The July 4th, 2019 Mw 6.4 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. The earthquake was located approximately 150 km northeast of San Andreas Fault - the major plate boundary in the region. At this latitude, the Pacific plate is moving 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.

This region of eastern California has hosted numerous moderate sized earthquakes. Over the past 40 years, 8 other M5+ earthquakes have occurred within 50 km of the July 4th, 2019 earthquake. The largest of these was a M 5.8 event on September 20, 1995, about 13 km to the west-northwest of today’s event, which was felt strongly in the China Lake-Ridgecrest area, and more broadly from Los Angeles to Las Vegas. As of 1 hour after the event the July 4th, 2019 earthquake has been followed by numerous aftershocks, the largest of which was a M 4.7 earthquake 2 minutes after the M 6.4 event. Most aftershocks align in a SW-NE trend around the M 6.4 earthquake, though some also align on a NW-SE trend. The M 6.4 event was also preceded by a series of very small earthquakes (foreshocks) over the previous hour, including a M 4.0 event about 30 mins earlier.

Update 2019/07/05 - 11:17am Pacific Time
The aftershocks sequence of the July 4 Mw 6.4 earthquake appears so far standard for that region, with event productivity that is somewhat higher than average for California. The aftershocks delineate two structures that are nearly orthogonal and include so far several M > 4 events. 

The standard expectation is to have in the next few weeks, as part of the aftershocks sequence, 1 event with a magnitude of about 5.4, 10 events with a magnitude of about 4.4, 100 events with a magnitude of about 3.4, etc.  It is also possible, although not likely, that the July 4 Mw 6.4 earthquake will be followed by a larger event in the area in the next few weeks.

Several teams from the USGS, CGS, EERI GEER, SCEC and other institutions are currently conducting various studies in the field. Given the size of the July 4 event, location, and good regional coverage of seismic stations, there is no strong reason at present for rapid deployment of additional sensors in the area.  We continue to monitor the situation and will adapt the response as needed.  

Update 2019/07/05 - 03:14pm Pacific Time
Photographs from social media showed left lateral offset of CA 178 at a location approximately four miles east of Ridgecrest. Surface rupture mapping teams from the USGS, CGS, and various academic institutions converged on Ridgecrest on the evening of the 4th. Highway repairs, conducted within an hour after the event, show no evidence for afterslip. Most of the surface rupture apparently lies to the NE of the highway, within the China Lake Naval Weapons Center. Access to the base is under negotiation.

Update 2019/07/05 - 11:51pm Pacific Time
An earthquake with magnitude 7.1 occurred around 8:25pm in the same area as part of the sequence of seismicity following the July 4 M 6.4 earthquake. The M 7.1 July 5 event is now followed by its own aftershock sequence. The seismicity of the last 2 days delineates two nearly orthogonal fault zone structures, one trending NW-SE with right lateral slip and the other trending NE-SW with left-lateral slip. Several surface ruptures have been documented including a scrap with ~1m vertical offset on highway 178. The seismicity associated with this sequence of events will continue in the coming days to weeks with numerous additional felt earthquakes.

We continue to monitor the situation and will post more updates tomorrow.

Update 2019/07/06 - 11:59pm Pacific Time

The aftershock sequences of the M6.4 and M7.1 events continue following approximately generic evolution of aftershocks for the region. Plots of locations and preliminary focal mechanisms of many events are available. There are now also preliminary finite source models. 

Geologic mapping of the rupture has been limited by access to the Naval Weapons Center. Teams from the USGS and CGS were able to access to the base and conduct helicopter overflights. Stereo photographs were taken yesterday of the M6.4 rupture traces. Teams from the USGS and CGS conducted overflights of the M7.1 rupture today, starting from the epicenter and moving north and south. Some parts of the base cannot be accessed due to damage. Right lateral slip of 2m and up to 4m of vertical was reported at one point on the M7.1 rupture trace.

Endpoints of rupture are not yet completely known. CGS team was not able to reach the northern extent of seismicity and surface rupture, but did trace rupture to within 15 km of the end of seismicity. USGS team did cover the entire aftershock zone but have not yet compiled information. Teams did not observe rupture north of the base boundary. Southern ends of rupture are better defined. Left lateral strand that slipped in M6.4 shows surface rupture to the power line road. M7.1 right lateral strand shows evidence of slip almost to Searles Valley road, within 6 km of the trace of the Garlock fault.

Initial rapid deployments of seismic sensors and campaign GPS are underway by USGS and SCEC teams, and plans are in progress to follow up in a few days with more extensive deployments of seismic nodes.

Update 2019/07/07 - 08:27pm Pacific Time

The aftershock sequences of the M6.4 and M7.1 events continue following approximately generic evolution of aftershocks for the region. Plots of locations and preliminary focal mechanisms of many events are available. Preliminary finite source models show large slip to the northwest of the fault intersection. 

Drone footage showed complex left-lateral rupture.  Mapping by the GEER team on the southern part of the rupture found 1.5 to 3 meters of rupture that was very widely distributed - in places up to 1000’.  The USGS team worked to constrain the northern extent of the rupture, but had not reported back by the time of the 7:00 pm clearinghouse call, so the northern extent of the surface rupture is still not completely mapped. JPL is flying UAVSAR to map rupture and monitor post seismic deformation.  Drones on the base are prohibited, and they would like to be advised if anyone is flying drones near the base. USGS aftershock forecasts are highly appreciated.  They are being updated daily and are available at: https://earthquake.usgs.gov/earthquakes/eventpage/ci38457511/oaf/commentary.

Initial deployments of seismic sensors and campaign GPS are underway by USGS and SCEC teams, and plans are in progress to follow up later this week with more extensive deployments of seismic nodes.