#14,167
Thirty-four hours after an M6.4 earthquake rocked the Mojave desert - which has been followed by thousands of small to moderate aftershocks - the same region was hit overnight by a much larger M7.1 temblor.
Damage in the more populated regions of Southern California appears light, but heavier damage has been reported in the small towns closer to the epicenter.This was the largest quake to hit Southern California in 20 years. Luckily the epicenter was more than 120 miles north of Los Angeles, and so damage reports from the highly populated coastal cities have been few.
There are reports of structure fires, and other damage, in the smaller cities nearer the epicenter. We'll get a better idea of the extent after daylight returns.
From Dr. Lucy Jones' twitter account, we are reminded that there is about a 1 in 20 chance that an even larger quake will occur in this sequence in the next week or two.
The USGS describes the event:
Tectonic Summary
The July 6th, 2019, 03:19 UTC (July 5th 20:19 locally) Mw 7.1 earthquake in eastern California, southwest of Searles Valley, occurred as the result of shallow strike slip faulting in the crust of the North America plate. Focal mechanism solutions for the earthquake indicate rupture occurred on a steeply dipping fault as the result of either right lateral slip on a plane striking NW-SE, or as left lateral slip on a plane striking SW-NE.
At the location of this earthquake, approximately 150 km northeast of San Andreas Fault - the major plate boundary in the region – the Pacific plate moves to the northwest with respect to the North America plate at a rate of approximately 48 mm/yr. The location of the earthquake falls within the Eastern California shear zone, a region of distributed faulting associated with motion across the Pacific:North America plate boundary, and an area of high seismic hazard. More detailed studies will be required to precisely identify the causative fault associated with this event, though seismic activity over the past 2 days has been occurring on two conjugate fault structures in the Airport Lake Fault Zone.
This earthquake occurs approximately 34 hours after and 11 km northwest of a M 6.4 event in the same region, on July 4th, 2019, at 17:33 UTC. The July 4th event was preceded by a short series of small foreshocks (including a M4.0 earthquake 30 minutes prior), and was followed by a robust sequence of aftershocks, including almost 250 M 2.5+ earthquakes (up until the M 7.1 event). Those events aligned with both nodal planes (NE-SW and NW-SE) of the focal mechanism solution of the M 6.4 event, which was very similar in faulting style to today’s M 7.1 earthquake. The sequence includes two other M5+ earthquakes, one of which occurred 20 seconds before the M 7.1 event. The M 7.1 earthquake occurred at the NW extension of the prior sequence.
While commonly plotted as points on maps, earthquakes of this size are more appropriately described as slip over a larger fault area. Strike-slip-faulting events of the size of the July 6, 2019, earthquake are typically about 70x15 km (length x width).
While the odds greatly favor that this M7.1 quake will be the largest in this sequence, the same thing was being said about July 4th's M6.4 temblor.
This region of eastern California has hosted numerous moderate sized earthquakes. Over the past 40 years, prior to the July 4th event, 8 other M5+ earthquakes have occurred within 50 km of the July 6th, 2019 earthquake. The largest of these was a M 5.8 event on September 20, 1995, just 3 km to the west of today’s event, which was felt strongly in the China Lake-Ridgecrest area, and more broadly from Los Angeles to Las Vegas.
Even without a bigger quake, the region will undoubtedly be struggling with hundreds or thousands of aftershocks in the days and weeks ahead.Yesterday morning, in Southern California: Shaken and Hopefully Stirred To Action, I posted a long blog on the prolonged earthquake drought in California, and the need to prepare for when that drought ends.
