The Causes of Earthquakes

Loretta F. Kasper, Ph.D.

History of Earthquake Study

Questions about the causes and the nature of earthquakes have occupied the minds of people since ancient times.  Earthquakes have been attributed to a variety of things from subterranean winds to fires deep within the earth.  However scientific theories about the causes of earthquakes are relatively recent.

            In 1859, the Irish engineer Robert Mallet proposed that earthquakes occurred by a sudden flexing and constraint of the elastic materials forming a portion of the earth's crust or by the crust giving way and becoming fractured.  In the 1870's, the English geologist John Milne devised a forerunner of today's earthquake recording devise, the seismograph.

            The modern seismograph was invented by the Russian seismologist Prince Boris Golitzyn in the early 20th century.  This device used a magnetic pendulum suspended between the poles of an electromagnet.

Theories About the Cause of Earthquakes

            There are two major theories which have been advanced to explain the cause of earthquakes.  The first theory states that the snapping back of distorted and ruptured rocks is the cause of earthquakes.  This means that the rocks within the earth break apart, release seismic waves, and then "snap back" much like a rubber band does when we stretch and then release it. 

            Seismic waves have an extremely complex nature.  The sudden force that generates them is not an actual blow, but a snap of billions of tons of bedrock as they are twisted and strained out of shape by an accumulation of geological forces built up over many, many years.  The bedrock ruptures along a fault and then snaps back, relieving the stress.  This "snapping back" causes the earthquake.  This phenomenon, called "elastic rebound," was first described by Harry Fielding Reid, the man who led the investigation into the San Francisco earthquake of 1906. 

            The second theory regarding the cause of earthquakes is based on the science of plate tectonics.  This theory proposes that earthquakes result from stresses set up by movement in the major and minor plates that make up the crust of the earth.  This theory describes the earth as being divided into many different pieces, much like a jigsaw puzzle.  These tectonic earthquakes are very destructive and cause the most devastation.

            Earthquakes can also be caused by volcanic activity, as in Hawaii, or even by human activity.  When people fill new reservoirs or detonate atomic explosives underground or pump fluids deep into the earth through wells, they can sometimes induce earthquakes.

Plate Tectonics

            Plate tectonics is the study of the movement of the earth's plates.  Plate tectonics is an important area of study because plate movements generate a large proportion of all earthquakes.  When plates glide along each other sideways, it is called a strike-slip fault.  When the plates snag, pressure builds until one lurches forward, producing an earthquake.  The San Andreas fault in California is an example of a strike-slip fault.

            When one plate pushes over another, it is called a thrust fault.  It too will rupture if the plates lock, but its presence may be unknown until then.  The Davis fault below Los Angeles is an example of a thrust fault.  The rupture of an unknown thrust fault running east-west in the San Fernando Valley appears to be the cause of the 6.6 magnitude Northridge earthquake that struck near Los Angeles in January, 1994.

            Earth Waves

            The interior of the earth, called the earth's crust, is known to us by studying the records of earth waves.  Earth waves carry away some of the energy released when earth materials rupture.   Waves are generated through the earth as a consequence of a rupture of the cracks in the earth or a constantly expanding wave front.

            There are two major types of earth waves: body waves and surface waves.  Body waves travel through the interior of the earth.  Surface waves travel along the surface.   There are three sets of earth waves--P, S, and L waves--evident in earthquakes. 

            P or primary waves go through anything: gas, liquid, or solid.  The wave particles move in a push-pull, or compressional wave.  These waves move out from the quake's origin and alternately compress and stretch the material they travel through.

            S, or shake or shear, waves go through solids only, and are responsible for the breaking of solids.  The particles move at right angles to the direction of the waves advance.  They produce a ripple effect, an up-and-down and side-to-side oscillation like the snapping of a rope.

            L waves are surface waves that travel along the surface to the recording station from the area directly about the focus of the earthquake.  These waves are more complex.  One type, known as Love waves, whips back and forth horizontally.  The second type, called Rayleigh waves, moves along like an ocean breaker, rotating the rock and soil in an elliptical pattern.

            All waves travel at a constant velocity through different materials.  We can measure the velocity of the waves to determine the type of material they travel through.  As waves pass through different materials there is a change in their velocity.  This change is called a seismic discontinuity.

            Time-distance graphs are the bases for analyzing the history and speed of earth waves.  Travel times are governed by the materials through which waves have traveled.  Locating earthquakes requires data from several stations.  Earthquake prediction may be possible primarily by observing and interpreting preceding, or precursor, events, including crustal movements, tilt of the earth's surface, and changes in fluid pressure and in electrical and magnetic fields.

The Structure of the Earth

            The earth is made up of three layers, crust, mantle, and core.   The earth's crust is separated from the mantle at a discontinuity called the Mohorovic, or the Moho, or the M discontinuity.  The crust of the continental U.S. varies between 20-60 km in thickness.  The crust under the oceans averages about 5 km in thickness. The mantle extends to 2900 km in depth.  The lithosphere contains the crust and the upper, colder part of the mantle.  The asthenosphere extends downward from its border with the lithosphere to about 500 km.  The core is thought to have a liquid outer zone and a solid center; the outer zone behaves as a liquid in not allowing S waves to penetrate.

Vocabulary

Directions:  Using the context of the reading passage, write a definition for the following words.

            1. attributed

            2. distorted

            3. accumulation

            4. induce

            5. strike-slip fault

            6. thrust fault

            7. oscillation

            8. velocity

            9. discontinuity

            10. precursor

Comprehension Questions

Directions:  Using your own words and the material provided in the reading passage, write an answer to the following questions.

            1.  How were earthquakes explained in ancient times?

            2.  Who was Robert Mallet?

            3.  Who was responsible for inventing the seismograph?

            4. What is the elastic-rebound hypothesis for the cause of earthquakes? Which scientist proposed the elastic-rebound hypothesis?

            5.  What is a second theory about the cause of earthquakes based on?

            6.  Why are tectonic earthquakes so dangerous?

            7.  In addition to elastic-rebound and plate tectonics, what other types of activity can cause earthquakes?

            8.  Plate movements generate a ______________of all earthquakes.

            9.  Describe an earthquake in a strike-slip fault.

            10.  Describe an earthquake in a thrust fault.

            11.  What are the principal earthquake waves?  How are they similar?  How do they differ from one another?

            12.  What are some of the principal kinds of earthquake precursors?

13.    What does a seismic discontinuity tell us about the structure of the earth?

            14.  What have earthquake studies taught us about the structure of the earth's interior?  Make a sketch to show the various parts of the earth.