Understanding Earth Chapter 13: Earthquakes Copyright © 2007 by W. H. Freeman & Company Grotzinger, Jordan Press and Siever Fifth Edition

Concepts Earthquake Stress Strain Elastic Rebound Theory Epicenter Foreshocks, aftershocks P, S and Surface waves (Love and Rayleigh) Moment Magnitude Earthquake Intensity Tsunami First Motion

An earthquake is caused by a sudden rupture (break) of the earth An earthquake generates vibrations that travel through the earth, a.k.a earthquake waves What is an Earthquake??

1906

Why do Earthquakes happen? Plate Motion

How do earthquakes occur? Elastic Rebound Theory is one explanation of how earthquakes take place.

Elastic Rebound Theory Pressure or stress, builds up in the rock, much like a bending rubber eraser in your hands. You can break a rubber eraser if the pressure or stress you apply to it exceeds the natural strength of the rubber.

Elastic Rebound Theory There are two important concepts you should be aware of. Stress in earth sciences refers to the pressure the rock experiences and strain is the actual damage or resultant deformation that we can measure. While the pressure builds up in the rock the rock DOES deform or strain. However, it does so elastically. That is if you release the rock from under stress it will BOUNCE back without any signs of damage. Normal rocks are usually only able to take a small (1-2%) of elastic strain. Stretching or thinning beyond 1-2% can cause permanent damage.

Elastic Rebound Theory When the rock breaks or ruptures some of the energy goes into breaking the rock and a lot of it goes into moving the surround rocks. The rupture itself may remain buried, invisible underground (a blind fault). A rupture may or may not reach the surface. After the stress or pressure is release the rock seals ahd again starts to accumulate stress.

Elastic Rebound Theory Ruptures or breaks in the rock are planar cracks that grow in all directions The rupture itself may remain buried, invisible underground (a blind fault). A rupture may or may not reach the surface.

Foreshocks and Aftershocks are smaller earthquakes that precede and postdate the main shock. What is a foreshcok or aftershock depends on when the main shock comes. But, this is really only known AFTER the big earthquake occurs!

A seismograph is a tool that measure ground motion. During the 1988 Auburn-LSU football game LSU scored a late and winning touchdown. The crowd cheered so loudly that the “earthquake game” made local history because the ground motion appeared on the LSU seismograph. The “EARTHQUAKE GAME”

A seismograph consists of a real or virtual mass that is so large that it moves totally out of “sync” with the frequency of the ground motion, that is as IF IT WERE STILL. The mass is not actually still but its large inertia allows the faster motions of the earthquake groundshake to be registered accurately. Seismograph

3 Types of Earthquake waves (P and S) When an earthquake occurs it starts at depth, in the BODY of the earth. At depth there are only two types of particle motion possible. (P and S waves) P and S waves are known as BODY waves because they always originate inside the earth. These are not the most damaging but they are the first to be detected because they travel at very fast speeds ( e.g. >6000 m/s.)

3 Types of Earthquake waves P waves are the fastest, and first waves to arrive. S waves are the second fastest (~50%) and Surface waves (45%) are the slowest of the three major types of earthquake waves.

--Image 1 Particle Motion

--Image 2 Particle Motion

--Image 3 Particle Motion

--Image 4 Particle Motion

--Image 5 Particle Motion

3 Types of Earthquake waves (Surface) The most damaging earthquake vibrations occur when the P and S waves reach the surface. At the surface they convert into horizontal S waves and a combination of vertical S and P waves called Rayleigh waves.

The focus of an earthquake is the origin at depth of that earthquake. The closest place on the surface of the earth to the focus is called the epicenter. An epicenter is located by triangulating between 3 or more seismograph stations, using the arrival times of two or more of the 3 types of earthquake waves.

Size of an Earthquake An earthquake size can be determined qualitatively or quantitatively and there is an equivalence between the two types of scales.

Size of an Earthquake (1) The Modified Mercalli Intensity scale describes the perceived intensity and damage, in 12 degrees, of the shaking and is useful when there are no seismographs, as in historical earthquakes (e.g. New Madrid 7.5, 1811, Missouri). Damage depends on whether there are people to witness the earthquake effect and on local conditions.

Size of an Earthquake (2) The quantitative measure of the strength of an earthquake is the moment magnitude. Moment magnitude is proportional to the logarithm of the area faulted and seismic energy released during the rupture. It can be measured directly from the seismograms.

Earthquakes generate different first motions in different directions depending on the type of fault that generates the earthquake. A first motion for a normal fault in a horizontal direction is OUT from the fault during the rupture. These tend to occur along divergent plate margin boundaries. ACTIVE FAULTS GENERATE EARTHQUAKES (1)

Earthquakes generate different first motions in different directions depending on the type of fault that generates the earthquake. A first motion for a thrust fault in a horizontal direction is IN to the fault during the rupture. These tend to occur along convergent plate margin boundaries. ACTIVE FAULTS GENERATE EARTHQUAKES (2)

A first motion for a strike-slip fault in a horizontal direction is IN to the fault in certain orientations and OUT from the fault in other orientations. The first motion changes every 90 degrees around the epicenter. ACTIVE FAULTS GENERATE EARTHQUAKES (3)