September 15, 2007

Experts explain seismic shaking in O.C.
The until recently relatively obscure Elsinore system packs the potential for 7.5 quakes in its arsenal, scientists say.
By GARY ROBBINS
The Orange County Register

Did you feel it?

Seems like that's all that most people in northern Orange County have been talking about since Sept. 2, when a magnitude 4.7 earthquake erupted just across the border in Riverside County, in the normally quiet stretch between Corona and Lake Elsinore known as Temescal Valley.

A series of aftershocks followed, all in the same area of the eastern Santa Ana Mountains. None of the shaking has been strong enough to cause significant damage. But it was widely felt, making some people nervous. And it's generated lots of questions.

We raised your concerns with three veteran geophysicists – Bob Dollar of the U.S. Geological Survey in Pasadena, Mark Legg of Legg Geophysical in Huntington Beach and Dave Bowman, chairman of geological sciences at Cal State Fullerton. We've summarized and expanded upon what they had to say.

Q:Which fault is producing the quakes?

A:They're coming from a spur of the Elsinore fault zone, which is a potentially dangerous system that's not as well understood as some of Southern California's other big faults. The Elsinore system 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 in Orange and Riverside counties. It actually poses more of a potential threat to Orange County than the San Andreas.

Q:Is the specific spur that's causing the recent shaking 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 coming from the Glen Ivy spur, which is a 22-mile-long segment of the fault just west of the Corona Freeway in Temescal Valley. The Glen Ivy is capable of producing a 6.8 quake. Such an event could prove hugely damaging to Orange County, which sits on the other side of the Santa Ana Mountains.

Recent research has shown that earthquakes on small spurs like Glen Ivy may occasionally jump onto the main fault and run away to become much larger events. For instance, the magnitude 7.9 Denali, Alaska, earthquake in 2002 started on a previously unknown spur now known as the Susitna Glacier fault. The rupture jumped from that fault onto the much larger Denali Fault and ruptured for another 185 miles. Granted, this is something of a worst-case scenario. However, it is possible.

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

A: Yes. There was a 6.0 quake in that area in January 1910 and a 5.2 in May 1938. We had to estimate where the quakes began because not as much data was collected then. But it's the same general area. There also were a couple of 4.0's in the early and mid-1930s. All of this occurred before the region was heavily populated.

Q:Since the 4.7 quake on Sept. 2, we've had several other quakes measuring 3.0 or higher. The largest (on Sept. 12) was 3.7. Are these just 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 on Wednesday 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 whether it is 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.

Aftershocks are just earthquakes that occur in fairly close proximity to a larger, usually more notable event. How much time must pass before an event is no longer an aftershock depends upon the size of the main shock and its aftershock productivity.

Once the rate of earthquakes falls to approximately a "background" rate – say, the year before the main shock, for example – additional earthquakes are no longer considered aftershocks. The 1952 Kern County magnitude 7.5 event was clearly still producing aftershocks 30 years later.

Q: The 4.7 quake on Sept. 2 and one of the aftershocks 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: A large quake can cause quakes elsewhere. The 7.3 Landers quake in 1992 produced microseismicity in Mammoth Lakes. This is called triggered seismicity. But the Indonesia quake didn't cause the quakes here. It's not reasonable to assume that an event of that size would only cause small quakes in a place as far away as Southern California.