Experts Divided on Whether Recent Quakes Fit a Pattern

Historically, California has had one earthquake of at least magnitude 6.5 about every four years, but the state has had six temblors that strong or stronger since 1980, and over the weekend it suffered two in just two days.

In the last 20 years, there have been 18 earthquakes of at least magnitude 6.0, and four of those came in just five days in the past week.

But does this mean that California has entered a more destructive earthquake cycle? Most scientific experts interviewed in the last few days respond: not necessarily.

In the natural, random order of things, “it seems like we go for a long time without any big quakes, and then it seems we have a lot of quakes that get a lot of attention,” said Bruce Presgrave, a geophysicist at the National Earthquake Information Center in Golden, Colo. “In the long run, the statistics are pretty uniform.”

Lucile M. Jones, seismologist at the U. S. Geological Survey’s office in Pasadena, called recent statistics “chance.” She explained, “If they never clustered, it wouldn’t be random.”

The view is not unanimous. Lori Dengler, professor of geology at Humboldt State University, said Tuesday she thinks the seismic events of the last 20 years represent a return to what she considers the more normal pattern of the 19th Century, when there were “a lot of big and very damaging earthquakes.”

Following the tremendous burst of energy in the magnitude-8.3 San Francisco quake of 1906, there seems to have been an unusually quiet period lasting, with some interruptions, more than 60 years, she said.

Dengler also notes that “geological times and human times are different. It’s very hard to draw an inference from patterns when you’re only looking at a piece of that pattern. We would have to have 10,000 years of experience to be more certain.”

Reliable records in California go back less than 200 years.

Worldwide, since 1900, the Geological Survey’s records show that there have been an average of about one magnitude-8.0 quake each year, 18 magnitude-7.0 quakes and 120 magnitude-6.0 quakes (or about 1 every 3 days).

Caltech geology professor Kerry Sieh noted this week that in the four days beginning April 23, there were seven quakes of at least magnitude 6, including the four in California. There were two on the Myanmar-China border and one in Pakistan.

“So over the last several days, there have been more than usual,” Sieh said. “But what does that mean?”

He said there have been many cases in which there was a sudden flurry of significant earthquakes within a short time. “The year 1812 was one for big earthquakes, with the eighth at New Madrid, Mo.,” he said, and “we had two above 7 in California just 13 days apart, one in Santa Barbara and the other in Wrightwood.”

Most earthquake scientists, including Sieh, say there is no physical mechanism that could explain a connection between widely distant quakes, even when they occur within a short time.

So they discount any connection between the Joshua Tree earthquake last Wednesday and the Humboldt County temblors a few days later. They similarly dismissed as coincidence the quakes in the summer of 1986, when sizable temblors struck North Palm Springs, Chalfant Valley near Bishop and in the ocean off San Diego County within a few weeks of each other.

Taking some exception to this view, however, is Caltech geophysicist Don Anderson. He said Tuesday he would like to see more investigation of the possibility that stress on the various tectonic plates that underlie California might somehow be transmitted over long distances.

“Quakes may be just a very trivial byproduct of very large plate tectonic performance, like a sandpile that reached a critical mass and went all at once,” he said, adding, however, that he does not believe a quake in Southern California would directly trigger one in Northern California.

As California grows in population and new housing and business areas proliferate, there is an ever bigger chance that the strong earthquakes that do occur here will affect urban areas and, despite updated building codes, do some damage.

But in the past 60 years only four earthquakes caused loss of life and substantial damage in the Los Angeles area--the 1933 Long Beach, the 1971 Sylmar-San Fernando, the 1987 Whittier Narrows and the 1991 Sierra Madre.

Even those damaged only comparatively small parts of the area. A person living in West Los Angeles or in many parts of Orange County could have gone through their entire lives here and not seen their homes suffer any appreciable damage.

In this century, there has not been a magnitude-7.0 earthquake centered in Los Angeles, Orange or San Diego counties, although at least four centered outside have been felt in some parts of those counties.

Yet there could be one tomorrow, and state authorities have prepared damage maps outlining conceivable patterns of destruction if a magnitude-7.0 quake or stronger occurs either on the San Andreas Fault, where it passes through Los Angeles County, or the Newport-Inglewood Fault, where it passes through Orange and Los Angeles counties.

Big Quakes

In the last 20 years, California has had 18 earthquakes of at least magnitude 6.0, considerably more than had traditionally been expected. Date: Magnitude Nov. 8, 1980, Offshore Eureka: 6.6 Nov. 26, 1976, Ferndale: 6.0 Apr. 25, 1992, Petrolia: 6.9 Apr. 26, 1992, Offshore Petrolia: 6.5 and 6.0 Aug. 1, 1975, Oroville: 6.1 Apr. 24, 1984, Morgan Hill: 6.2 May 25, 1980, Mammoth Lakes: 6.3 and 6.0 May 27, 1980, Mammoth Lakes: 6.0 July 21, 1986, Chalfant Valley: 6.1 Oct. 17, 1989, Loma Prieta: 7.1 May 2, 1983, Coalinga: 6.5 Aug. 4, 1985, Avenal: 6.0 Apr. 22, 1992, Desert Hot Springs: 6.1 Nov. 23, 1987, Westmoreland: 6.2 Nov. 24, 1987, Westmoreland: 6.6 Oct. 15, 1979 Calexico: 6.6 Since 1971, including the Sylmar-San Fernando earthquake, all California temblors have killed 134 persons and injured approximately 6,900.