Here in the Los Angeles area we've had a bunch of little earthquakes in the last day or so, kicked off by a magnitude 3.6 foreshock followed by a magnitude 5.1 quake centered in the La Habra/Brea area that caused some minor damage. Since then there has been a whole swarm of smaller aftershocks, including a 4.1 we felt this afternoon. Here's a map, showing some of the known faults in red and the earthquake epicenters in orange:
Now, before I continue, let me disclaim: I am not a seismologist, so take my ideas here with a grain of salt. So far, there has been some discussion that the earthquakes may be related to the Puente Hills Fault, which is a north-dipping blind thrust fault (it is not observed at the surface) that has lifted up the row of hills near where I've labeled that fault. This fault has been a troublemaker in the past, causing the M 5.9 Whittier Narrows earthquake in 1987, and the M 4.4 Pico Rivera earthquake in 2010.
But I don't think it's responsible for the current spate of quakes. Notice that the aftershocks are not lined up along the (approximately shown) Puente Hills fault, but along a NE-SW-trending lineament. Could the seismic data be consistent with a different fault?
Well, here are the moment tensor and focal mechanism solutions for the main shock (found here):
What's a left lateral fault doing here?? Well, as it turns out, a little further to the north, we find the San Jose Fault, which is known to be a NE-SW striking fault that dips steeply northwest, with left-lateral/reverse slip. Just like the hypothetical Brea Canyon Fault. In the map above, on the basis of some suspicious linear ridges, I've also penciled in where some additional little faults might help transfer slip between the San Jose Fault and a hypothetical Brea Canyon Fault.
So maybe this latest earthquake swarm is actually on an unmapped extension of the San Jose Fault. I suspect I'm not the first to think of this, likely the official seismologists want to be really sure before they announce a new fault may underlie people's homes! But new faults are showing up in southern California all the time . . . .
Now, before I continue, let me disclaim: I am not a seismologist, so take my ideas here with a grain of salt. So far, there has been some discussion that the earthquakes may be related to the Puente Hills Fault, which is a north-dipping blind thrust fault (it is not observed at the surface) that has lifted up the row of hills near where I've labeled that fault. This fault has been a troublemaker in the past, causing the M 5.9 Whittier Narrows earthquake in 1987, and the M 4.4 Pico Rivera earthquake in 2010.
But I don't think it's responsible for the current spate of quakes. Notice that the aftershocks are not lined up along the (approximately shown) Puente Hills fault, but along a NE-SW-trending lineament. Could the seismic data be consistent with a different fault?
Well, here are the moment tensor and focal mechanism solutions for the main shock (found here):
For non-geologists, these take a bit of explanation. To simplify a bit, they both essentially show the directions in which the earth was compressed (orange areas) or extended (white areas) during the earthquake, but based on different data treatments. Since these are directions in three-dimensional space, a beach-ball-like diagram is used to project the spherical directions onto a flat screen, as if you were looking into the bottom of a bowl. Because rocks on either side of the fault move in opposite directions during the earthquake, one of the two lines separating the colored quadrants usually approximates the orientation of the fault that caused the earthquake.
Most of the aftershocks have similar focal mechanisms. So, the fault could either strike NW-SE, dipping southwest, with right-lateral/reverse oblique slip; or, it could strike NE-SW, dipping steeply northwest, with left-lateral/reverse oblique slip.
The Puente Hills fault does strike NW-SE, and right-lateral faults are much more common than left-lateral faults in southern California. But the Puente Hills Fault dips (quite shallowly) to the north. And if you look at the aftershocks, they seem to follow a NE-SW trend.
So maybe there's an unmapped fault here? If the aftershocks are several kilometers deep on a NW-dipping fault, the surface trace of the fault should be a few km to the SE of the aftershock line. Lo and behold, there is a parallel NE-SW lineament in the topography in about the right place, running along Brea Canyon (the route of the 57 freeway). I'll call this the Brea Canyon Fault; it would have to be a left-lateral/reverse fault based on the focal mechanisms:
What's a left lateral fault doing here?? Well, as it turns out, a little further to the north, we find the San Jose Fault, which is known to be a NE-SW striking fault that dips steeply northwest, with left-lateral/reverse slip. Just like the hypothetical Brea Canyon Fault. In the map above, on the basis of some suspicious linear ridges, I've also penciled in where some additional little faults might help transfer slip between the San Jose Fault and a hypothetical Brea Canyon Fault. So maybe this latest earthquake swarm is actually on an unmapped extension of the San Jose Fault. I suspect I'm not the first to think of this, likely the official seismologists want to be really sure before they announce a new fault may underlie people's homes! But new faults are showing up in southern California all the time . . . .
