Geologists digging near Palm Springs have found evidence of one of the biggest displacements ever seen along the mighty San Andreas Fault: a 72-foot gap likely created by two major quakes.
The quakes probably rattled the earth along the fault's Mission Creek segment hundreds of years ago, the scientists said in interviews.By digging trenches into soil and rock in the Coachella Valley south of Palm Springs, the geologists discovered evidence that the temblors may have sliced apart a stream channel, shifting the segments 72 feet apart.
At least two quakes - one about 300 years ago and another 250 to 400 years earlier - moved the earth that distance, said Thomas Fumal and Michael Rymer, geologists with the U.S. Geological Survey in Menlo Park.
Each quake would have had to be powerful - magnitude 7.7 to 7.8 - to trigger such a seismic shift, they said. A magnitude 7 earthquake is major, capable of producing widespread, heavy damage.
Each earthquake would have had to displace nearly 36 feet, "about as large as we've seen before," in San Andreas quakes farther north, Fumal said.
Additional quakes also might have powered the 72-foot shift, Dan Ponti, another geologist with the U.S. Geological Survey, said Friday.
"We can see at least two. There may be another one or two in there," he said, adding that the preliminary findings leave many unanswered questions about when another temblor might jolt the Mission Creek segment.
The excavated area, about 150 feet by 50 feet, contains a series of intersecting trenches that cut across and parallel to the fault. Fumal and Rymer estimated the dates of the past quakes by analyzing layers of carbon-based peat at the site.
Ponti said there is good evidence that there was another quake along the fault before water cut the channel, which would mean there were at least three quakes in the past 1,000 to 1,200 years.
The new evidence suggests the Mission Creek segment probably breaks in fairly large earthquakes somewhat regularly.
If more research suggests such a pattern, "we're probably looking at an average rate of recurrence of around 400 years," Ponti said.
With about 300 years since the last one, the next "could be another hundred years or maybe tomorrow," he said.
Seismologists have long suspected that the communities of San Bernardino and Palm Springs are at significant risk of a San Andreas quake in the high magnitude-7 range, Ponti said.
The San Andreas fault stretches 500 miles from San Francisco to the Mexican border.