By David Jacobson, Temblor
Updated: 11 a.m. Tuesday March 14
Last night, at 10:06 p.m. local time, a M=3.6 shook San Bernardino, and was also felt in eastern Los Angeles County. The exact epicenter of the quake was in Loma Linda, just south of the city of San Bernardino, which is home to over 200,000 people. While the quake was not strong enough to cause damage, over 3,000 people reported feeling the quake on the USGS website. However, many more would have felt the earthquake, which registered moderate shaking near the epicenter.
This earthquake did not occur on the San Andreas Fault, but rather along the San Jacinto Fault. While not as famous as the San Andreas, the San Jacinto Fault is also a major strike-slip fault in the region, on which many earthquakes occur, and which poses significant hazards to Southern California.
Running from north of San Bernardino, where it branches off the San Andreas, to south of the Salton Sea, the San Jacinto Fault traverses the Inland Empire, and near major development. Because of this, many people are directly on the fault, and lifelines such as electricity, water, and fuel cross it. Furthermore, San Bernardino is home to a high concentration of brick buildings that would be at risk in the event of severe shaking. Therefore, even though a San Jacinto earthquake likely wouldn’t be as large as a San Andreas Fault event, the impact would be significant. The reason why a San Jacinto quake wouldn’t be as large is because the fault does not undergo as much compression as the San Andreas does, according to Egill Hauksson, a Research Professor of Geophysics at Caltech.
The San Jacinto Fault is also of interest to scientists, as some believe that it may be possible for an earthquake to start on the San Jacinto, and continue north onto the San Andreas in a M=7.5 event. Last year, in a study by Julian Lozos, an Assistant Professor at California State Northridge who completed the work while at Stanford, it was examined whether the M=6.9 1812 Southern California earthquake was the result of rupture along both faults. If such an event is truly possible it would alter the seismic hazard of Southern California. Therefore it is important for Californians, especially those in the southern part of the state, to be reminded that it’s not just the San Andreas that can cause shaking.
Update: At 10:14 a.m. on March 14, there was another M=3.6 along the San Jacinto Fault, this time at the southern end near the Salton Sea. This quake registered strong shaking near the epicenter according to the USGS, though not as many people would have felt it as it occurred in a lower-populated area. The map below shows the location of both M=3.6 quakes. Should any more quakes occur, we will continue to update this post.
Los Angeles Times
Julian C. Lozos, A case for historic joint rupture of the San Andreas and San Jacinto faults, 11 Mar 2016: Vol. 2, no. 3, e1500621, DOI: 10.1126/sciadv.1500621