How deep below the Earth’s surface do earthquakes start?

The western coast of the United States is one of the most earthquake-vulnerable areas in the world. Throughout the 20th century, several earthquakes have ravaged the area, including a 2.9 magnitude quake that happened this past Monday in Truckee, a small town which is along California’s eastern border near Reno, Nevada. Unfortunately for residents of California, not every earthquake was as harmless or insignifcant as the one on Monday. According to, California has fallen victim to two 7.9 magnitude earthquakes in 1857 and 1906. An earthquake even postponed game 3 of the 1989 World Series between the San Francisco Giants and the Oakland Athletics; as both teams prepared for the big game a 6.9 earthquake rattled Candlestick Park and the surrounding areas.

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The scene at Candlestick Park after a 6.9 magnitude earthquake rattled California’s Bay Area prior to Game 3 of the 1989 World Series between the Oakland Athletics & the San Francisco Giants (FOX Sports)

A new study by the California Institute of Technology, also known as Caltech, shows that earthquakes originate deeper below the earth’s surface than we thought. For years, seismologists believed that earthquakes originated anywhere between 12-15 feet below the Earth’s surface, but this study found that earthquakes can originate well deeper than 15 feet below the surface. Three scientists from Caltech studied the Newport-Inglewood Fault Zone, one of the most dangerous in Southern California. According to the LA Times, this particular fault zone is responsible for the 6.4 magnitude Long Beach earthquake of 1933, which caused $40 million in property damage and 115 deaths, according to the US Geological Survey. The findings of Caltech’s study were found based on six months worth of data from over 5000 sensors installed underground in Long Beach, CA. It studied each earthquakes’ density, seismicity and helium ratios, and also looked at the earthquakes’ size distribution in Long Beach only.

The Earthquakes’ size distribution in Long Beach (Asaf Inbal, Jean Paul Ampuero, Robert W. Clayton, California Institute of Technology)

So what does this mean? Why is it important that earthquakes originate below 15 feet under the Earth’s surface? Earthquakes on the surface of the Earth can now travel much deeper below the Earth’s surface, which means that they will be bigger, more violent and cause more damage and fatalities. While there is almost no way to stop earthquakes from happening, this leads to many other questions about the state of our planet. Why do earthquakes travel so deep below the Earth’s surface? What else could earthquakes traveling deeper below the surface cause?

Jean Paul Ampuero, one of the three authors of the study by Caltech, first got the idea that something was wrong when an 8.6 magnitude earthquake hit the Indian Ocean. The current belief of earthquakes originating 12-15 feet below the surface wasn’t consistent with an earthquake as strong as that. As a result, Ampuero, Asaf Inbal and Robert W. Clayton all got to work, and found that the belief that earthquakes originated 12-15 feet below the surface of the Earth was wrong, which shows that, as seen multiple times in class, scientists are wrong most of the time.

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