Earthquakes

A. There are two types of vibrations produced by earthquakes

1. Body waves travel through the Earth's interior

a. Primary waves (or P waves) are compressional waves that vibrate in the direction of propagation; think of "push" waves. These waves move sort of like a wave moving along a slinky.

b. Secondary waves (or S waves) are waves that vibrate at 90° to the direction of propogation. S-waves travel slower than P-waves; think of "shake" waves. These waves move sort of like a wave moving down a rope that you might shake.

2. Surface waves travel along the outer surface of the Earth. These are the slowest and often produce the worst damage. (one type of surface wave is a Love wave).

B. A seismograph is an instrument used to measure the intensity and location of earthquake by measuring ground motion. When a seismograph measures ground motion during the period of an earthquake, it measures the arrival of the different waves mentioned above.

1. By measuring the amount of time between the arrival of P and S waves in an earthquake, it is possible to determine the distance of the epicenter from the seismic station.

2. An earthquake needs to be recorded at three seismic stations in order to pinpoint the epicenter of the quake.

3. The focus of an earthquake is the region of maximum offset in a fault ruptured by an earthquake.

4. The epicenter is the location directly above the focus at the surface

C. The Richter scale is the most common method of describing an earthquake's magnitude.

1. It uses the amplitude of the largest wave to reach station to estimate the intensity of an earthquake.

2. The Richter scale is logarithmic

a. An earthquake of magnitude 5 produces an amplitude ten times greater than an earthquake of magnitude 4.

b. A magnitude 5 earthquake produces 30x the energy of a magnitude 4 quake.

3. The largest earthquake ever recorded was ~8.6. Other major earthquakes include:

a. Alaska, 1964: 8.4-8.6

b. Chile, 1960: 8.5-8.7

c. San Francisco, 1906: 8.3

A. What is the worldwide distribution of earthquakes?

1. Plate boundaries are common locations of earthquakes. Plates usually move several cm per year. One example is the San Andreas fault zone which marks thee boundary between the Pacific and North American Plates.

2. Earthquakes occur along faults.

a. Faults represent a zone where rupture has occurred repeatedly; it is a zone of weakness.

b. Faults are usually characterized by broken and pulverized rock.

B. What is the elastic rebound theory?

1. The crust is deformed by forces within the Earth.

2. Rocks behave elastically up to a point as stress builds up, then rupture occurs.

3. When rupture occurs at one point along a fault, then the stress is passed down to the next section of the fault. For instance, the area around Loma Prieta is relatively quiet since the 1989 earthquake. The area around Parkfield is expected to rupture at any time because of the buildup of stress.

4. Aftershocks may occur along a fault as the tension settles out and adjusts to the new stress environment.

A. On dry land, shaking ground itself does not usually kill or seriously injure people. It is usually the falling buildings and structural ruptures caused by the shaking that do.

1. The 1906 earthquake in San Francisco caused most of its casualties in the fires from ruptured gas lines.

2. In the 1989 Loma Prieta earthquake, most of the casualties were caused by the collapse of the Cypress Structure freeway in the East Bay area.

B. Different ground types will causes different amounts of damage to buildings.

1. Loose, unconsolidated sediment tends to magnify the amplitude of an earthquake.

2. Water saturated sediment will actually tend to behave like liquid in an earthquake. This results in mud volcanoes, sinking buildings, and additional damage to buildings. This is called liquefaction.

3. Crystalline, and well-cemented rock tend to attenuate seismic waves. These usually afford the best foundation for buildings in an earthquake.

C. Tsunamis usually form in earthquakes around marine areas.

D. Landslides are often initiated by ground shaking.

E. Ground rupture may tear up structures (roads, fences, buildings) that lie across the fault.