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In a major new report on earthquake scenarios in the Mammoth area, geologists say they have identified five faults in Long Valley capable of producing temblors of 6.7 or greater—roughly as strong as the 1994 Northridge quake.
Along with scientific data, the scientists say planners in the area should include the data in preparing future scenarios that would impact everything from property destruction to emergency health care.
“The scenarios are both large enough and likely enough that emergency planners should consider them in regional emergency response plans,” said Chris Wills, the California Geological Survey supervising geologist, who co-wrote the report.
Planners in Mono County said they agree.
“We have a safety element in our General Plan, and we also have more in our overall environmental assessment documents,” said Wendy Sugimura, the associate analyst in the Mono County Planning Division.
She said the county currently is in the process of updating its General Plan, and she is eager to see the new research.
Wills, in a telephone interview from his office in Sacramento, said the report was not meant to be alarmist.
“We’re talking about events that might happen once every hundred years, or even 10 years,” he said. “But that’s what planning is about—the long range.
“You don’t want to spend time planning for what happens every other Tuesday.”
Wills said the California Geological Survey, unlike the U.S. Geological Survey in Menlo Park (San Mateo County), works closely with government entities in preparing geological impact reports, but has concentrated in recent years on urban areas such as San Francisco and Los Angeles.
This report, he said, came at the request of the USGS.
In it, Wills along with longtime Long Valley specialist David Hill of the USGS, said future earthquakes in the area would not be limited to mere shaking, as is the case in most earthquakes.
Rather, the report indicates that earthquakes in the area could potentially cause landslides and widespread liquefaction.
“Shaking is the obvious concern, but there are lesser-known effects that pose threats to public safety and property,” Wills said.
“This report identifies areas prone to surface fault rupture—that is, the ground on one side of a fault moving horizontally or vertically relative to the other side—landslides, and also liquefaction, which means the ground temporarily losing its ability to support buildings or infrastructure.”
Wills said the report does not include avalanche scenarios.
The 102-page report, called “Scenario Earthquake Hazards for the Long Valley Caldera-Mono Lake Area, East-Central California,” is available online at www.conservation.ca.gov.
It is part of the U.S. Geological Survey’s (USGS) multi-hazards project for this volcanic and heavily faulted part of California.
“The analyses in the new report can be useful in estimating the extent of potential damage and economic losses in possible future earthquakes and in preparing emergency response plans,” Hill said.
“This area is very seismically active. There was a swarm of small earthquakes in February around Mammoth Mountain, but large and damaging quakes must be expected.”
The CGS developed earthquake scenarios for the five faults in the study area considered most likely to produce a Northridge-sized or larger earthquake—Fish Slough, Hartley Springs, Hilton Creek, Mono Lake, and Round Valley Faults—as well as the White Mountains Fault to the east.
Significantly, the scenarios portrayed are not worst-case events.
Each scenario is illustrated with maps of seismic shaking potential and fault displacement, liquefaction, and landslide potential.
All the scenarios show the possibility of widespread ground failure, which would have major consequences.
“There are not a lot of detailed studies measuring ground strength on the Eastern side of the Sierra,” Wills said, “but emergency planners need to be alert to what impact a strong quake might have on direct lifelines and transportation networks, such as U.S. 395.
“If that highway were cut in any way, you have a problem,” he said.
Liquefaction damage would likely occur in the areas of higher ground shaking near the faults where there are sandy or silty sediments, and the depth to groundwater is 20 feet or less.
Generally, he said, this means damage is most common near lakes and streams in the areas of strongest shaking.
Landslide potential exists throughout the study region, particularly where there are steep slopes, according to the report.
The landslide hazard zones also are likely sources for snow avalanches during winter months and for large boulders that can be shaken loose and roll hundreds of feet downhill, which happened during the 1980 Mammoth Lakes Earthquakes, a magnitude 6.2 and two magnitude 5.9s.
Fault rupture did significant damage to the Mammoth Elementary School, and nine people were injured during the 1980 sequence.
The CGS is part of the California Department of Conservation (DOC).
In addition to studying and mapping geologic phenomena such as earthquakes and landslides, DOC categorizes mineral resources; administers agricultural and open-space land conservation programs; ensures the reclamation of land used for mining; and regulates oil, gas and geothermal wells.