant large earthquake. The amount of movement also depends on the local geology. Soft sediment and sand tends to amplify seismic waves and create more shaking (and consequently damage). Intensity is measured on the Mercalli intensity scale, which goes from 1 to 12.

EARTHQUAKES AND PLATE TECTONICS

Most earthquakes occur along plate boundaries, as the constant movement of the plates causes faults to slip.   The map above shows all earthquakes above magnitude 4 recorded in the world for the year 1996.   The plate boundaries are shown as thin black lines.   Below is a map of Southern California with all earthquakes that occurred between 1996 and 1997.   The plate boundary between the North American plate and Pacific plate lies along the San Andreas fault but we can see that considerable earthquake activity occurs along the San Jacinto and Elsinore faults as well.   Earthquakes provide a good way to locate plate boundaries.

LAB EXERCISES

Earthquake Location Earthquake Magnitude
HALLWAY DISPLAY

In the hallway of the Chemistry-Geology building are three seismographs showing the seismic signals are three seismic stations in the San Diego area (Palomar mountain, Barrett Dam, and Glamis). These recorders show only the vertical component of the seismic wave that travels past it. The PC map display shows the location of earthquakes that have occurred in Southern California in the past few days.

 

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