Today’s Magnitude-4.1 Joshua Tree, CA, quake and seismic swarm appear to be delayed aftershocks of the 1999 M=7.1 Hector Mine earthquake

24 Jan 2016  |  Quake Insight

The M=4.1 is part of a very shallow 12-day-long seismic swarm

A magnitude-4.1 shock struck at very shallow depth (2-5 km, or 1-3 mi) today. The event is the largest shock in a seismic swarm that begin 12 days ago; all the events in the swarm are equally shallow. The M=4.1 quake locates 4 km (2 mi) NW of a short unnamed fault oriented SW-NE, and 15 km (10 mi) east of the Lavic Lake fault rupture of the 1999 M=7.1 Hector Mine earthquake. Focal mechanisms for the event are inconsistent; it could be a ‘normal’ event parallel to the unnamed fault, or possibly a right-lateral/reverse event striking parallel to the Lavic Lake fault. Neither are what one would have expected in this fault system.

Temblor map of today’s M=4.1 quake (red dots), the preceding 12-day-long seismic swarm (green dots) and to surrounding active faults (red lines) and the 1999 M=7.1 Hector Mine, CA, rupture.

M=4.1 was probably promoted by the M=7.1 Hector Mine quake that struck 17 years ago

What is fascinating about the Joshua Tree swarm is its relationship to the M=7.1 shock that struck 17 years ago. The swarm lies in a stress lobe that was brought closer to ‘Coulomb’ failure by the M=7.1 mainshock. The Coulomb stress assumes that faults are most likely to fail when they are sheared and unclamped.  A study by Fialko et al in Science in 2002 calculated that the site of today’s quake was strongly stressed by the 1999 mainshock. Fialko et al assumed that the surrounding faults were oriented similar to the main rupture, an assumption currently uncertain. But the M=4.1 event nevertheless points to the importance of Coulomb stress change calculations to forecast where subsequent quakes might be more likely to strike (red zones in the maps below), and less likely (blue zones).

The stress imparted by the 1999 M=7.1 Hector Mine, CA, rupture brought the site of the 24 Jan 2016 M=4.1 quake closer to failure. The permanent or ‘static’ Coulomb stress change is shown at left, and the peak dynamic stress carried by the seismic waves is shown at right. The dynamic stresses are 5-10 times larger at the site of the M=4.1 event than the static stresses, but they lasted about a minute 17 years ago, whereas the static stresses do not diminish, and so likely continue to exert an influence on seismicity. (1 MPa is about 1/4 the typical car tire pressure; 5 MPa is about the pressure in a bicycle tire). Figure modified from Yuri Fialko, David Sandwell, Duncan Agnew, Mark Simons, Peter Shearer, and Jean-Bernard Minster (Science, 2002).

Ross Stein and Volkan Sevilgen, Temblor

Data from USGS, Caltech/USGS Southern California Seismic Network, California Geological Survey, and Fialko et al. (Science, 2002)

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