5.7-5.9 Aftershock in Southern California

Big aftershock to the El Mayor - Cucapah Earthquake, just over the border in California, at 5.7 or 5.9 (awaiting confirmation from the USGS). It has been followed by more than a dozen aftershocks of magnitude 3.0 or higher. I would imagine it has been felt widely in southern California.

This is a good reminder to be prepared for the larger quakes to come. The San Andreas system has a great deal of accumulated stress that has to be relieved pretty soon.

Update: 5.7, after review by seismologists

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....

When a Magnitude 6.9 Earthquake is an Aftershock....

There wasn't a whole lot of information when I wrote about the 8.8 earthquake in Chile in the earliest hours of the morning. Information is starting to flood in now from many sources, and my first impression is media coverage is a bit better than it was for the Haiti quake and other earlier events. When I say "better", I don't mean more cameras, I mean evidence of an effort by the reporters to educate themselves about the basic science of earthquakes.

My first thought is for the people who have been affected, both in Chile and across the Pacific basin. A quake this big is going to have huge impacts. The aftershocks are going to be an issue: my USGS notifications are set for any quakes bigger than 5.5 and there have been 11 of them so far, with the largest at 6.9. For perspective, 6.9 is the size of the Loma Prieta earthquake here in California from 1989. It killed 4 dozen people and did around $10 billion in damage. The quake in Haiti was a somewhat larger 7.0. Aftershocks are going to continue for a long time, over a region about 600 miles long (the size of the fault zone that shifted in this quake).

The tsunami is a huge threat. The size of a tsunami in any particular place is dependent on many factors, including the size of the earthquake (which was colossal), how the quake transferred energy to the ocean water (unknown to me at this point), and the shape and depth of the coastline where the tsunami strikes. All I can say to my Pacific Basin friends is that when the Civil Defense folks set off the sirens, take it seriously! Hawaii in particular has a tragic history with tsunamis, and there has been a vast amount of coastal development since the last major tsunami (in 1960, also caused by an earthquake in Chile). There will be a big problem of people wandering down to the beach to watch the tsunami, which can only be described as an act of idiocy. Get to higher ground or higher floors. If the warnings are inaccurate, you've only lost a bit of time, but if they are accurate, your life will be saved. Don't go to the coast until the "all clear" is given, because there will be more than one surge of water. You can't swim your way out of a tsunami; they are one of the most dangerous of geologic events, as the Indonesian tsunami of 2004 demonstrated.