California: Small Fault, Big Quake!

Jul 15, 2019 By Pallavi C, Writer
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On July 4 and 5, 2019, a pair of powerful earthquakes shook Southern California with a magnitude of 6.4 and 7.1!

Fortunately, they took place in a sparsely populated area and did not cause major damage to infrastructure or injuries among residents. These were followed by thousands of aftershocks, with at least 70 of them measuring 4.0 magnitude or more.

For those living in California, earthquakes are a way of life with many small tremors periodically rocking the state. However, this recent set of quakes was different.

A Smaller Fault Line

California lies on the Ring of Fire, a 25,000-mile-long region along the Pacific Ocean that is home to some of the world's most active volcanoes and largest earthquake activity.

As explained in our earlier article here, our Earth is made up of tectonic plates that are constantly moving and grinding against each other and causing the ground to shake. The boundaries where these plates meet are known as fault lines.

California's largest fault line -- the 800-mile long San Andreas fault, forms the boundary between the Pacific Plate and the North American plate. While 80% of the quakes in California are along the San Andreas fault, the recent tremblors were on lesser-known fault lines.

Scientists from NASA’s Advanced Rapid Imaging and Analysis team concluded that these earthquakes took place in an active fault zone known as Eastern California’s Shear Zone. An image shared by NASA shows ground on either side of the fault line moving in opposite directions and causing significant displacement of the Earth. The images also show that the 6.4 and 7.1 magnitude quakes were perpendicular to each other. 

Unusual Aftershock

Usually, an earthquake consists of a foreshock, earthquake, and aftershocks. But, the likelihood of a large, 6.4 earthquake/foreshock triggering an even larger 7.1 earthquake is 1 in 20 according to Susan Hough, a USGS seismologist.

It is quite possible that this wasn’t a conventional earthquake, and the two events were completely separate but were caused in the same area or that a single event led to the rupture of multiple fault lines. In 1992, an earthquake in  Landers, California fractured along 5 fault segments and even has its own name- The Landers Sequence. The total length of the earthquake’s rupture was around 80 kilometers. It was one of the first complex earthquakes and led scientists to conclude that multiple faults could be responsible for one tremendous earthquake. 

Geologists believe California is due for a big quake, as the last destructive one was back in 1989. The recent quakes are a reminder for California’s residents that preparation is key. The state provides information on how to keep homes safe, what to do in case of a tremblor, and recommends keeping an earthquake kit ready.

Sources: LA Times , National Geographic, Forbes, USGS.gov, NPR