Underwater Epicenter Complicates Scientists’ Studies of Earthquake

The magnitude 5.3 earthquake Sunday that was centered 30 miles off the San Diego coast occurred in an underwater region that may have many small faults but which has largely been unexplored.

Part of the problem is that these faults are underwater, while most of the geologist’s sensors are on land. Perhaps more significant, however, is that geologists have concentrated heavily on the main fault systems, which pose the greatest hazard, and have spent less time looking at smaller faults.

“We know there are faults out there, but they haven’t been precisely located and they haven’t been named,” said geologist Kate Hutton of Caltech.

Collision of Plates

Virtually all the earthquake activity along the Pacific Coast of North America is associated with the collision of the North American plate--a large surface region that is slowing being propelled westward by the earth’s molten core--with the Pacific plate, which is moving east and plunging under the North American plate.

The San Andreas fault, which runs inland from San Francisco south to the Salton Sea, represents a major segment of the area where the plates come together, but the entire fault zone is much larger and more diffuse. Great pressure builds up in the earth as the two plates unsuccessfully try to slide past each other.

When they do slip past each other, an earthquake results, and stress is relieved. “About 60% of the stress between the two plates is relieved by the San Andreas fault system,” Hutton said, “and the rest by smaller faults.”

Tuesday’s temblor near Palm Springs was on the Banning fault, which is part of the San Andreas fault system, but at magnitude 5.9 on the Richter scale it was too small to relieve significant amounts of stress and reduce the risk of a future large quake.

Less Stress Relief

Sunday’s magnitude 5.3 quake was only about one-fifth the intensity of the Tuesday temblor, and thus relieved even less stress. Each increase of one number on the Richter scale represents a ten-fold increase in ground motion.

Hutton said that there is “almost 0% probability” that the two quakes were related. The fact that the two quakes came within days is coincidental, she said.

Historically, the San Diego County area where the quake occurred has not had a great deal of seismic activity. Since Caltech scientists began keeping records in 1932, the largest offshore earthquake in San Diego County was a magnitude 4.8 in 1984.

The largest earthquake in San Diego itself was a magnitude 4.6 quake in June, 1983. One aftershock of Sunday morning’s quake, at 7:02 a.m., was recorded at magnitude 4.5. By mid-afternoon, Caltech had measured nine aftershocks ranging from magnitude 3.5 to 5.

Geologists were not yet certain of the precise location of the quake’s epicenter and what fault it was associated with. “Normally after a quake, you go out and see which fault has fractured,” said Hutton. “When it’s underwater, you can’t do that.”

More Difficult Process

The scientists will thus have to use the more tedious process of triangulating in on the quake’s location by using land-based sensors many miles away. Because the nearest of those are on San Clemente and Santa Catalina islands, the process requires time and is not as accurate as a visual inspection.

Scientists said, however, that Sunday’s quake did not appear to be associated with the fault that runs from Santa Monica through Inglewood to Newport Bay and then offshore along the coast to Baja California. The fault has been associated with small quakes for hundreds of thousands of years.

The 1933 magnitude 6.3 quake in Long Beach occurred at the north end of the fault. Some experts have argued that quakes along this area might imperil the San Onofre nuclear power station.

Large undersea quakes can produce seismic waves, but Sunday’s was not large enough to trigger any significant waves.