Resource: Earthquakes: Los Angeles

Print Background Essay

The majority of earthquakes and volcanoes occur along the boundaries of tectonic plates, where slabs of solid rock up to 30 kilometers (19 miles) thick are pulled apart, slide past one another, or collide with other slabs. The Pacific Ring of Fire, so called for the prevalence of volcanic and seismic activity in the area, is a series of such boundaries. There, the Pacific Plate comes in contact with other plates along the west coasts of North and South America, across the Aleutian Islands, and down the east coasts of Asia and Indonesia.

Plate boundaries are defined by the direction plates move in relation to one another. At divergent boundaries, plates move away from each other; at convergent boundaries, they move toward each other; and at transform boundaries, they slide past each other. All three types of movement apply stress and strain to the rocks that make up the plates. When the strain becomes too intense, rocks fracture and shift, resulting in a separation, or fault, as well as the telltale vibrations of earthquakes.

Where the Pacific and North American plates meet along the west coast of North America, both transform and convergent boundaries can be found. Transform boundaries produce strike-slip faults, such as the infamous San Andreas Fault. Along the 1,300 kilometers (800-miles) of the San Andreas, the two plates roughly slide past each other in opposite directions. When the plates become temporarily locked, forces that drive plate motion cause stress and strain to build up in rocks that make up the plates. When this strain exceeds the strength of the rocks involved, the rocks fracture along faults, temporarily relieving the strain and causing the vibrations we feel as earthquakes.

Of greater concern for the city of Los Angeles are thrust faults, which sometimes occur at convergent boundaries. At these faults, which extend directly under the city, rocks can give way to immense pressure causing one slab to move over the top of the other. The lower plate then pushes the other plate upward. Within the last ten years, geologists have discovered that this upward thrusting can sometimes happen very quickly, increasing the height of mountains several feet in a single event. While earthquakes generated by this type of fault might lack the intensity of those associated with the San Andreas Fault, they carry greater potential for catastrophe in Los Angeles because they originate directly beneath the city.

To learn more about the effects of earthquakes, check out Earthquake Prediction and Earthquakes: San Francisco.

To learn more about interactions between tectonic plates, check out Mountain Maker, Earth Shaker and Tectonic Plates, Earthquakes, and Volcanoes.

To learn more about the instruments scientists use to detect earthquakes, check out Earthquakes: The Seismograph.

Design and build an earthquake-proof house of cards in this NOVA classroom activity.

Print Questions for Discussion

• Describe the plate motion of the Pacific Plate and the North American plate throughout California. Is it a smooth boundary? Explain.
• In how many years do scientists expect Los Angeles to be across from San Francisco?
• How does a thrust fault compare with the San Andreas fault? Discuss the different causes of these two kinds of faults.
• Why are there so many small, fractured thrust faults near Los Angeles? What are the implications for the city?
• Discuss the statement: "We'd look like Kansas if we didn't have our faults and our earthquakes."