Tuesday's earthquake here was placed variously at 5.8 to 6.2 on the Richter scale. But it didn't take a seismologist to tell veterans of California temblors that the quake was ''strong.'' Buildings swayed wider and windows and doors rattled louder than they usually do in the routine rumblings beneath California.
What a seismologist can tell residents, however, is that the quake was not unexpected and that there are likely to be more just like it over time. Further, emphasizing the area's quake-prone position, Tuesday's temblor came just before next week's anniversary of the major Coalinga quake in 1983 and following last week's anniversary of the huge San Francisco earthquake of 1906.
Emanating from the Calaveras Fault, where it runs through Morgan Hill 50 miles south of here, this week's quake was a near duplicate of a 5.9 quake 30 miles further south in 1979, says Gerald Eaton, a seismologist with the US Geological Survey. (That quake received less attention because it occurred in a more isolated area.) The two quakes are evidence of the strike-slip pattern of activity along the fault, which heads in a northwesterly direction, ending near the western shore of San Francisco Bay, Dr. Eaton says. The Pacific side of the fault is sliding toward the northwest as it grates along the eastern side of the fault, he explained.
''The break (on Tuesday) was to be expected. The only question was when it would occur. Historically there have been some large-ish earthquakes here, and somewhere along the whole stretch of faulting it will indeed break again in the range of 6 to 6.5 (on the Richter scale),'' Eaton says.
Where the ''breaking'' will occur is not certain. But he notes that much of the rest of the Calaveras Fault lies north of here, in the heavily populated East Bay, where damage would be worse.
Although Tuesday's quake caused some minor injuries to San Jose and Morgan Hill residents, knocked homes off foundations, sparked fires, cracked highways, and interrupted phone and electric service, the damage was considered light compared with last year's 6.5 quake, which flattened Coalinga.
Eaton explains that if a quake were to hit closer to the metropolitan area of San Francisco, damage would be much worse. He says most of the structures in the farming community of Morgan Hill were new, low-lying buildings made of relatively limber wood and nails. The masonry and multistory construction of older buildings in the bay area, he says, would incur more damage in a quake.