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