SAN DIEGO (FOX 5/KUSI) — Different parts of the country have different natural disasters, like tornadoes in the Midwest or hurricanes in the South. Those in the western U.S. mostly deal with wildfires, earthquakes and droughts. But why is there more seismic activity in one part of the country than the other?

The reason — the western U.S. sits on an active plate boundary while the eastern U.S. is in the stable interior of a tectonic plate, according to the U.S. Geological Survey (USGS). The West also sits over a thin crust with high heat flow that contributes to the rapid movement of the Earth’s surface and faults.

Southern California rattled by earthquake swarm

USGS reports the western U.S. has been tectonically active since the supercontinent Pangea broke up about 200 million years ago, before the San Andreas Fault was formed about 30 million years ago.

Rocks in eastern North America are older and have been exposed to extreme temperatures, making them harder and denser, USGS reports, while rocks in the West are younger and broken up by faults that are also younger.

Another factor as to why earthquakes cause more damage in the East is vulnerable infrastructure that was not made to endure earthquakes.

The West also has more active faults and areas with sparse vegetation, so earthquakes can leave clear markings for scientists to research earthquake history, size and effects.

USGS says the western United States sits along the boundaries of major tectonic plates that make up the Earth’s crust — the North American Plate and the ocean plates to the west.

Here in San Diego County, we are bordered by several fault zones — the Rose Canyon fault along the coast and beneath downtown San Diego, and the Elsinore and San Jacinto faults that go through East County.

Which active fault zones are in San Diego County?

All of San Diego County is located within Seismic Zone 4, which is the highest Seismic Zone and is prone to ground shaking, according to the County of San Diego.

The region’s high seismic activity was brought back into the spotlight after a series of more than 300 earthquakes rattled Southern California over the weekend. The earthquake swarm struck in the Brawley Seismic Zone in Imperial Valley, a high-seismic-risk zone that connects to the Imperial Fault and the San Andreas Fault.

The San Andreas Fault and the San Jacinto Fault are more than 60 miles away from the more populated parts of San Diego County.

The Earthquake Country Alliance says the Rose Canyon Fault is the only major active earthquake fault in the urban San Diego area, which has not produced a major earthquake since before European settlers arrived.

Most San Diegans live less than 15 miles from the Rose Canyon Fault, and ground shaking from a major earthquake could cause damage throughout the entire county, the Earthquake Alliance says. If a major earthquake were to happen during fire season, the fire damage could exceed the damage caused by shaking.

The Southern California Earthquake Data Center says the Brawley Fault Zone in Imperial County usually produces a magnitude 6.0 earthquake every 30 to 40 years, with the last earthquake of that magnitude in that area recorded in 1979.

While earthquakes don’t strike very often in San Diego County, the Earthquake Alliance says one of the many hundreds of faults in the region has a significant earthquake about every decade and urges residents to stay vigilant for a large earthquake along the Elsinore, San Jacinto and San Andreas faults.

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