The U.S. Geological Survey on Tuesday told California disaster planners that Monday's Chalfant Valley 6.1-magnitude earthquake may be repeated in the next few days with additional quakes of similar magnitude and that conceivably a larger quake might also occur.
In a letter addressed to William M. Medigovich, director of the California Office of Emergency Services in Sacramento, and released here, Dallas L. Peck, Geological Survey director, said:
"It is our assessment that the region of Chalfant Valley, from Bishop north to the Nevada border, may experience additional earthquakes similar to the July 21 event during the next few days.
"A less likely larger event, if it occurs, could be strong enough to be widely felt outside the immediate epicentral region and possibly cause damage in Bishop, Big Pine and north to Mono Lake. We will continue to monitor the situation. We shall update this advisory on July 24 (Thursday) unless the situation changes significantly before that time."
The unusual advisory came as more aftershocks, the largest of them 4.6 on the Richter scale, rumbled through the area 15 miles north of Bishop and as cleanup work proceeded in the hamlet of Chalfant Valley, where a local state of emergency was declared by the Mono County Sheriff's Department.
Residents of Chalfant Valley swapped stories about what they were doing when the 6.1 quake hit Monday. Later in the afternoon, the Geological Survey letter raising the possibility of a larger quake, which was widely publicized by local radio, also generated considerable discussion and concern.
Mobile home repairmen were busily placing some of the 53 mobile homes knocked off their foundations on Monday back atop them at a cost of about $800 each. However, authorities said between 18 and 20 of the homes were ruined beyond repair.
Fifty homeless people were temporarily put up in Bishop motels.
Meanwhile, state and Mono County authorities gave a preliminary damage estimate of "more than $1 million" for the earthquake in the largely rural area. The figure compared with the latest estimates of $8 million in damage for the 5.9 earthquake on July 8 north of Palm Springs and $203,000 for the 5.3 quake July 13 off the San Diego County coast.
As the Geological Survey issued its advisory, the state Office of Emergency Services sent two large CH-47 Army National Guard helicopters, a medical evacuation helicopter, a communications van and a fuel tanker to Bishop to stand by just in case there are more quakes.
A phrase in the Geological Survey advisory, to the effect that the Chalfant Valley region "has been discussed by geologists and recognized to have the potential for generating a magnitude-7 event," generated some alarmed speculation in Bishop and surrounding communities. At one point Tuesday afternoon, rumors spread in Bishop that the Geological Survey had actually predicted a 7.0 earthquake for 3 p.m.
About the same time, the Geological Survey issued a statement clarifying the earlier letter.
"Peck emphasized that he was not predicting any additional earthquakes in the area but merely saying that based on the seismic history of the region, additional earthquakes in the next few days are likely," the clarification said. "Although the region has a potential for earthquakes in the magnitude 7.0 range, Peck said the USGS has no evidence that would suggest that an earthquake of that magnitude will occur in the area in the immediate future."
The clarification nonetheless repeated, "Additional quakes in the magnitude 6.0 range may occur during the next several days."
Bishop Mayor Don Talmage responded, "I think we have a fairly decent plan in place which will deal with whatever comes down the pike. Personally, I hope the reports are wrong."
David Hill, the Geological Survey seismologist who has the responsibility for monitoring the nearby Long Valley caldera northwest of the present quake zone, explained by telephone from Menlo Park, Calif., that the Geological Survey assessment is based on its historic appreciation of what has happened in the past throughout the Eastern Sierra region, where multiple quake sequences have not been uncommon.
Hill noted, for example, that four moderate earthquakes, the strongest of which were in the 6 range, in the Mammoth Lakes area May 25, 1980, were followed by a 6.3 quake just two days later. He also recalled that in 1954, "there was quite an extended sequence of large quakes east of Reno."
In the last several years, the Geological Survey has issued advisories similar to the one it issued Tuesday on a few other occasions. For instance, Caltech seismologist Kate Hutton recalled that after a magnitude-4 quake off San Diego last year, it warned that there was a small chance, perhaps 1 in 20, that a larger quake might occur within five days. Nothing happened, however.
Hutton said she thinks that the basis for the Geological Survey's latest advisory "is that that (the Eastern Sierra) area is known for having multiple earthquakes."
"The statement about a 7 quake is only due to Chalfant Valley's proximity to the White Mountains seismic gap (where no big quake has occurred in recorded history)," she added. "It would certainly cause one to be more alert. It doesn't mean that there's a likelihood (of it happening). There's a possibility."
Meanwhile, authorities in Bishop issued a statement attributed to the "Mono and Inyo Counties Unified Command" that said, in part:
"The third day of earthquake activity in the Eastern Sierra finds all the involved agencies still in readiness to respond to needs as they arise. Since the peak of earthquake activity on Monday morning, no new major problems have arisen, and all agencies are working to achieve stability and to be prepared should more temblors occur.
"In the 24 hours since the largest temblor Monday morning, there were a total of 1,003 quake events recorded . . . by U.S. Geological Survey scientists. Of these, 72 measured 3.0 Richter or larger."
In another development Tuesday, the largest seller of auto and home insurance in California, State Farm, announced that it has imposed separate 30-day moratoriums on putting new earthquake insurance policies into effect within 100-mile radiuses of the Chalfant Valley, Palm Springs and San Diego County earthquakes. Each moratorium is pegged to the date of the particular quake.
A company spokesman said the firm normally takes such a precaution after all quakes measuring 5.0 or more on the Richter scale. He said new earthquake policies will be written, but their effective date will be delayed until after the 30 days passes in each case.
Assistant California Insurance Commissioner Richard J. Ross explained that many companies have such moratoriums. He said that one reason is that they fear aftershocks and another is that they were concerned that someone buying a policy right after a damaging quake has occurred will attribute damage that occurred in the original quake, when they were not covered, to these aftershocks, thereby confronting adjusters with a difficult situation.
Checks with several other companies Tuesday showed Allstate has imposed a seven-day policy issuance moratorium on the Chalfant Valley quake, within a 100-mile radius, and is making individual judgments in other areas. Twentieth Century imposed a three-day moratorium in a 100-mile radius of each of the recent quakes and the Farmers group of companies has taken no action as yet but is studying the situation.
Ross said all of these companies are within the law in imposing moratoriums, because while California law requires companies selling home insurance to sell earthquake insurance, it says they have 60 days to actually issue it.
George Ramos reported from Bishop and Kenneth Reich from Los Angeles.