September 2, 2007

Sciencedude: San Andreas fault in “seismic lull”?

Gary Robbins

Is greater Los Angeles in a “seismic lull” that has spared us the sort of catastrophic earthquake scientists have warned us about?

It’s a distinct possibility, says Cal State Fullerton geophysicist Dave Bowman, who helped flesh out the provocative idea in a recent issue of the journal Geology.

Bowman and researchers James Dolan and Charles Sammis of the University of Southern California say the region, which includes Orange County, appears to be in an “ongoing lull that has persisted for about the past 1,000 years.”

The period coincides with decreased activity on the San Andreas fault system and increased seismicity on the Eastern California Shear Zone fault system in the Mojave Desert. The faults, Bowman says, could be taking turns shaking Southern California.

“The idea is a little controversial since the geology only goes back 12,000 years,” Bowman says. “But it might be happening.”

We dug into the matter with Bowman, who deftly used baseball analogies to explain things.

Q: What exactly do you mean when you say that greater Los Angeles is in a seismic lull?

A: Some background will help. The most important thing to realize is that earthquakes – whether on a single fault or a group of faults – do not repeat regularly. Geologists may say that a fault has an earthquake about every 150 years, but there is enormous variability in that number. Sometimes the earthquakes will be 50 years apart, sometimes they’ll be 500 years apart.

What we’re beginning to see is that faults tend to go through “hot streaks” and “cold streaks” just like a baseball player. There may be periods of time when a fault, or group of faults, has lots of big earthquakes. Seismologists call this clustering. These periods are punctuated by relatively quiet times which have gotten the unfortunate name “lull.”

I say unfortunate because faults in a lull can still have earthquakes, just like a baseball player in a slump may still get the occasional base hit. Let’s continue with the baseball analogy. … When a player is on a hot streak, it’s reasonable to assume that he’ll not only get more base hits, but will also get more home runs.

Now imagine that this tendency to go on hot or cold streaks applies not to a single player, but to the whole team. You can imagine that when the whole team is “hot,” then they’ll score lots of runs. When the team is “cold,” their scoring production will drop off.

This is a good analogy to what we’re seeing with earthquakes. When the faults in a region – fault system is the fashionable term – are on a hot streak, this is an earthquake cluster. When they’re in a slump, this is a lull.

The problem we have in looking at the geologic record of earthquakes is that we can only see the biggest earthquakes – the home runs. The 1992 Landers (magnitude 7.3) and 1999 Hector Mine (7.1) earthquakes in the Mojave (Desert) fall into this category. In contrast, Northridge and San Fernando were ground rule doubles, not home runs.

Q: So were the San Fernando (6.7) and Northridge (6.7) quakes blips in an otherwise quiet period for the San Andreas?

A: Northridge is sort of like Chone Figgins – lots of hits, but few home runs. Even in a slump, you’d expect him to get a couple hits.

It’s the same with the faults in L.A. We expect moderate earthquakes like Northridge and San Fernando to continue to occur, even in a seismic lull.

Q: Could we have a couple of 7.0 quakes on the San Andreas, or a fault further west, over a 10-year period and still be considered to be in a lull?

A: The San Andreas also has clusters. But instead of being Chone Figgins, the San Andreas is more like Barry Bonds. Even in a slump, you’re still going to get the occasional home run.

Q: Your paper seems to say that the San Andreas fault system has suppressed quakes in the Eastern California Shear Zone. Then things switched around, and the shear zone is dominant. Is that accurate?

A: Yes, that’s more or less it. The other part of the equation is that when the San Andreas is highly active, so are the faults in the L.A. basin. So what we appear to see is that the Mojave faults take turns with the San Andreas/L.A. basin.

The “why” is the hard part. We propose a mechanism in the paper, but to be fair it’s just an educated guess. The idea is that the deep portions of big faults – below the depths where earthquakes can occur – controls whether or not the shallow, earthquake-producing portion of the fault is in a cluster or a lull.
If the deep portion of the fault is moving rapidly, then you’re in a cluster. If it’s moving slowly, you’re in a lull.

So now the question becomes: how come the deep portion of the fault speeds up and slows down? Our idea is that there is a feedback between the shallow and deep part of the faults. Earthquakes are very dynamic, violent events that tend to produce “damage” throughout the fault zone. If you’re in the middle of a cluster, with lots of big earthquakes, then there is a lot of “damage” accumulating on the fault at depth. This inhibits the deep part of the fault from easily sliding, causing the fault to slow down and bring the whole system back into a lull. Given sufficient time, the deep part of the fault will heal, allowing it to pick up speed again leading to another cluster.

Q: If the Mojave system is suppressing the San Andreas, does that mean Orange County shouldn’t worry about a large quake erupting locally?

A: Absolutely not. L.A./Orange County still needs to worry about all the smaller faults. Even a modest 6.5 event can be a killer if you are unprepared.