When the Wrong Worry Produces the Right Reaction

With the release of San Andreas, a far-over-the-top disaster fantasy (yes, that is what this genre should be called), there has come renewed focus on earthquake safety, in part because people sort of believe the stuff they see on the screen. (As an aside, you can see the tweets that Lucy Jones, a seismologist with the USGS in Pasadena, sent out from the premiere). We’ll save beating the dead horse of the science in San Andreas for another day (feel free to visit such beatings here and here and here and here); today let’s contemplate what this means in a broader sense.

Fivethirtyeight, which is often fairly thoughtful on this sort of thing, kind of missed the memo on what really matters.  They focus on the probability of a M8+ earthquake in California, noting that such an event is thought to only have a ~7% probability in the next 30 years. So, the piece impugns, folks in California are too worried; why prepare for such a low probability event?

Here’s the problem: it probably isn’t the M8 that is going to be the worst case scenario in terms of personal danger (it might be for overall infrastructure damage).  The Hayward Fault is probably getting towards the point where rupture is likely; a M6.8 or so ripping through the heart of the East Bay will cause a huge amount of damage and, if folks aren’t prepared, death and injury.  A M7 on the blind thrust fault under Los Angeles could be absolutely devastating: the ground motions in LA such an event would produce would be higher than from a M8+ on the San Andreas.

Consider the past hundred years or so in California during which no M8 earthquakes have occurred (indeed, it isn’t clear that a M8 has ever struck California: the largest events went unrecorded or were recorded in the very infancy of instrumental seismology).  Significant, damaging earthquakes have included Long Beach in 1933, Arvin-Tehachapi in 1952, Coalinga in 1983, Loma Prieta in 1989, San Fernando in 1971, Northridge in 1994, Landers in 1993 and several in the Imperial Valley region (1940, 1979, 2010).  None exceeded M7.5 and the worst (except for Loma Prieta) were under M7.  Eight died in the M5.9 Whittier Narrows earthquake. Several of these were on unrecognized faults.  Keep in mind that hazard maps in California are based on recognized faults, and all too frequently the magnitude and frequency of earthquakes on many of these faults are guesswork. [A similar and larger problem lurks in the national hazard map, which is fundamentally a retrospective map that probably exaggerates the risk in places that did have historic earthquakes and underestimates it in places that didn’t].

The overall problem is that there are a lot of faults capable of M6s and we don’t necessarily know much about a lot of them–and probably there are still a few hiding out that we don’t know about. While these faults almost certainly are landing in the once in 1000 to once in 10,000 year range for recurrence, if you have 100 such faults you are now looking at significant earthquakes every decade to century–and the numbers we have in California are a lot closer to every decade (the past 2 decades have been unusually quiet, but that happens with stuff like this).

So it probably isn’t the San Andreas and statewide disaster that will befall Californians, it will be smaller but closer ruptures.  And the rule of thumb is that you won’t see outside help for the first 72 hours after a large earthquake.  So if fear of the “Big One” causes folks to have an emergency plan, bolt their bookcases to the wall, have a small stock of emergency food and water, know how to turn off their gas, etc., it is a good thing.  After all, in the 20 years since LA last got rocked and the 25 since the Bay Area took a real shot, a lot of folks have moved in, been born, or grown up and so have no experience with this sort of things. Sure, they may be preparing for the wrong reason–that 7% chance over the next 30 years of the “Big One” pales against the far greater chance that they’ll need that stuff for something equally crippling where they live–but they will be in better shape because of it..

Unfortunately, it seems plausible that the exaggerations in San Andreas could hurt rather than help disaster preparations despite a few useful tidbits in the movie. Oh well.  Its not like it was a public service announcement or anything….

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