Every earthquake that comes, comes as a surprise. But the quake that hit the little town of Coalinga Monday afternoon in California's San Joaquin Valley threw seismologists for an extra loop.
In a sense, the Coalinga quake - which was recorded variously between 6.0 and 6.5 on the Richter Scale - fell between the cracks.
It did not hit in a spot that earthquake watchers particularly had their eyes on. Although the temblor was centered only about 15 miles east of the notorious San Andreas Fault, it was not near any of the ''earthquake gaps'' - areas thought to be due for a shaking because of geological pressures.
In fact, the quake was not on any known network of faults.
''If you had asked me the day before all the places where an earthquake might occur, I would never have thought of the Coalinga area,'' says Clarence Allen, a geophysicist at California Institute of Technology and a dean of earthquake researchers.
''It was a little off the beaten track,'' he adds.
What Dr. Allen is most eager to glean from the recording instruments over the next few days and weeks is why they didn't know that a quake was imminent in the Coalinga area - and whether they should have.
''In a sense,'' he says, ''it's a little discouraging if we can have a major earthquake on a fault line we don't even know about.''
Discouraging, of course, because it makes planning and preparedness more difficult.
A pressing question for the experts is how this temblor may be related to the major earthquake that geologists say could come in the next 100 years or so.
The connection is not obvious. Yet all California earthquakes, Dr. Allen points out, are related to the San Andreas fault somehow. The San Andreas is the major fault line that runs from north of San Francisco into the Gulf of California.
The question for seismologists is whether the Coalinga quake, which was still producing aftershocks well into Tuesday, might trigger another, bigger temblor along the San Andreas itself or might instead relieve some of the pressure on the main fault and forestall a big earthquake.
According to most reports, the extensive damage reported in the small town of Coalinga was chiefly to brick buildings, with some damage to wood-frame buildings that were not firmly attached to their foundations.
Brick buildings without reinforcing beams, most built in the 1930s and earlier, are perhaps the greatest hazard in earthquakes.
Richard Andrews, executive director of the state Seismic Safety Commission, hopes that this quake will encourage Californians to rehabilitate unfit buildings.
He notes that the only city to have undergone a full-scale project to make old buildings earthquake safe is Santa Rosa, north of San Francisco.
Santa Rosa has a history full of earthquakes. The city was hit harder than San Francisco in the devastating 1906 earthquake, and crumbled badly in 1969 under a quake measuring 5.7 on the Richter Scale.
The city had to knock down 100 buildings in its commercial center immediately after the 1969 quake. As a result, it has combined its redevelopment efforts with a thorough effort to reinforce its brick buildings.
The city can now hold up under a quake of 7.5, says City Manager Kenneth Blackman.
The city of Santa Ana, south of Los Angeles, is beginning a similar project, and Los Angeles moved to reinforce some of its 8,000 unreinforced brick buildings, too. But it's a difficult process.
''It takes a long time,'' says Mr. Blackman. ''Even now we're still getting some of the more reluctant owners'' to do this expensive work on their old buildings.