7.7 Earthquake Zones

What caused the earthquake in Northridge, CA, in 1994?

The city of Northridge,CA, experienced a 6.7 magnitude earthquake in 1994. Roads, bridges and elevated highways, like this one, were damaged and 72 people died. Northridge lies on a blind thrust fault that was only discovered as a result of the quake. The fault is part of the San Andreas Fault system, which is part of the Pacific Ring of Fire (and ground shaking).

Annual Earthquakes

In a single year, on average, more than 900,000 earthquakes are recorded and 150,000 of them are strong enough to be felt. Each year about 18 earthquakes are major, with a Richter magnitude of 7.0 to 7.9, and on average one earthquake has a magnitude of 8 to 8.9.

Magnitude 9 earthquakes are rare. The United States Geological Survey lists five since 1900 (see Figure below and Table below). All but the Great Indian Ocean Earthquake of 2004 occurred somewhere around the Pacific Ocean basin.

Earthquake Zones

Nearly 95% of all earthquakes take place along one of the three types of plate boundaries.

• About 80% of all earthquakes strike around the Pacific Ocean basin because it is lined with convergent and transform boundaries (Figure below).
• About 15% take place in the Mediterranean-Asiatic Belt, where convergence is causing the Indian Plate to run into the Eurasian Plate.
• The remaining 5% are scattered around other plate boundaries or are intraplate earthquakes.

Summary

• Small earthquakes are extremely common, but the largest earthquakes are extremely rare.
• The vast majority of earthquakes happen at plate boundaries.
• The Pacific Ocean basin has the most earthquakes due to the plate boundaries that line it; the Himalaya region has the second most due to the convergence of India and Asia.

Review

1. Why are most earthquakes at plate boundaries?
2. Why are some earthquakes away from plate boundaries?
3. What two types of plate motions occur along the Pacific Rim? Where would you find each type along western North America?
4. What type of plate motions cause the Mediterranean-Asiatic quakes?

Explore More

Use the resource below to answer the questions that follow. 

1. What was the magnitude of the Chilean earthquake?
2. What two plates are converging near Chile that caused the earthquake?
3. What is the Ring of Fire? What occurs along this Fing of Fire?
4. What was the strongest earthquake ever recorded? When did it occur?
5. Why are scientists urging Memphis to adopt building codes similar to Chile's?