Geology 111G Lecture 20 16 April 2005

Earthquakes

Earthquakes

Seismology

Seismic Waves

Records of Earth Waves

Travel Times

Locations of Earthquakes

Interpreting Earthquakes

I. Seismology: the scientific study of earthquakes

A. Typically, the discipline is included in the field of geophysics, which merges geology and physics.

B. Seismograph: instrument that measures and record earth waves

C. Seismogram: record of wave amplitude generated by seismograph

II. Seismic Waves

A. Earthquakes represent the energy generated when rupture occurs along a fault; the failure event releases energy that travels through the earth as waves, like ripples on a pool of water.

B. There are two general classes of seismic waves.

1. Body Waves travel through the interior of the earth.

a. Compressional waves move particles forward and backward relative to their travel path; they travel by changes in volume caused by expansion and contraction and can move through solid, liquid, or gas. They have the highest velocity of seismic waves - 6 km/sec in the shallow crust - and thus arrive first at the seismograph station. For that reason they are also called P or Primary waves.

b. Shear waves move particles at right angles to their direction of travel they only travel through materials that resist a change in shape. They travel more slowly through the earth - about 3.5 km/sec in the shallow crust - than P waves and so are also referred to as S waves or Secondary waves.

2. Surface Waves travel along the surface of the earth; a good analogy is ripples on the surface of a pond. Last to arrive at seismograph station because of their slow speed and also because they travel a longer path (along the surface, rather than through the earth).

III. Record of Earth Waves

A. Travel Times: Earth waves traveling through a medium of uniform elasticity will travel in straight lines. If the elasticity increases gradually, as it does in the earth, the waves move faster and faster and travel in a smooth arc.

1. Distance traveled through the earth is described by the number of degrees of arc between the earthquake and the recording station or its equivalent distance on the surface.

2. Earthquake waves traveling 11,00 km or less move along a predictable path.

3. Compressional earthquake waves traveling more than 11,000 km arrive late, while shear waves don't arrive at all.

a. This evidence indicates that the earth has a liquid core.

B. Locations of earthquakes

1. Focus: source of a set of earthquake waves; location where earth failure generates the waves. Usually occurs at some depth in the earth, up to about 700 km depth.

2. Epicenter: location on earth's surface directly above the focus.

C. Locating Epicenters

1. The time difference between the P and S waves is calculated and plotted on a time-distance graph; distance is them directly read from the graph.

2. An arc is drawn around the recording station, indicating that distance.

3. Three stations are required to uniquely define the epicenter.

IV. Interpreting Earthquakes

A. Earthquake magnitude: Index of earthquake's energy at its focus based on the amount of motion caused by certain waves.

1. Richter magnitude: logarithmic scale based on measured wave amplitude at 100 km from epicenter.

a. Each whole number on scale represents 10 times the wave amplitude of the previous number.

1) 2.5: large enough to be felt in local area

2) 4.5: (100 times the wave amplitude) large enough to create local damage.

3) 7 or greater: major destructive earthquake

b. An increase in one unit of Richter scale represents 31 times more energy release.

c. Effect of surface material on wave amplitude: example of Cypress structure on Nimitz freeway.