How unusual is the recent pattern of earthquakes in California?
March 31, 2014 4:02 AM   Subscribe

Within the last month there's been a 6.9 off the Northern California coast which I did not feel, a 4.4 near Westwood which I felt, a 5.1 near La Habra which I felt, and a swarm of aftershocks (?) of the La Habra earthquake which I haven't felt. That seems like an unusually large number of earthquakes to me. Has anyone crunched the numbers to find out if the earth has been shaking in California more than usual?
posted by rdr to Science & Nature (7 answers total) 2 users marked this as a favorite
See this NY Times article.

The short version is that the period of inactivity for the last decade+ is the unusual thing. These earthquakes could be the start of a return to normalcy.
posted by Betelgeuse at 4:17 AM on March 31, 2014 [5 favorites]

The problem with Earthquakes is that there is no 'usual'- these are dynamic systems with hugely complex variables. What has definitely changed is what's been previously reported. Sensing equipment is way more sophisticated now and new travels fast so the smaller aftershocks that possibly wouldn't have previously been noticed are being picked up. Also movement in different regions are now connected by media reports: in the past they may have been by laypeople on the ground as separate, small events rather than a range of connected aftershocks.

However we do know that, statistically speaking, a large event in this region is actually overdue so Betelgeuse is also right, the recent lack of seismic activity may actually been a period of quiet within the general 'normal'.
posted by freya_lamb at 4:30 AM on March 31, 2014 [1 favorite]

The USGS is the number-cruncher for stuff like this, and they say no, not unusual. You can see their So-Cal-specific seismicity data and so on here.
posted by rtha at 5:55 AM on March 31, 2014 [2 favorites]

A decade is the blink of an eye in geologic time. California is a tectonically active area, so weekly and yearly activity can vary greatly without meaning a damn thing. Tectonic plates are moving, and earthquakes will occur. The plate motion is at a relatively steady rate, so earthquakes will happen at an average rate that is somewhat proportional to the rate of plate motion. The thing about that average is that we're talking an average over centuries, not years. So a few earthquakes in a few weeks is meaningless in terms of the overall average.

From a natural disaster standpoint, however, it could be meaningful. Earthquakes happen because stresses are building up in the earth. The rocks are somewhat elastic -- which means they can bend and "store" some of that stress. The recent low in earthquake frequency means that stress has been building up. Because earthquake waves transmit through the Earth and move rocks, when they hit an area of built up stress, it could be the trigger needed to release the stress. Think of it like shaking a soda bottle - once the pressure builds up, the cap only needs to be released for a fizzy explosion to happen. When you twist open the cap, there's a critical point that you pass where the pressure gets released. So these earthquakes could be triggering each other, or they could just be part of the statistical average.
posted by DoubleLune at 5:58 AM on March 31, 2014 [1 favorite]

As others have said, it's the "earthquake drought" that's unusual. Also, ten-plus years ago you didn't have the constant news cycle and social media, so you didn't have notifications turned on for @earthquakesLA or texts from USGS so all this onsey-twosey stuff was a footnote on the evening news, but only on a slow news day. So it makes all this feel like the most activity ever when it is actually just the most activity in a few years plus vastly advanced technology.

I'm sure the USGS is doing all kinds of crunching. Their website is actually a pretty great way to lose an afternoon now and then.
posted by Lyn Never at 8:00 AM on March 31, 2014

Out of curiosity, I pulled some data from a commonly used earthquake catalogue and made some plots (PDF) of monthly activity for the past six months. The plots show a histogram of earthquake count by magnitude in the map area, and the dots show locations (dot size is magnitude). There's month-to-month variation (generally at the higher magnitudes), but March doesn't look particularly out of line, and the Gutenberg-Richter Law remains in effect (the histograms are more or less straight lines on a log scale).
posted by irrelephant at 8:12 AM on March 31, 2014 [2 favorites]

After thinking about it I believe the fact that I was near the epicenter of the March 17th 4.4 quake caused me to think of it as a larger quake than it actually was. If I'd been a few miles further away I wouldn't have paid much attention to it. If I hadn't thought much about the 4.4, then an unrelated 5.1 a couple of weeks later wouldn't have seemed worth noticing. Thanks all of you for answering.
posted by rdr at 2:45 AM on April 1, 2014

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