California rocks again

The last week or two have seen a sudden increase in earthquakes in California.   For the first time in several years California is once more having more eathquakes than Oklahoma.

13 OK earthquakes >M2.5 in past 7 days

48 CA earthquakes >M2.5 in past 7 days

While this is  worrying for people living in California it is somehow reassuring for a geoscientist.   Earthquakes are supposed to happen in California, that is what we have been teaching kids for the past 40 years or so since plate tectonics was discovered.

If we look in a bit more detail (USGS Earthquakes website is great for doing this )  we see that the vast majority of these Californian earthquakes have happened in a place called Bombay beach and are associated with a NE-SW trending fault under the lake.

Most earthquakes happening at Bombay beach on a NE-SW fault

This fault is related to the great San-Andreas fault system which runs through Calfornia, so people are inevitably asking ... is this the precursor to "THE BIG ONE", which in California means an earthquake >M7  which is likely to happen every 10 years or so (that Gutenberg-Richter relatioinship again) .   Unfortunately such swarms of small earthquakes are just as likely to not be followed by a big earthquake as to be the precursor, this particlur fault at has a history of producing small swarms with no big one following but that does not mean this will be the case now.  Seismologists in California  try to adjust their short term risk forecasts to take account of such swarms maybe being a precursor but the overall risk is still low (less than 1% ) so not  a usable piece of information to help people in their day to day lives.

Why worry more about earthquakes in California than Oklahoma ?

It is all to do with maximum likely earthquake magnitude and fault lengths.    Earthquakes start to become dangerous once their magnitude goes over M5, once they reach M7 they can be devastating.   California, sitting as it does on a tectonic plate boundary has a massive fault system running through it (the San Andreas) which is thousands of km long.   It has a history of M7+ earthquakes (6 in the last 40 years) and there is a possibility that they could reach M8 if the whole San Andreas fault slips at once.  

Fault length is the reason.  The thing that really decides how big an earthquake is is the total size of  the fault that ruptures (well really fault area x slip length)  this leads to another useful rule of thumb (seismologists seem to have a lot of thumbs !)

Empirical (fancy name for rule of thumb) relationship between earthquake magnitude and rupture length

      So a M4 earthquake ruptures a fault less than 1km long  whereas a M7 earthquake ruptures a fault about 100km long.   Or from a geoscientists point of view you need to have a pre-existing fault system at least 100km long to enable a M7 earthquake to happen.   There are plenty of these in California but not so many in Oklahoma (or they are much older and well buried).   So in California we are pretty much guaranteed to have an M7+ earthquake in the next twenty years or so,   maybe the next one will be preceded by a swarm of smaller earthquakes or maybe it will happen out of the blue.  No wonder seismologists are starting to pay lots of attention to California at the moment.