San Andreas Fault

Thanks to a new method of modeling earthquakes, scientists may now understand why the Parkfield segment of the San Andreas fault - a carefully studied region known for producing moderate temblors every 20 years or so - has been behaving unexpectedly since around the time Ronald Reagan was in the White House. Taking data collected by sensors on the ground and in space and combining them with observations from laboratory physics experiments, Caltech researchers conducted a computer simulation of tectonic events at Parkfield and discovered that a series of small quakes there may have staved off a larger shaker that geologists predicted would occur in the late 1980s or early 1990s.

Thanks to a new method of modeling earthquakes, scientists may now understand why the Parkfield segment of the San Andreas fault - a carefully studied region known for producing moderate temblors every 20 years or so - has been behaving unexpectedly since around the time Ronald Reagan was in the White House. Taking data collected by sensors on the ground and in space and combining them with observations from laboratory physics experiments, Caltech researchers conducted a computer simulation of tectonic events at Parkfield and discovered that a series of small quakes there may have staved off a larger shaker that geologists predicted would occur in the late 1980s or early 1990s.

Buried beneath the tracts of desert-colored homes with shingled roofs, wide streets and big yards, rests the mighty San Andreas fault, a geological menace responsible for two of the greatest earthquakes in California history. For Karmen Garcia and her two dogs, Dino and Sparky, this is home. "You can't see it here, but I know it's there," said Garcia, 45, as she walked her dogs along Bayberry Street. I think about it every day." Garcia laughed at the irony.

The earthquake may have measured only a 3.9, but it still could make history in Orange County. Monday's temblor, centered in the southern suburb of Laguna Niguel, could be the first measured on a fault discovered only 13 years ago and running along the coast from Newport Beach and Costa Mesa to San Juan Capistrano - close to the San Onofre nuclear power plant. The little-known fault - called the San Joaquin Hills thrust - is similar to the fault that triggered the deadly Northridge quake in the San Fernando Valley 18 years ago. Unlike the famous San Andreas fault, which can be seen on the surface, the fracture in the earth's crust that makes up the San Joaquin Hills thrust fault is entirely underground.

Under the central part of the San Andreas fault, the deep seismic whisperings known as tremors have increased after two recent large earthquakes, and a surge in tremors appears to have foreshadowed one of them, according to a study to be published today in the journal Science. "It reaffirms the need to be ready," said Robert Nadeau, a research seismologist at UC Berkeley who led the study.

The U.S. Geological Survey has recorded a series of small aftershocks southeast of Hollister in the aftermath of a 4.7-magnitude earthquake Friday on the San Andreas fault. A 3.0 aftershock struck about 7:15 a.m. Sunday, the Geological Survey's Menlo Park office reported. No damage was reported in the rural area, officials said. Sunday's aftershock followed a series of gentle jolts Saturday, including a 3.7 shaker--all aftershocks to a 4.

New earthquake research confirms the southern end of the San Andreas fault near Los Angeles is overdue for a Big One, according to an analysis published in the current issue of the journal Nature. The lower section of the fault has not produced a major earthquake in more than three centuries.

Parkfield, the Central California hamlet on the San Andreas Fault where scientists have deployed an elaborate network of seismological instruments since 1985, had a 5.0 temblor early Tuesday, but scientists did not view it as a likely precursor of a larger quake. The moderately strong 2:28 a.m. jolt was centered three miles west of Parkfield and was felt in a wide area, including Paso Robles and San Luis Obispo, causing only a few minor rockfalls that did no damage.

For years, geological evidence has suggested that large earthquake systems, such as California's San Andreas Fault, are driven by powerful forces that are held in check only by friction along the fault that is nearly as strong as the forces themselves. Textbooks used to train generations of earth scientists have stressed that concept, and most geologists today have had little reason to doubt it. But now it appears that the textbooks probably are wrong.

Scientists increased their watch along the San Andreas fault near Parkfield in Central California Monday after a 3.5 earthquake occurred in a particularly sensitive zone where a magnitude 6 quake has been predicted since 1985. Bill Ellsworth of the U.S. Geological Survey said the quake at 4:28 p.m. Sunday was centered on the San Andreas fault under Middle Mountain, seven miles northwest of Parkfield. The predicted strong temblor under the mountain is at least four years overdue.

Rocky and mostly barren, the San Jacinto and Santa Rosa mountains southwest of Palm Springs offer terrain hospitable only to king snakes, lizards and tortoises. Shade is almost nonexistent, and jagged rocks and barrel cactuses welcome visitors with stinging jabs to the feet and shins. Still, this is ideal habitat for bighorn sheep and mountain bikers. In the scrubland outside Palm Springs, the sheep are endangered but the bikers are flourishing. For mountain bikers, the more rugged and treacherous the terrain, the better.

Three days after the earthquake and tsunami devastated northeastern Japan, Gary Fuis walked across the San Andreas fault under a moonlit sky. The desert was quiet. A breeze fanned through the creosote. To the west, he could see the Salton Sea, and to the east, the headlamps of the night crew taking up their positions. In a little more than an hour, they would start detonating their explosives, generating seismic waves that would be recorded by seismometers buried throughout these sandy hills and positioned on the floor of the Salton Sea. A geophysicist with the United States Geological Survey, Fuis is overseeing an ambitious project to create an underground image of one of the most seismically active and geologically complex regions of the country, a triangle of land extending from Palm Springs to the Mexico border.

The "Big One" that has been forecast for the San Andreas fault could end up being bigger than earthquake experts previously thought. Recent research showing that a section of the fault is long overdue for a major earthquake has some scientists saying the southern portion of the fault is capable of a magnitude 8.1 earthquake that could run 340 miles from Monterey County to the Salton Sea. That's significantly stronger and longer than the...

Iwaited out Chile's recent earthquakes and highway-snapping aftershocks from deep inside the country, down along the serrated coast of Patagonia. From my remote reaches I didn't feel a thing. When I left from Santiago's airport a couple of weeks ago, I saw shattered windows boarded up with plywood. Most of the terminals had been moved outside into tents on the ramp beside one of the runways. I got back to the U.S. just in time for the fresh quakes ripping up the crotch of Baja. Is it just springtime in the Americas, earthquakes in bloom?

A predawn earthquake sent a sharp jolt across the Los Angeles area Tuesday, but the magnitude 4.4 temblor was barely strong enough to knock items off shelves. It was, however, sharp enough to frazzle residents, many of whom felt a "strong bang." The epicenter was 10 miles southeast of downtown Los Angeles in Pico Rivera, and the quake was felt as far away as San Diego and Ventura County. Los Angeles County fire official Ed Pickett, who was in East Los Angeles, said the jolt at 4:04 a.m. felt "like the building dropped."

For all the attention generated by the massive earthquakes in Haiti and Chile, experts in California remain skeptical that residents of this quake-prone region are any better prepared for the inevitable Big One. California saw a rise in quake awareness and retrofitting after the state recorded a series of major temblors over seven years: Whittier in 1987, Loma Prieta in 1989 and Northridge in 1994. But there hasn't been a devastating temblor in the state since the Northridge quake, and experts are concerned that quake preparedness may have declined in recent years.

The geological forces along the Pacific Ocean floor that were responsible for last Thursday's massive earthquake in Mexico are different from those at work in California along the San Andreas fault. And, scientists say, the occurrence of the Mexican quake, involving a different section of the Earth's crust, has not increased or decreased the potential for quakes in California.

This map, from a 1982 scenario prepared by the state Division of Mines and Geology, provides a hypothetical look at the degree of ground shaking and ground failure that might result from an 8.3 magnitude temblor along the San Andreas Fault. It would be equivalent to the greatest earthquake ever recorded in California. The scenario's authors chose the San Andreas Fault because it is the longest, most famous fault in the state.

The 2004 Sumatra earthquake that set off a deadly tsunami also seems to have caused more earthquakes along the San Andreas fault in the last few years, according to a study from UC Berkeley. The study analyzed 20 years of data in the Parkfield area, which sits on the fault, and found that the disastrous earthquake weakened the fault, changing both the frequency and strength of earthquakes in the area. "So you will have many earthquakes, but the magnitude will be smaller than expected," said Taka'aki Taira, a seismologist at UC Berkeley who headed the study.

By bouncing sound waves off the floor of the Salton Sea, researchers have discovered more than a dozen previously unknown earthquake faults, leading to a new theory of how the ground is sinking and stretching near the infamous San Andreas fault. Danny Brothers, lead author of a study published Sunday, said the new understanding of the area's seismic mechanics does not appear to suggest that a massive quake on the San Andreas is more imminent than previously believed.