Normal aftershocks

I have been deluged with questions since the Ridgecrest earthquake that are some version of “What does [quake A at location B] mean?”

  • “Why are there so many aftershocks near Coso?”
  • “What does it mean that there are earthquakes near the Garlock fault?”
  • “We just had another M4.5? Does this mean we need to be ready for a big one?”
  • “Are the Bay Area quakes related to Ridgecrest?”
  • Etc.

First, for the basics about how one earthquake triggers another, look down in this blog to my post from February 2018. It describes what seismologists know about earthquake triggering. We actually know quite a bit and can describe the rate of earthquakes reasonably accurately. The variables are time from the largest earthquake (the rate goes down approximately as 1/time), the magnitude of the triggering event and the triggered event (small earthquakes are always much more common than big ones and big earthquakes trigger more new events), and distance (most triggered events are very near the fault that produce the mainshock and the rate dies off as 1/distance^2). If you want the full details, go to the 2018 post.

In other words, we have equations to describe the rate and that we can use to compare one aftershock sequence to another.  But this is a rate.  The time of an individual earthquake is randomly distributed about that rate. So for instance we can say there will probably be 25 to 35 earthquakes of M3 or greater in the next week, but no idea whether the next one will be in 1 minute or tomorrow.

The USGS is creating and posting daily updates of these predictions here.

People are also asking about the risk because we have the big Garlock fault near these aftershocks. Does that make our risk worse?  If another M7 were to be triggered (the current weekly chance of that is about 1 in 300), it would likely be on the Garlock fault so that risk is already included in the probability numbers.

People also ask about the San Andreas. It is much farther away from the Ridgecrest earthquakes so the chance of an earthquake on it has not changed.

Compared to the average California aftershock sequence, the Ridgecrest sequence is about average in total number but is dying off more quickly than average. That means the chance of bigger ones is going down fast which is why the USGS probabilities are quite a bit lower now than they were last week.

All of the questions are some version of “Are these aftershocks normal?” and the answer is “Yes.”

But don’t make the mistake of thinking that “normal” means another big earthquake can’t happen. A large late aftershock is also pretty normal. Magnitude 6 earthquakes every couple of years somewhere in Southern California are normal. “Normal” does not mean it cannot trigger another earthquake. We are trying to make patterns in a random distribution. Just like constellations, we can make the patterns but they are not going to be any use in figuring out what is coming next.

You can’t live in Southern California worrying which day will have the big earthquake. You wear your seat belt every day, not knowing when the next accident will be. You need to make your house safe, get your water supply and be ready for the earthquake on any day.