Imperial seismic swarm lights up the southern tip of the San Jacinto fault in an area of extreme hazard

22 April 2016  |  Quake Insights

A seismic burst, so far topped only by a M=3.4 shock, struck beneath the city of Imperial yesterday. It’s a fascinating location, where the San Andreas fault system converges with the San Jacinto fault system.

The great San Andreas and San Jacinto fault systems somehow merge in the deep, sediment-filled Salton trough. The size and year of historical earthquake ruptures are shown in blue.

The site of the recent swarm is surrounded by the rupture zones of recent large earthquakes, and so has among the highest Temblor seismic hazard ranks in the U.S. Those shocks include the overlapping 1940 M=6.9 and 1979 M=6.4 Imperial Valley earthquakes, the 1987 M=6.6 Superstition Hills earthquake, the 2010 M=7.2 El Mayor-Cucapah quake, which was largely in Baja California but just crossed the border into the US (the Mexican geologists jokingly referred to it as an ‘illegal fault’), and the 26 August 2012 M≤5.4 Brawley Seismic swarm, on a linking cross fault.

In this Google Earth image, the sites where the San Andreas and San Jacinto systems diverge (north of Riverside) and converge (south of Imperial) are seen. Julian Lozos’ interpretation of the 1812 rupture is also shown. This event collapsed or highly damaged Missions San Juan Capistrano and San Gabriel, and caused minor damage at Missions San Fernando and San Buenaventura. The image is 325 km (175 mi) across.

Julian Lozos, now at California State University Northridge and formerly a USGS post-Doc, published an article last month in Science Advances arguing that the enigmatic 8 Dec 1812 M~7.5 shock most likely ruptured the San Jacinto fault north of Riverside, but then jumped onto the San Andreas fault. This and another evidence suggests that fault jumps in the San Andreas system are not only possible but likely. This jump took place 150 mi (235 km) northwest of where the two faults converge near Imperial, site of the recent swarm.

The 21 April 2016 swarm struck on the periphery of Imperial (grey area), with 16,000 residents.

Could another large quake re-rupture the Superstition Hills fault, or jump on to the Brawley seismic zone?

The Brawley is an unusual section of the San Andreas without surface expression, but the site of many swarms and fault creep events at depth. Although how these faults connect at depth is unclear, what is indisputable is that major faults and perpendicular ‘cross-faults’ can be activated simultaneously as occurred in 1987 (Hudnut et al., 1989), so there are a myriad of potential linkages that put the cities of Imperial, El Centro, Brawley, Westmoreland, and Mexicali, and the rich surrounding agricultural areas, at risk.

Ross Stein and Volkan Sevilgen, Temblor

Data from Caltech/USGS Southern California Seismic Network, USGS, and California Geological Survey, Southern California Earthquake Center, and:

Julian C. Lozos (2016), A case for historical joint rupture of the San Andreas and San Jacinto faults, Sci. Adv., 2, doi: 10.1126/sciadv.1500621.

Kenneth W. Hudnut, L. Seeber, and J.Pacheco (1989), Cross-fault triggering in the November 1987 Superstition Hills earthquake sequence, southern California, Geophys. Res. Letts., 16, doi: 10.1029/GL016i002p00199

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