Moderate Quakes Often Little Noticed in State

A 4.6 quake that struck California’s North Coast on Wednesday night would probably have caused damage had it occurred in an urban area.

But because the 11:11 p.m. temblor was centered 57 miles off the seaside town of Trinidad, it was barely felt by anyone. Only 19 people reported it on the Web site of the U.S. Geological Survey.

Such is a fairly routine case in earthquake country.

Despite California’s growing population and considerable urban sprawl, most quakes, even those centered on land, occur in areas too remote to cause much damage.

Even major jolts, like the 1992 Landers quake, magnitude 7.3, or the 1999 Hector Mine quake, magnitude 7.1, were in deserts or other rural areas, and there were few casualties.

One person died in the Landers quake, and there were no fatalities in the Hector Mine temblor.

Yet, according to Caltech’s Tom Heaton, an earthquake engineering professor, if either quake “had happened in the Los Angeles Basin, it would have killed hundreds or thousands of people.”

“It would have been much more serious than Northridge in 1994,” Heaton said. That temblor, a magnitude 6.7, killed 57 people and did more than $40 billion worth of damage.

The shaker directed an estimated 70% of its energy into the sparsely populated Santa Susana Mountains.

Caltech seismologist Egill Hauksson calls the Northridge event “in pilot’s language, a ‘near miss.’ ” Still, it was one of the most costly natural disasters in U.S. history.

As Ross Stein, a scientist with the U.S. Geological Survey, said Thursday, “Earthquakes follow the rule of real estate: location, location, location.”

As an example, Stein cited the magnitude 8.0 quake that occurred last month 65 miles off Japan’s Hokkaido island. It was the biggest quake recorded anywhere in the world in 2 1/2years, but it killed no one and caused little serious damage.

On the other hand, Stein said, the equivalent-sized Yokohama quake of 1923, which was centered much closer to shore, next to one of the world’s most heavily populated urban areas, killed 143,000 people, devastating about a third of Tokyo and all of Yokohama.

Lucy Jones, scientist-in-charge of the Geological Survey’s Pasadena office, said Thursday that the “the overwhelming majority of earthquakes, perhaps nine out of every 10, occur where no damage is likely.

“Two thirds of the Earth is covered by water,” she said. “Then you have the mountains, the deserts ....A relatively small percentage of the Earth has a dense population.”

There are about 10 quakes a year in the magnitude 7 range, Jones said.

So, about 100 magnitude 7 quakes every 10 years. But of those, only about 10% are really damaging, she added.

Still, scientists such as Hauksson and Heaton point out, it remains vitally important to prepare for the uncommon quake that does devastate an urban area, such as the 1999 temblor that struck in western Turkey, killing thousands.

And damage can vary widely, depending on the exact conditions when the quake strikes.

Haresh Shah, director of the quake mitigation firm RMS in the San Francisco Bay Area, recalled Thursday that, after the Northridge quake, his company worked up some quake scenarios for the Los Angeles and San Francisco Bay areas.

“We felt that, if a 7.0 quake were to strike on the Newport-Inglewood fault in the Los Angeles area on a day when there were 10-mile-per-hour winds blowing and there was not a drought underway, the loss due to fires alone would be between $22 billion and $32 billion,” Shah said.

If higher, Santa Ana-style winds were blowing, the loss could be far greater.

Officials said it’s important to keep in mind that, in its recorded history of 220 years, Los Angeles has not had a magnitude 7 quake in the basin. But, over the very long term, such a quake is held to be possible.

The quake that occurred 57 miles off Trinidad on Wednesday night was several hundred times smaller than a magnitude 7.0, and well removed from any place it could do any damage.