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Earthquakes at Transform Plate Boundaries

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Would you like to live in San Francisco?

Lots of people live in California for the weather. Transplants from snowy climates think they've found paradise in the state's warm sunshine. What if you got your dream job in San Francisco? Would you take it? Are you afraid enough of the region's potential for large earthquakes that you wouldn't? Look at the map of faults in the Bay Area ( Figure below ) before you decide.

Transform Plate Boundaries

Deadly earthquakes occur at transform plate boundaries. Transform faults have shallow focus earthquakes. Why do you think this is so?

California

As you learned in the chapter Plate Tectonics, the boundary between the Pacific and North American plates runs through much of California as the San Andreas Fault zone. As you can see in the ( Figure below ), there is more than just one fault running through the area. There is really a fault zone. The San Andreas Fault runs from south to north up the peninsula, through San Francisco, gets through part of Marin north of the bay, and then goes out to sea. The other faults are part of the fault zone, and they too can be deadly.

The faults along the San Andreas Fault zone produce around 10,000 earthquakes a year. Most are tiny, but occasionally one is massive. In the San Francisco Bay Area, the Hayward Fault was the site of a magnitude 7.0 earthquake in 1868. The 1906 quake on the San Andreas Fault had a magnitude estimated at about 7.9 ( Figure below ). About 3,000 people died and 28,000 buildings were lost, mostly in the fire that followed the earthquake.

Map of faults in the San Francisco Bay Area

(a) The San Andreas Fault zone in the San Francisco Bay Area. (b) The 1906 San Francisco earthquake is still the most costly natural disaster in California history.

Recent California earthquakes occurred in:

  • 1989: Loma Prieta earthquake near Santa Cruz, California. Magnitude 7.1 quake, 63 deaths, 3,756 injuries, 12,000+ people homeless, property damage about $6 billion.
  • 1994: Northridge earthquake on a blind thrust fault near Los Angeles. Magnitude 6.7, 72 deaths, 12,000 injuries, damage estimated at $12.5 billion.

In this video, the boundaries between three different tectonic plates and the earthquakes that result from their interactions are explored: http://www.youtube.com/watch?v=upEh-1DpLMg (1:59).

New Zealand

New Zealand also has a transform fault with strike-slip motion, causing about 20,000 earthquakes a year! Only a small percentage of those are large enough to be felt. A 6.3 quake in Christchurch in February 2011 killed about 180 people.

Summary

  • Transform fault earthquakes have shallow focus because the plates meet near the surface.
  • The San Andreas Fault is actually a fault zone made up of a number of other active faults.
  • New Zealand also has a transform plate boundary.

Practice

Use these resources to answer the questions that follow.

1. What is a transform boundary?

2. Give an example of where a transform boundary is found.

3. How far does the San Andreas Fault extend?

4. What two plates form the fault?

5. What type of fault is it?

6. What causes an earthquake?

7. What is a creepmeter?

8. How many earthquakes occur at the San Andreas fault each year?

Review

1. Why are earthquakes at convergent plate boundaries sometimes deep, while those at transform plate boundaries are always shallow?

2. Are the earthquakes that take place along the other faults in the San Andreas Fault Zone always smaller than the earthquakes that take place on the San Andreas Fault itself?

3. Do you expect that the quiet along the San Andreas Fault near San Francisco since 1906 means that earthquake activity is calming down along that plate boundary?

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