September 13, 2007

Sciencedude: Scientists explain why quakes are shaking O.C.

Gary Robbins

The two small earthquakes that jostled northern Orange County early Wednesday were aftershocks of the 4.7 quake that erupted Sept. 2 on a little-known spur of the Elsinore fault zone, scientists say.

The spur is just across the Orange County line in Riverside County, between the Corona (I-15) Freeway and the eastern flank of the Santa Ana Mountains. Lake Elsinore is just to the southeast.

There was a 3.7 quake in this area at 4:30 a.m. Wednesday, followed by a 3.2 at 7:21 a.m. More shaking is expected, perhaps as early as today.

The quakes haven’t caused much damage. But they’ve made some people nervous, and have generated lots of questions. We’ve raised those concerns with three veteran geophysicists — Bob Dollar of the U.S Geological Survey, Mark Legg of Legg Geophysical, and Dave Bowman, chair of geological sciences at Cal State Fullerton.

Q: Which fault is producing the quakes?

A: They’re coming from a spur of the larger Elsinore fault zone, which is a potentially dangerous system that’s not as well understood as some of Southern California’s other major faults. The fault runs from the Chino Hills down to El Centro, east of San Diego – making it about 110 miles long. The Elsinore is capable of producing quakes up to 7.5 in magnitude, which means it could cause tremendous damage.

Q: Is the spur dangerous?

A: Yes. It’s not unusual for a section of a major fault to break and produce a damaging quake. We’re still looking at exactly where these quakes are coming from. But it seems to be an area of the fault that’s near Glen Ivy, just west of the Corona (I-15) Freeway. There’s a section of the fault known as the Glen Ivy spur that’s capable of producing a 6.8 quake. But scientists are still examining the source of the shaking.

Recent earthquakes have shown that quakes on small “spurs” like this may occasionally jump onto the main fault and run away to become much larger events. For instances, the magnitude 7.9 Denali, Alaska, quake in 2002 started on a previously unknown “spur” now known as the Susitna Glacier Fault. The rupture jumped from this fault onto the much larger Denali fault and ruptured for another 300 kilometers on the Denali and Totschunda faults.

Q: Has this area of the fault produced quakes before?

A: Yes. There was a 6.0 quake in that area in 1910, and a 5.2 in 1958. We had to estimate where the quakes began because not as much data was collected then. But it’s the same general area.

Q: Since the 4.7 quake on Sept. 2, there have been several other quakes measuring 3.0 or higher. The largest (on Sept. 12) was 3.7. Are these aftershocks, or are they separate quakes?

A: They appear to be aftershocks because they’re occurring in the same spot. It’s common for the main quake to produce aftershocks for several weeks.

Q: But the 3.7 quake was bigger than one of the earlier aftershocks. Doesn’t that make it a separate quake?

A: It’s not unusual to have an aftershock that’s bigger than all but the main shock. What we don’t know is whether what’s been happening is simply a quake with aftershocks or a precursor to a larger event. Scientists have yet to come up with a consistent, repeatable way to predict when and where quakes will occur.

Q: The 4.7 quake on Sept. 2 and one of the after shocks occurred during the big heat wave. Was heat a factor in the quakes?

A: There’s no such thing as earthquake weather. Most quakes occur at least a mile or two below the Earth’s surface, far beyond the affects of weather.

Q: What about the 8.4 earthquake that occurred this week off Sumatra Island in Indonesia? Did seismic energy from that event cause the 3.7 and 3.2 quakes that happened here on Wednesday?

A: There’s no belief we experienced something as random as a big quake in Indonesia causing small quakes in this one part of Southern California. But a large quake can cause seismic activity elsewhere. The magnitude 7.3 earthquake at Landers in 1992 produced micro-seismicity in Mammoth Lakes. This is called triggered seismicity. But Indonesia didn’t affect us.