Monday, August 3, 2009

Baja Inches Northward

There was a fairly substantial earthquake in the Gulf of California this morning, with a preliminary magnitude of 6.9. The diagram below, from Arizona Geology, is primarily intended to illustrate the predicted maximum ground motions from quakes in the GofC/SoCal area, but also shows the regional tectonic structure nicely, as well as the location of this morning's largest event.The (generally shorter) lines running NE-SW are small segments of mid-ocean ridge, which through basaltic volcanism and emplacement of magma in the lower to middle crust, actually create new ocean floor. So picture arrows pointing outward from the segments with the described orientation, and you can see that the (generally longer) red line segments must represent areas where the blocks of ocean crust are sliding past each other: transform faults. Each one of those NW-SE trending lines is like a mini-San Andreas Fault.

When I read about this quake earlier, I quickly jumped to the conclusion that it was most likely a strike-slip quake, there would be little damage from it, and that the risk of tsunami was pretty low. Spreading earthquakes will tend to be relatively small, since the crust in those areas is hot and fairly ductile; it won't accumulate as much stress without plastic deformation. So it seemed almost certain to me that this would be a strike-slip earthquake. By definition, strike-slip quakes do not involve much vertical displacement- and it's vertical motions that displace water to create tsunamis. I don't know much about the bathymetry of the G of C floor, but a quake-triggered submarine landslide could have triggered one. Finally, since this was well out to sea (the closest town is 76 mles away), it didn't seem likely there'd be too much damage- even though a 6.9 is definitely a powerful earthquake.

The first I heard of this was from Garry Hayes' post at Geotripper, who also noted, "A 5.8 shock preceded the 6.9 event, which was followed by aftershocks at 6.0 and 5.0 magnitude." Even the fore- and aftershocks were pretty substantial. A few minutes later Arizona Geology chimed in for the first time with a nice Google Earth image to show the location and the region around it. Another piece from Arizona Geology asks readers who felt the earthquake to report their experiences to USGS- the quake was felt at least as far as Phoenix, Az, and these sorts of first-hand reports help seismologists fine-tune their motion and damage models.

So this one was a fine show, with little human suffering involved- I haven't read any reports of damage or injuries. Would that they could all be so innocuous.

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