Paul Somerville, Chief Geoscientist, Risk Frontiers
An M 6.4 earthquake occurred near Ridgecrest, Southern California, about 180 km north of Los Angeles, on July 4th, 2019, preceded by a short series of small foreshocks (including an M 4.0 earthquake 30 minutes prior), and was followed by a strong sequence of aftershocks, whose epicentres aligned with both possible fault planes (NE-SW and NW-SE) of the focal mechanism solution of the M 6.4 event, as shown in Figure 1. On July 6th UTC (July 5th 20:19 locally) an Mw 7.1 earthquake at the northwest extension of the M 6.4 event was preceded by 20 seconds by a magnitude 5.5 earthquake.
M 6.4 Earthquake Ruptured Two Orthogonal Faults
The epicenter of the M 6.4 earthquake is located near the intersection of its two possible fault planes, and the distribution of aftershocks on two orthogonal planes, shown on the left of Figure 1, suggests that it ruptured both of them. As shown on the left side of Figure 2, an earthquake is represented by a shear dislocation on a fault, shown by the opposing thick arrows, which has a force representation consisting of two equal but opposing couples, represented by the two pairs of thin arrows shown near the outer edges of the cloverleaf pattern. The opposing couples maintain dynamic equilibrium by yielding no net force and no net torque. This means that, observed from a distance and only using information on the direction of first motion of the seismic waves (up or down), we cannot identify on which of the two possible fault planes the earthquake occurred.
As shown in the centre panel of Figure 2, the same force system – north-south compression and east-west extension, shown by the wide double arrows, can produce strike-slip movement on either the northwest (left) or northeast (right) striking fault plane. This explains why the M 6.4 event could rupture both of the two possible fault planes in the same earthquake. Seen from afar using first motions of P waves, the two potential fault planes are demarked by up (compressional) and down (rarefactional) quadrants that cover the earth’s surface, as shown on the right of Figure 2.
To resolve which plane or planes hosted the earthquake, we need to look at aftershock locations (Figure 1), look for surface faulting (Figure 3), or analyse the mainshock waveforms to identify where the seismic waves actually came from (Figure 4). The latter has been done of the M 7.1 event (Figure 4), which shows horizontal rupture of up to 2.5 metres over a length of 60 km of the fault extending from the ground surface to a depth of about 10 km. The road that was ruptured by both earthquakes is State Highway 178, whose location is shown partially in Figure 5; it extends eastward from Ridgecrest.
Relationship to the San Andreas Fault
The San Andreas fault, which runs diagonally from northwest to southeast on the left side of Figure 6, forms the main and long-established boundary between the North American plate to the east and the Pacific plate to the west. It runs through San Francisco and lies close to Los Angeles and other Southern California cities, so news of a large earthquake in California raises alarm among the general public.
However, another incipient part of the boundary is forming to the east of it, running in a north-northwesterly direction, and it hosted the M 7.6 Owens Valley earthquake of 1872 and the July 2019 Ridgecrest earthquakes, shown on the right side of Figure 6. The orange lines in Figure 6 indicate faults that have not ruptured in historical time, and the red lines are ones that have. The three big historical earthquakes in California are the M 7.9 1857 Fort Tejon, south-central San Andreas earthquake (roughly from the latitudes of San Luis Obispo to Riverside, Figure 6, left), the 1872 M 7.6 Owens Valley earthquake (Figure 6, right), and the 1906 M 7.8 San Francisco earthquake on the Northern San Andreas fault (roughly from the latitude of San Luis Obispo past San Francisco, off the map in Figure 6, left). Since the 1872 Owens Valley earthquake, other earthquakes that have occurred recently on this incipient eastern plate boundary are the 1992 M 6.3, Joshua Tree, 1992 M 7.2 Landers, 1992 Big Bear, 1995 M 5.8 Ridgecrest, and 1999 Hector Mine earthquakes. The rupture zones of the Landers and Hector Mine earthquakes are shown by red lines east of Victorville on the left side of Figure 6.
Reported Damage and Future Warnings
The MMI Intensity shakemap of the earthquake is shown on the left side of Figure 7. The main damage to the towns on Ridgecrest (population 29,000) and Trona (population 1,900) appears to have been incurred by older houses and trailer homes (which readily topple from their foundations, right side of Figure 7); some house fires also started but were soon extinguished. There were no reported deaths or serious injuries.
The Governor of California announced that the estimated losses are about $US100 million, but the USGS made a preliminary estimate of at least $1billion. It is possible that this larger estimate may include damage to the Naval Air Weapons Station China Lake, a 1.1 million acre weapons testing facility, the Navy’s largest, whose location is shown in Figure 1 and within which the epicentres of both the M 6.4 and 7.1 earthquakes were located. The facility’s Facebook page announced that it is “not mission capable until further notice,” but officials said that security protocols “remain in effect.”
According to the current USGS forecast, over the next one week, beginning on July 6, 2019 at 2:20 p.m. Pacific Time (5:20 p.m. ET), there is a 2% chance of one or more aftershocks that are larger than magnitude 7.1. The number of aftershocks will drop off over time, with the largest expected to have a magnitude of about 6, but a large aftershock can increase the numbers again temporarily.
The recently installed early earthquake warning system ShakeAlert, which detects earthquakes and announces that they have occurred in near real time, worked as planned but not as some people would have liked. The designers of the ShakeAlert LA app decided that most people would not want to be notified of large earthquakes that are too distant to cause damaging shaking near them. Consequently, the ShakeAlert LA app was designed to only send an alert if the magnitude is above 5 and the MMI intensity is 4 or higher somewhere in Los Angeles County. The Ridgecrest earthquakes met the magnitude criterion but not the intensity criterion in Los Angeles, and so did not result in alarms. However, many people were alarmed because they clearly felt the long “rolling” motions (surface waves) of the distant large earthquakes and were concerned that the alarm was not working properly. The designers of the app may now consider providing alarms, perhaps nuanced, of large distant earthquakes that people may feel but that do not present local damage potential.