California’s coast was struck by seven underwater earthquakes on Sunday—the largest reaching a magnitude of 5.8 on the moment magnitude scale (MMS), the U.S. Geological Survey (USGS) reported. Californians in the area reported weak to moderate shaking but no damage after the seismic activity.
The first took place at 11:24 a.m. PST, 46 miles (74 kilometers) west of Petrolia, and measured 2.6 on the MMS. It was followed some hours later by the second and largest quake at 7:59 p.m. PST, which struck 53 miles (86 kilometers) west of Petrolia.
A string of earthquakes then took place later that evening, from 9:10 p.m. PST on March 8, 2020 to 12:24 a.m. PST on March 9, 2020, measuring between 2.5 and 4.9 on the MMS.
The quakes occurred along the Mendocino Fracture Zone at the junction between the San Andreas Fault and Cascadia Subduction Zone.
The Mendocino Fracture Zone is part of the Mendocino Ridge, which extends more than 3,100 miles (5,000 kilometers) westwards from California into the Pacific, separating the Gorda Plate in the north from the much larger Pacific Plate in the south.
The Gorda Plate lies on a Great Subduction Zone (GSZ), called the Cascadia Subduction Zone, which underthrusts North America.
According to the Pacific Northwest Seismic Network (PNSN), GSZs produce the largest earthquakes in the world and the Cascadia Subduction Zone has produced earthquakes of magnitude 9.0 or greater in the past. It will likely produce the same in the future.
The last megathrust earthquake that has been recorded in the region took place in January 1700. However, PNSN states that the geological record shows that similarly large earthquakes have taken place at least seven times in the previous 3,500 years and tend to take place once every 400 to 600 years.
The U.S. West Coast lies in a region of high seismic activity with more than 500 active and 15,000 known fault lines in the state of California alone. However, most earthquakes are too small to be noticed unless you are reading the results of a seismograph.
According to the USGS, there are approximately 10,000 earthquakes in southern California each year but just a few hundred are a magnitude 3.0 or higher. There only tends to be 15 to 20 greater than a magnitude 4.0, considered light and involving little damage.
According to the New Yorker, geologists predict there will be another “big one” in the Pacific northeast that could cause an area from the continental shelf to the Cascades running from Canada to California to drop by as much as six feet and rebound 30-100 feet to the west, also triggering a massive displacement of ocean water.