By Shinji Toda, Ph.D., IRIDeS, Tohoku University, and Ross Stein, Ph.D., Temblor, Inc.
The Magnitude 7.1 earthquake abruptly halted aftershocks on the M 6.4 cross fault. It also produced a far-flung aftershock sequence that touched the San Andreas, but refused to cross the Garlock Fault.
Citation: Shinji Toda, and Ross Stein (2019), Ridgecrest earthquake shut down cross-fault aftershocks, Temblor, http://doi.org/10.32858/temblor.043
A remarkable illustration of how stress controls seismicity was handed to us by the Ridgecrest earthquake sequence. Two perpendicular faults ruptured in the M 6.4 earthquake on July 4 (left panel below), both lighting up in aftershocks. But when the M 7.1 struck 34 hours later, seismicity along the cross fault abruptly stopped (middle panel below). Why?
The answer seems to be that the M 7.1 rupture cast a ‘stress shadow’ over the cross fault, inhibiting failure by several bars, and so shutting aftershocks off. In our calculation of the stress imparted by the M 7.1 (right panel above), the cross fault is ‘shadowed,’ (blue zones) while the stress increased to the northwest (red zones), to southeast extending across the Garlock Fault. The isolated side lobes also experienced aftershocks to the M 7.1 event. The seismicity shutdown along the cross fault is dramatic when viewed as a time series for M≥1 shocks (below), with the seismicity rate dropping by a factor of 10.
A few bars are not a lot of stress. In the photo below, Ross puts a half a bar of Coulomb stress across the base of his palms by pushing as hard as he can.
Seismic shaking can promote seismicity but not inhibit it, so the shutdown illustrates that small decreases in Coulomb stress can inhibit earthquakes, a phenomenon first discovered by Ken Hudnut, Leonardo Seeber and Javier Pacheco (Hudnut et al., 1989). They found that the 1987 Superstition Hills earthquakes, a M 6.2 followed by a M 6.6 some 11 hours later, also shut down seismicity on a cross fault. This phenomenon was seen again in 1997 Kagoshima couplet in Japan, adjacent M 6 shocks 48 days apart (Toda and Stein, 2003), and in the 1992 M 7.3 Landers shock, which shadowed part of the M 6.1 Joshua Tree aftershock zone that had struck 66 days beforehand (Toda et al., 2012). The Joshua Tree shutdown began several days after the Landers shock, the Kagoshima shutdown began within about a day, but at Ridgecrest, it was immediate. Stress shadows were named by Ruth Harris and Robert Simpson of the USGS, which they argued explained the paucity of large shocks in the century or so after the great 1857 in southern California (Harris and Simpson, 1996), and after the great 1906 quake in northern California (Harris and Simpson, 1998). Stress shadows were also hypothesized to explain the shutdown of the southern Hayward Fault after the 1989 M 6.9 Loma Prieta shock (Simpson and Reasenberg, 1994; Stein, 1999).
If one steps back from the cross fault, one sees what appear to be aftershocks as much as 150 km (90 mi) from the M 7.1 epicenter in three of the four stress trigger (red and yellow) lobes in the panel at left below, and these appear to correspond to the stress trigger lobes. But are these aftershocks, or just shocks that would have occurred in those remote locations anyway?
We can answer that question by plotting the change in the seismicity rate, comparing the first 11 days after the M 7.1 to the preceding year (above right). Here, the warm colors are places where the seismicity rate increased after the M 7.1, light blue areas are where it decreased. Now, all four stress trigger lobes appear to correspond to seismicity rate increases (e.g., aftershocks), and they extend all the way to the San Andreas Fault at Tejon Pass and Cajon Pass.
San Andreas in play?
What do the subtle, or perhaps apparent, seismicity increases at the two big bends on the San Andreas Fault mean for future great quakes? We do not know. We can say only that the effects, while promoting failure, are very small, about the same as the twice-daily tidal stresses.
The Garlock Wall
But if the shutdown and remote shocks can be explained by stress transfer, another feature of the aftershocks is mysterious. If you glance again at the Coulomb stress lobes, you can see that the stress trigger zone extends well to the south of the Garlock Fault, but the M 7.1 aftershocks do not. That is even clearer in the map below: The Garlock is a wall.
Why would a fault be a barrier to aftershocks? We can think of two explanations, but we can’t prove either. The first is that Coulomb stress changes amplify the background seismicity rate (this follows from the theory of rate/state friction developed by Jim Dieterich at the USGS; Dieterich, 1994). If the background rate is high (those are where there are many blue shocks above), then the seismicity will be very responsive to the stress changes. That seems to be what happened at the Coso Volcanic Field, which was active before the M 7.1, was stressed by the M 7.1, and produced abundant aftershocks. But if its near-zero, as it appears to be south of the Garlock, the stress changes have almost no effect.
Even if that explanation were correct, that would not explain why the northern Mojave Desert, south of the Garlock Fault, was so quiet in the first place? Perhaps because it is composed of different rocks than the material to the north. The Garlock has slipped a total of ~100 km (60 mi), juxtaposing different chunks of crust. To the north lies mostly granitic rocks, and to the south lies mostly volcanic rocks (see the map below). Somehow, rock type might modulate seismicity.
So, what did we learn?
The Ridgecrest sequence demonstrates how Coulomb stress controls seismicity. Ridgecrest has also produced an exceptionally distributed aftershock sequence that just kisses the San Andreas, but refuses to cross the Garlock Fault.
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