Chapter 8
Earthquakes
Vibrations of the Earth caused by the sudden release of energy, usually as a
result of displacement of rock along faults. An earthquake is always followed by a
series of smaller earthquakes (aftershocks) which represent further adjustments of
rock along the fault. Approximately 13 million people have died as the result of
earthquakes during the past 4,000 years. There are currently no reliable methods for
predicting when earthquakes will occur.
Cause of Earthquakes
Earthquakes are caused by the sudden release of energy along faults.


Elastic-Rebound Hypothesis - The idea that strain builds up in rock until the
elastic limit (strength) of the rock is exceeded. The rock then ruptures (fails) at
a point, snapping back toward an unstrained position. releasing the elastic
energy as seismic waves radiating outward from the break. The greater the
stored strain, the greater the release of energy.
Seismic and Volcanic Activity - The coincidence of many active volcanic
belts with major belts of earthquake activity indicates that volcanoes and
earthquakes may have a common cause. Plate interactions commonly cause
both earthquakes and volcanoes.
Seismology
Seismology is the study of earthquakes, and is a subdiscipline within geophysics.
Instrumental Observations
Seismoscopes
Earliest instruments designed to detect an earthquake did not provide any written
record of an earthquake.
Seismographs
Instruments which detect, measure and record seismic waves. On old
instruments, movement is recorded on a rotating
paper or magnetic tape strip (seismograms), but modern instruments record data
electronically into a computer. Two basic types:
1. Horizontal movement recorder - Need two recorders at right angles to each
other (N-S and E-W).
2. Vertical movement recorder.
Earthquake Focus and Epicenter


Focus (hypocenter) - Point within the Earth from which energy is released.
The size of the earthquake source cannot be accurately determined. Deepest
earthquakes are at the bottoms of active subduction zones.
Epicenter - The point on the Earth's surface directly above the focus. Given a
geographic location.
Earthquake Categories
About 90% of all earthquakes have depths < 100 km. Earthquakes can be
grouped into three categories based on the depth of their foci:
1. Shallow focus - Foci are less than 70 km depth. Most destructive earthquakes.
2. Intermediate focus - Foci are between 70 and 300 km depth.
3. Deep focus - Foci are greater than 300 km.
Benioff Zones
Patterns of increasing earthquake focal depths found at convergent plate margins.
The earthquake foci mark the descent of the subducting lithospheric plate.
Frequency and Distribution of Earthquakes
Most earthquakes (about 95%) occur in seismic belts along the margins of tectonic
plates. Enormous pressure is built up at plate boundaries, particularly convergent and
transform boundaries. The pressure is ultimately released as earthquakes.


Frequency - More than 150,000 earthquakes strong enough to be felt by
someone are recorded annually worldwide. An additional 900,000 earthquakes
occur annually that are too small to be felt or recorded as separate events.
Earthquake Distribution - Occur at convergent (primarily) and transform plate
boundaries. May occur in clusters as plates shift position.
o
o
Seismic Belts - There are two major earthquake belts:
1. Circum-Pacific Belt - About 80% of all earthquakes occur in
this belt, including the world's most devastating in terms of life
and property loss.
2. Mediterranean-Asiatic Belt - Accounts for about 15% of all
earthquakes.
Intraplate and Spreading Ridge Only about 5% of all earthquakes.
Most are not very strong, but some intraplate earthquakes are among
the largest known.
Seismic Waves
Some of the energy released by an earthquake travels through the Earth. The
speed of a seismic wave depends on the density and elasticity of the materials through
which they travel. There are two general types of seismic earth waves:

Body Waves - Speed decreases with increasing density of rock and increases
with increasing rock elasticity. Rock elasticity increases faster than density
with depth. Two kinds of body waves:
1. Primary (P) Waves - Compressional waves that vibrate parallel to the
direction of wave movement. n-avel through solid, liquid and gas.
Fastest seismic wave.
2. Secondary (S) Waves (also known as Shear Waves) - Vibrate
perpendicular to the direction of wave movement. Only travel through
solids (liquids have no shear strength).

Surface Waves - Travel along the outer layer of Earth. Rolling, shaking
motion causes most of an earthquake's damage. Slowest seismic waves. Two
major types:
1. Rayleigh (R) Waves - Behave like water waves with elliptical motion
of material in wave. Generally slower than Love waves.
2. Love (L) Waves - Involve shear motion in a horizontal plane. Most
destructive kind of seismic wave.
Locating an Earthquake
Seismograms show that P waves always arrive at a recording station first,
followed by S, and finally surface waves (L before R).
Time-Distance Graphs
Plots of average P- and S-wave travel times against distance from the focus.
Allow the distance to the earthquake source to be calculated. The farther the distance
between the seismograph station and the earthquake source, the greater the difference
in arrival times between the body waves.
Travel Times
If the Earth were composed of homogeneous material, seismic waves should
travel in straight line paths. When there is a Such variations can give clues to the
Earth's interior layering.

Distances less than 11,000 km - Seismic waves have predictable travel
schedules. P wave speed = 1.8 x S wave speed. L wave speed = 0.9 x S wave
speed.

Distances greater than 11,000 km - P waves are observed to be delayed and S
waves disappear completely. Experiments have shown that P waves are
slowed down by liquids, and S waves cannot pass through liquids. Strange
behavior of seismic waves could be explained if the Earth's interior contained
a liquid layer (outer core).
Time-distance graphs allow the source of an earthquake to be calculated. Determined
from the time elapsing between the arrival of P and S waves at a minimum of 3
different seismograph stations. Time of the earthquake can be determined by knowing
the speed of P waves. Intersection of distance arcs locates epicenter.
Measuring Earthquake Intensity and Magnitude



Worldwide Standard Seismograph Network - A worldwide seismograph
network was begun in 1961. Allows the accurate location of earthquakes
around the world.
Earthquake Intensity - Measure of an earthquake's destructive power (size and
strength). Measured by the Modified Mercalli Intensity Scale which
qualitatively links earthquake power to its effect on buildings. Somewhat
subjective. Intensity depends on:
1. Amount of energy released by the earthquake.
2. Duration of shaking.
3. Distance from the epicenter.
4. Focal depth of earthquake.
5. Type of rock and degree of consolidation.
6. Population density.
7. Type of building construction.
8.
Earthquake Magnitude - Quantitative measure of the energy released by an
earthquake at its source. Measured by the Richter Scale magnitude (each step
= 31 times increase in energy = 10 times increase in earthquake wave
amplitude). Magnitude 6 earthquake = Megaton nuclear bomb. Largest
recorded earthquake was magnitude 8.6 (1964 Alaska earthquake). Most
earthquakes have a magnitude of < 2.5. The largest earthquakes (> 8.0
magnitude) occur on average every 5 years. Magnitude determined by:
1. Measurement of size (amplitude) of seismic waves.
2. Distance from epicenter - Magnitude decreases with distance from
source.
3. Sensitivity of the seismograph.
Destructive Effects of Earthquakes
Of the hundreds of thousands of earthquakes that occur every year, only one or
two are likely to cause severe results. Destructive effects depend on earthquake
magnitude, distance from epicenter, time of day, geology of area, type of building
construction, and duration of shaking. The most destructive earthquakes occur during
working and school hours in densely populated areas. Earthquake hazards include:
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

Ground Shaking - Causes the most damage and loss of life. Buildings on
bedrock move as a unit with the ground and suffer the lower damage during an
earthquake. Worst damage occurs to buildings on poorly consolidated material
or water-saturated sediment. Structural damage depends on:
1. Earthquake magnitude - Determines the intensity and duration of
vibrations.
2. Underlying geology - The nature of material supporting the foundation
determines how much damage occurs. Poorly consolidated material is
subjected to longer shaking and greater S-wave amplitudes. Watersaturated sediments tend to behave like a fluid and flow when shaken
(liquefaction).
3. Design of the structure (materials and type of construction). Adobe and
mud-walled structures are weakest. Unreinforced brick and concrete
buildings have no flexibility and tend to collapse.
Fire - Often more dangerous that the earthquake itself. Severed gas and water
lines can lead to devastating fires. Valves can be installed to cut off lines from
breaks.
Tsunami (seismic sea wave) - Most result from vertical displacement of the
ocean floor or from submarine landslides during an earthquake. May also be
generated from volcanic explosions (Krakatoa). Can reach speeds up to 800
km/hr and heights up to 65 m. A seismic sea wave warning system was
established in 1948 for the Pacific basin after a 1946 tsunami killed 154 in
Hawaii.
Ground failures may include:
1. Landslides - Seldom occur farther than 40-50 km from the epicenter,
but can be the major cause of death in some earthquakes.
2. Land Movements - Earthquakes can be accompanied by significant
vertical and horizontal movement of the land surface. Fault Creep Slow periodic movement of the land on opposite sides of a fault. Over
time, fault creep can result in significant movement.
Earthquake Prediction
Successful prediction would include time frame, magnitude and location.
Remains an elusive goal, although measurement techniques have improved
dramatically
in
the
last
50
years:


Seismic Risk Maps - Indicate the likelihood and potential severity of future
earthquakes. Based on historical records of past earthquakes and the
distribution of known faults. Identify areas of seismic gaps (see below) and
earthquake periodicity.
Earthquake Precursors - Most earthquakes have precursors (short term and
long-term changes in the Earth that take place prior to an earthquake).
Unfortunately no reliable precursors have been identified:
1. Seismic Gap - A region in an area of seismic activity that has had little
earthquake activity for a number of years. Represent areas along a fault
that are locked and building up stress. Points out areas which are
"overdue" for an earthquake. Seismic gaps are considered to be
potential sites for major earthquakes. Long-range predictor.
2. Crustal Tilt and Elevation Changes - Can be caused by increasing
pressure in subsurface rocks in the region of the epicenter.
3. Measured by tiltmeters.
4. Changes in Rock Properties (Dilatancy Model) - Rock volume
(dilatancy), electrical and magnetic field changes can precede an
earthquake. Caused by high pressure producing numerous cracks in
rock prior to failure.
5. Changes in Well Water Levels, and Gases - Water levels can rise in
response to increasing pressure (decreases pore space in rock). Radon
levels increase.
6. Animal Behavior - Animals are more sensitive than humans to changes
in the Earth.
7. Radio Waves - Amplitude of ultralow frequency radio waves increased
just prior to the 1989 Loma Prieta earthquake.

Earthquake Prediction Programs - Include monitoring systems along major
fault zones as well as laboratory and field studies of the behavior of rocks
before, during and after major earthquakes. The U.S., Japan, China and Russia
have government-sponsored programs. The Chinese have successfully
predicted several earthquakes, but failed to predict the 1976 Tangshan
earthquake that killed 242,000.
Earthquake Control
Needs much more study. It was accidentally discovered that earthquakes can be
triggered by pumping fluids into fault zones. Water acts as a lubricant, allowing faults
to move. May be able to reduce stress on major faults by triggering small earthquakes
using fluid injection. The best defense currently is good emergency and evacuation
plans.
Structure of the Earth
Study of the travel of seismic waves through the Earth have given information
about its interior. Body waves are refracted (bent) and reflected at boundaries between
two different kind of materials (discontinuity) and travel more slowly through dense
material than through less dense material. From the study of seismic waves, the Earth
is
divided
into
three
major
layers:

Crust - Thin, outer layer of the Earth. Composed of solid rock. Base of the
crust is marked by the Mohorovicic Discontinuity (Moho), which is marked by
increased body wave speed below the discontinuity and by partial refection of
body waves at the boundary. Two basic kinds of crust are indicated by seismic
waves:
1. Continental Crust - Averages about 35 km thick, but ranges from 20-90
km. Average composition of granodiorite with low density.
2. Oceanic Crust - Ranges from 5-10 km thick. Composed of mafic rocks
(primarily basalt and gabbro).
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
Mantle - Thickest layer of the Earth (contains > 80% of the Earth's volume).
Increase in body wave speed below the Moho indicates high density, mafic
rock (upper mantle is believed to be composed of peridotite). The mantle is
essentially solid, but portions of it are capable of slow flow. Bottom of mantle
is about 2900 km deep.
Core - Composed of a liquid outer core (Shadow Zones - S waves disappear
when traveling deeper than 2900 km and P waves are slowed down) and a
solid inner core (P wave velocity increases again). Believed to be composed
primarily of iron mixed with nickel, cobalt, oxygen, sulfur, silica, and carbon.
The Earth's magnetic field is caused by the circulation of the outer liquid core,
but the exact mechanism is unknown. Core extends to the Earth's center at
6370 km depth.
The lithosphere is the rigid outer layer of the Earth, consisting of the crust and the
upper mantle (extends to 50-100 km depth). The asthenosphere is a zone within the
Earth's mantle that is particularly plastic and capable of flow. Boundary between the
lithosphere and asthenosphere is marked by a seismic discontinuity known as the lowvelocity zone (between 100-250 km depth). Tectonic plates are lithospheric plates that
move slowly over the asthenosphere.