Sunday, April 4, 2010

7.2 Magnitude Earthquake in Baja California: What's the Difference a Foreshock and an Aftershock?

This is follow-up on the 7.2 magnitude earthquake today in Baja California... The fault responsible for the quake is not yet officially determined, but the Laguna Salada fault is a strong candidate, given the pattern of aftershocks running northwest from the epicenter along the fault, and the history and size of quakes in the area (including a 7.2 quake in 1898). The Laguna Salada fault lines up with the Elsinore fault, which has been mostly quiet in historic time, but which has some features suggestive of a fairly active fault system (the Lake Elsinore graben, for instance). I will be very interested in learning whether this quake produced surface offsets

The USGS record for the region records seven earthquakes exceeding magnitude 3 in the week leading up to the quake, including a magnitude 4.2 on March 31st, and a magnitude 4.3 on April 3rd. I don't know how this pattern compares to "normal", but I would expect that these events will be considered foreshocks to the big quake today. Foreshocks can be considered as something like the cracking and popping one hears while bending a stick, just before it snaps. It would be wonderful thing to identify a particular earthquake as being a foreshock in order to give warnings, but the only way we have to tell a foreshock from a main shock from an aftershock is...hindsight. It would be good to keep in mind that a moderate earthquake in California, say, a magnitude 4 or 5, could be a foreshock to a larger impending event. Or not. The chances are pretty low, typically 1 in 20, but it is always a good idea to check on your earthquake preparations when such moderate quakes occur: do you have emergency supplies in place? Water, non-perishable food, first aid? Do you have a family plan for what you will do, and do you have a relative or friend outside the region that everyone knows to contact?

Aftershocks are another matter. They will always occur, there will be a great many of them, and some will be large enough to cause some serious damage. There will be instances when buildings sustain damage in the main quake, but will still be standing afterwards. Residents will often want to go back into the buildings to retrieve their belongings, but emergency workers will often prevent them from doing so. The reason for this is that structural damage is cumulative. The building may have survived the main shock, but just a small amount of additional shaking could very well bring the building down.

One other interesting aspect of today's quake is the heightened seismic activity in the region beyond the Laguna Salada fault zone. Large quakes may very well set off tremors along other nearby fault systems, especially if stresses are already high on the other faults. That appears to be happening today, as a number of small and moderate events are occurring across the region.

Follow up: We can't predict earthquakes, but the folks at XKCD comics note a unique quake warning system....

1 comment:

Gaelyn said...

Thanks Garry. I never think about living in earthquake country, but that's really not true. Good reminder to be prepared.