7.8 Earthquakes at Transform Plate Boundaries

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.

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.

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.

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? What does it mean and why?

Explore More

Use this resource to answer the questions that follow.

1. Where does the San Andreas Fault go offshore in the north? Where does it begin in the south?
2. Where is the most dangerous part of the San Andreas?
3. What is the Carizzo Plain expression of the San Andreas Fault like? Why is it so visible there?
4. He says the largest oceanic plate in the world is stitched to the continental plate. Which plates is he referring to and what type of lithosphere meets on each side of the fault?
5. What does Wallace Creek do at the fault?
6. Why was Daly City a dangerous place to build houses?
7. In the Bay Area what does the fault do that's dangerous for the future?
8. How are scientists mapping the San Andreas in the northern region?
9. Did developers know where the San Andreas Fault went when they built the community of San Bruno?