Physical Geology

Earthquakes

On the morning of April 18th, 1906, San Francisco police seargent Jesse Cook was on his beat, standing at the corner of Washington and Davis Streets. At 5:12 A.M. he heard a deep rumble and looked up to see the earthquake coming at him. He remembered later, "The whole street was undulating. It was as if the waves of the ocean were coming at me." The first shock lasted 40 seconds. Then there were 10 seconds of quiet followed by 25 more seconds of shaking. In the two minutes of the quake 80 million dollars worth of damage was incurred as buildings crumbled under the shaking. However, the majority of the damage followed as fires broke-out in the city and firemen discovered that the quake had broken the city’s water mains, rendering them useless. The great San Francisco fire burned out of control for three days, ultimately leveling four hundred and ninety city blocks and causing over 300 million dollars worth of additional damage. Although 700 people died in the disaster and hundreds of thousands were left homeless, aid was rushed into the city and refugee camps established quickly. Nobody starved, there were no outbreaks of disease, and within nine years the city was almost completely rebuilt.

Earthquake: shaking of the earth caused by vibrations that travel both through the earth and along its surface. Although earthquakes can be caused by volcanic eruptions and atomic explosions, most are caused by faults.

Fault: a crack or fracture in rock along which there is movement.

Normal Fault: movement is mostly vertical, as the hanging wall drops down relative to the foot wall.

Strip-slip Fault: movement is mostly horizontal, either right lateral or left lateral.

The rock of the Earth’s crust is riddled with faults. Many of these faults are no longer active (movement has stopped or ceased for a while). However, many faults are active or become reactivated by stresses within the crust, meaning that there is slow movement of rock along the fault.

Slow, steady movement along a fault is not what causes earthquakes. Friction along the fault surface can cause the moving rock to bind. When this happens movement stops along the fault - however, the rock continues to move away from the fault. This causes a buildup of strain as the rock is bent, storing more and more energy as would a wound rubber band. Eventually, enough strain is built up to overcome the friction along a fault and the strain is released in one sudden jumping movement, causing an earthquake.

Where do Earthquakes occur?

The vast majority of earthquakes are associated with tectonic plate boundaries where great masses of rock are moving relative to one-another. As we discussed studying plate tectonics, shallow earthquakes are associated with mid-ocean ridges and seafloor spreading, and shallow, intermediate, and deep earthquakes are associated with subduction zones.

Two of the most seismically active regions of the world are Japan and California.

In Japan, subduction of the Pacific plate beneath the Japanese island arc causes frequent large earthquakes, such as the recent one in Kobe.

In California, a large transform plate boundary, caused by the movement of two plates past one-another, has created the San Andreas fault. Recent California earthquakes, such as the Loma Prieta and Northridge quakes (San Francisco and Los Angeles) are examples of sudden movement along the San Andreas.

Eastern Earthquakes

It must be stressed, however, that few places are immune from earthquakes. Faults are everywhere, and very subtle stresses in the crust of the earth can accumulate over time to cause movement on these faults. For example, even though the eastern US is tectonically rather quiet, it does have its share of earthquakes.

Some of the largest earthquakes known in the US have occurred in the east. These include:

Boston, late 1600’s - extensive damage to buildings.

New Madrid, Missouri in the winter of 1811 and 1812 . On December 16th the first of three earthquakes struck, centered near the town of New Madrid. Two more followed, one on January 23, and one on February 7. The quakes all are estimated to have been greater than magnitude 7 on the Richter scale, with the final quake estimated at 7.8, the largest quake ever recorded in the United States. The shaking from these quakes altered the course of the Mississippi and other rivers. New Madrid was destroyed and at least one small town disappeared as the Mississippi River altered course to flow directly over the town. Chimneys fell down in Cincinnati and in Richmond Virginia. Residents of cities as far away as Boston felt the quake. Few people lost their lives, and property damage was minimal, due to the sparsely populated nature of the Mississippi valley at that time.

Charleston, South Carolina, August 1886. Large earthquake and fire that almost completely destroyed the city.

In New York City, earthquakes strong enough to topple chimneys have occurred in 1737, 1783, and 1884.

Earthquake waves

When an earthquake occurs, the sudden movement of the rock causes seismic waves to radiate out from the area where the movement occurred (the exact underground location is called the focus. The point on the surface of the earth directly above the focus is called the epicenter). These waves propagate by temporarily deforming the rock in various ways. After the wave passes, the rocks returns to its original shape and position.

Earthquakes produce two main kinds of waves:

Body waves: these waves can move through the interior body of the earth.

Surface waves: these waves can only travel over the surface of the earth

(in much the same way that ocean waves travel across the surface of the ocean).

Of the body waves, there are two general types:

P waves (primary waves) ? these are compressional waves (similar to sound waves) that propagate by alternating areas of expansion and compression. P waves are the fastest moving waves, travelling through the earth at about 6 km per second. Because P waves are like sound waves, when they reach the surface they can create sound waves in the air that are audible to humans and animals. P waves can travel through solids, liquids, and gases.

S waves (secondary waves) ? these are transverse waves that propagate by laterally displacing the medium through which they move. S waves move slower than P waves, at a velocity of about 3.5 km per second. However, S waves, because of their shearing motion, are far more damaging to structures than P waves. S waves only travel through solids.

.

Surface waves travel more slowly than either of the body waves. Also, because their motion is restricted to the surface of the earth they generally have a longer distance to travel to reach a particular point than do body waves. The two types of surface waves are:

Love waves ? these are a horizontal displacement at the surface, similar to that of an S wave, but with no vertical displacement. The horizontal shaking of Love waves is particularly damaging to building foundations.

Rayleigh waves ? these waves are similar to ocean waves. A rocking motion transfers the wave energy in the direction of travel.

Earthquake detection

The vibrations in the earth generated by earthquakes can be detected and measured using instruments called seismographs.

What we want to do to detect an earthquake is to measure the shaking of the earth. The problem encountered in trying to do this is that everything attached to the earth, including our measuring instruments, will move with the earth. We need a stationary frame of reference from which we can measure the shaking without being a part of it.

Although we cannot easily detach our instruments from the earth, we can take advantage of inertia to isolate them from earth movements.

For example, in a vertical motion seismograph, a heavy weight is suspended from a string in an airtight box. When the earth’s surface moves up and down due to earthquake waves the floor, table, and box move, but because of the inertia of the weight, it tends to remain still as the spring above it stretches and contracts. Thus, the box moves around the instrument and this motion is electronically recorded.

Different arrangements of weights and springs or pendulum wires are used to detect different components of horizontal and vertical motion.

In most seismographs, the movement of the sensor around the inertial mass is recorded electronically and the electronic signals are amplified and sent to a recording device.

The recording device often consists of a pen drawing a continuous line around a drum of paper. Vibrations cause the pen to bounce back and forth, creating a seismic trace that shows the up and down or side to side motion of the earth. This trace contains much information: it shows the arrival time of the earthquake waves, their frequency, and their amplitude (size).

Most seismographs can detect moderate sized earthquakes that occur anywhere in the world. Usually, a seismic observatory will maintain a battery of seismographs ? some extremely sensitive for detecting weak and distant earthquakes and some moderately sensitive to detect larger or closer earthquakes. In the event of a strong, close earthquake that might cause shaking that is violent enough to push the regular seismographs off of their scales, the moderate seismographs are set to trigger into action relatively insensitive strong motion seismographs.

Using the information from seismographs to locate and describe an earthquake

One can also recognize the different kinds of earthquake waves on the seismic trace. The first pulse of waves are the fast moving primary waves (P) waves. As they begin to fade, the second large pulse records the arrival of the slower secondary (S) waves. Finally, a mishmash of surface waves and reflected p and s waves arrive.

Now, what can we learn from this information?

1. Since we know how fast the fasted p and s waves travel, we can use the difference in their arrival times at the seismograph to determine how far away the earthquake was. Like two trains travelling at different speeds, but leaving the station at the same time, as the two types of seismic waves travel farther distances they get farther apart in time from one another.

2. Once we know how far away an earthquake was, we can determine the exact time that it happened. For example, p waves travel at a certain speed so that at a particular distance from the focus of the earthquake we can determine how long they took to get to the seismograph. We subtract this time from the time at which we first saw then on the seismograph and we know the time they began their journey ? the time the earthquake happened.

3. If we have the distances to the earthquake epicenter calculated from three or more seismic stations, then we can use triangulation to find the exact location of the epicenter.

4. The strength or magnitude of the earthquake is indicated by the amplitude or size of the spikes on the seismograph. However, the farther away the seismic station is, the more attenuated the waves become and the smaller are the spikes produced by a given earthquake. Fortunately, since we know how far away the earthquake was, we can compensate for distance to determine how large the earthquake was where it happened.

Earthquake strength is measured against a scale developed by the seismologist Charles Richter. This scale estimates the amount of energy released by an earthquake that would produce seismic waves of different amplitudes. This is a logarithmic scale, each number up the scale represents a 10X increase in amplitude, which itself is caused by a 100X increase in earthquake energy. However, when the total energy of all of the different waves are added up, it turns out that each jump on the Richter scale is equal to about a 30X increase in total energy. Thus, the difference between a Richter scale 5 and 8 earthquake is equal to 30 x 30 x 30 = 27,000 times increase in energy released.

The total amount of energy released in the largest earthquakes ever recorded (8.9) is roughly equal to the energy of 10,000 Hiroshima sized atom bombs.

Generally, a shallow earthquake must be larger than 5.5 before significant damage occurs at the epicenter.

Earthquakes of magnitudes less than 3 may not be noticed by human beings.

Some extremely sensitive seismographs can detect earthquakes less than magnitude -2, about the amount of shaking produced by a brick falling off of a table.

It may be that earthquakes much larger than this magnitude are not possible due to limits in the amount of strain that rock can withstand and limits in the area over which fault movement can occur at any one time.

Earthquake size can also be classified using estimates of how much and what kind of damage was done at different distances from the epicenter. The damage scale currently in use is the Modified Mercalli scale. This scale ranges from I (Not felt except by a very few under especially favorable circumstances), to VI (Felt by all, many frightened and run outdoors. Some heavy furniture moved; a few instances of fallen plaster and damaged chimneys), to XII (Damage total. Waves seen on ground surface. Lines of sight and level distorted. Objects thrown into the air.)

Anchorage Alaska, Good Friday, March 27th, 1964. Radio announcer R. Pate was home at 5:36 P.M. when he recorded his thoughts as the ground began to shake:

The sudden upward movement of the seafloor acted on the ocean water like a giant paddle, generating gigantic waves called tsunami. These waves reached the coast of Alaska less than a half hour after the quake. As the tsunami rushed onshore it destroyed waterfront areas along the Alaskan coast, particularly at Valdez and Seward. About 120 people were drowned. The total death toll for the entire earthquake was 130, with only 9 killed by building collapse due to shaking.

In addition to the collapse of buildings, the shaking from the Good Friday earthquake caused large portions of the coastline composed of loose soil and sand to liquify. The most impressive example of liquifaction was seen at Turnagain Heights in Anchorage, where 60 foot high soft clay beach cliffs collapsed causing the slumping of developed land up to 900 feet inland along more than a mile of coastline.