Topic 5: Earthquakes

This aerial photograph shows the destruction that occurred during the February 2, 1971 San Fernando earthquake. The freeway bridge and road were extensively damaged during this shaking of the crust.

New words:

(images courtesy of http://earthquake.usgs.gov/4kids//image_glossary/)

Earthquake: Earthquake is a term used to describe both sudden slip on a fault, and the resulting ground shaking and radiated seismic energy caused by the slip, or by volcanic or magmatic activity, or other sudden stress changes in the earth.

Seismograph: A seismograph, or seismometer, is an instrument used to detect and record earthquakes.

Bedrock: The bedrock is the relatively hard, solid rock that commonly underlies softer rock, sediment, or soil.

Richter Scale: The Richter magnitude scale was developed in 1935 by Charles F. Richter of the California Institute of Technology as a mathematical device to compare the size of earthquakes.

Seismic Wave: A seismic wave is an elastic wave generated by an impulse such as an earthquake or an explosion. Seismic waves may travel either along or near the earth's surface (Rayleigh and Love waves) or through the earth's interior (P and S waves).

P wave: A P wave, or compressional wave, is a seismic body wave that shakes the ground back and forth in the same direction and the opposite direction as the direction the wave is moving.

S wave: An S wave, or shear wave, is a seismic body wave that shakes the ground back and forth perpendicular to the direction the wave is moving.

Surface wave: A surface wave is a seismic seismic wave that is trapped near the surface of the earth. There are 2 types of surface waves.

• Rayleigh wave A Rayleigh wave is a seismic surface wave causing the ground to shake in an elliptical motion, with no transverse, or perpendicular, motion.

• Love wave A Love wave is a surface wave having a horizontal motion that is transverse (or perpendicular) to the direction the wave is traveling.

Aftershocks: Aftershocks are earthquakes that follow the largest shock of an earthquake sequence. In general, the larger the mainshock, the larger and more numerous the aftershocks, and the longer they will continue.

Focus: The focus is the point within the earth where an earthquake rupture starts.

Epicenter: The epicenter is the point directly above the focus at the surface of the Earth.

Resources: Text Book pages 395-405

• Animated site on earthquake development
• Recent earthquakes in California
• Cool earthquakes for kids (lots of info and pictures)
• What is a Geophysicist?
• Excellent information site - with animations
• About Seismic waves
• Effects of Earthquakes
• Richter scale
• Tsunami (with animation)
• Earth's Interior
• U.S. Earthquakes

Earth's central stucture

Tsunami wave

Faults

We classify faults by how the two rocky blocks on either side of a fault move relative to each other.

Normal Fault: A normal fault drops rock on one side of the fault down relative to the other side.

Reverse Fault: Along a reverse fault one rocky block is pushed up relative to rock on the other side

Strike Slip Fault: The rocky blocks on either side of strike-slip faults scrape along side-by-side. You can see in the illustration that the movement is horizontal and the rock layers beneath the surface haven't been moved up or down on either side of the fault.

Horst and Graben

Where normal faults slip down causing a valley: e.g. The Owens Valley and the Sierra Nevada fault zones are examples of high-angle normal faults.

Resources:

• Animated faults
• Fault development (USGS)