Massive Pressure Beneath the Earth Moved Mountains : Seismology: Long dormant Sierra Madre Fault buckled under tremendous stress related to the movement of two giant plates.

The earthquake that jolted Southern California on Friday morning boosted the San Gabriel Mountains up a couple of inches and demonstrated once again just how some areas of the Los Angeles Basin are more vulnerable to quakes than others.

In general, people closest to the quake felt it the most. Even at considerable distances, the quake caused extensive shaking because of variations in geology throughout the basin. Some structures fared better than others partly because they were farther away from the quake’s epicenter, and partly because they may have been on firmer ground.

As in any earthquake, the engineering quality of the homes and buildings of Southern California also made a difference. Earthquakes of a similar magnitude in other parts of the world have killed thousands.

Indeed, had it happened a few miles south, the consequences would have been far greater. Fortunately, the quake’s epicenter was 7.5 miles northeast of Sierra Madre, and that allowed much of the energy released by the quake to dissipate before reaching heavily populated areas, scientists said.

“If it had occurred under one of the cities in the San Gabriel Valley, we would have seen much more widespread damage,” said Egill Hauksson, a Caltech seismologist.

The quake also ended an “abnormally low level of seismic activity in Southern California” over the past year, he said. Many aftershocks are expected, but scientists said the chance that this magnitude 6.0 quake is a precursor to a larger quake is very unlikely.

Quakes such as Friday’s temblor are part of the price people must pay if they live in a geologically active region such as Southern California. It could also be argued that quakes make the area desirable because they are part of the process that created the mountains and valleys that give the region much of its character.

The quake struck on a long-dormant segment of the Sierra Madre Fault that runs along the foothills from Sylmar to Upland. That fault is part of a much larger system of faults that define the margin between two giant chunks of the Earth’s crust called tectonic plates.

The dominant feature of the margin between those plates is the San Andreas Fault, a sort of geophysical godfather that calls the shots for nearly the entire coast of California. Little happens here that is not influenced by that legendary rupture, which runs from somewhere near the border with Mexico to off the coast of Mendocino in Northern California.

The San Andreas was created by the movement of the Pacific Plate, the giant mass of sea floor that underlies nearly the entire Pacific Ocean, against the North American Plate, which underlies the continent. The Pacific Plate, driven partly by convection from deep within the Earth’s molten mantle, moves northwestward along the edge of the North American Plate, which is moving southeast.

The two plates slide horizontally along the margin, sometimes in fits and starts when they snag and then move forward through an earthquake. The San Andreas, which was created by the movement of those two plates about 20 million years ago, is a bit uncooperative in a way that has produced numerous mountain ranges.

Because the San Andreas bends near Parkfield in Central California, the plates cannot slide smoothly past each other. Instead, the Pacific Plate is driving into the North American Plate, like a fist into a glove, rather than sliding smoothly past.

“There’s the wrinkle,” said seismologist Lucile Jones of the U.S. Geological Survey’s Pasadena office.

It might seem reasonable to assume that strain in the Earth’s crust would run parallel to the fault, in this case, southeast to northwest, but scientists have found that is not the case. Instead, the bend in the San Andreas has created a complex maze of stress patterns because the Pacific Plate is driving into the North American Plate.

Strain measurements in the mountains east of Los Angeles show that huge chunks of the Earth’s crust on opposite sides of the San Andreas are pushing against each other rather than sliding sideways along the fault.

That creates what geophysicists call “compressional tension.” As the two chunks push against each other, something has to give. Horizontal movement alone will not remove the strain, so one block is forced to ride up over its neighbor.

That is what happened at 7:43 a.m. Friday.

The Sierra Madre Fault is a thrust fault, meaning that it dips into the crust--sort of like a very steep freeway on-ramp--at an angle of about 45 degrees. That segment of the fault has not been active for many, many decades, and strain had been building up along the fault all that time.

When the strain exceeded the fault’s ability to remain locked, the Earth trembled. The two chunks slid about three feet along the fault, and one side pushed up the “on-ramp” enough to raise the mountains about two or three inches, Caltech’s Hauksson said.

That made the San Gabriels a little taller in a process that has been going on for several million years.

It also made a lot of folks very uncomfortable.

One of those was Leon Knopoff, a geophysicist at UCLA.

With some trepidation, Knopoff and a colleague published an analysis several years ago suggesting that the alignment of the Earth, sun and moon might trigger some earthquakes. Whenever there is a full moon, or a new moon, the three bodies are aligned, and thus the gravitational influence of the sun and moon may be higher than normal.

Many scientists have wondered whether that gravitational tug could somehow pull faults apart, thus triggering earthquakes. In his research, Knopoff found that some large quakes on particular faults in Southern California did seem to occur more frequently when the moon was either new or full.

The findings are far from convincing to many scientists, and even Knopoff remains skeptical. Furthermore, because the theory sounds a little too much like astrology, Knopoff has tried to make sure his colleagues do not think he believes he can do what many experts believe no one can do--predict earthquakes.

Friday’s quake hit two days after a full moon, early in the day when the sun and the moon were tugging on the opposite sides of the Sierra Madre Fault.

“It doesn’t mean anything,” Knopoff said shortly after the quake struck. “It’s just another digit in the statistics.”

Chances are, he has not heard the last of it.

The Quake Zone: How It Happens

The magnitude 6 earthquake that ruptured along the Sierra Madre Fault on Friday ended a year of seismic quiescence in Southern California. The Sierra Madre Fault, which runs from Sylmar in the west to Upland in the east, is a bit player in a drama that has been unfolding here for millions of years.

Like hundreds of other California faults, the Sierra Madre was created by enormous forces caused by two giant tectonic plates grinding against each other along nearly the entire coast of California

The rubbing of two plates compresses the land, creating stresses in the crust that can only be relieved when one large block of land moves up and over its neighbor. The mechanism for that is a thrust fault, such as the Sierra Madre, that dips down into the Earth at a 45-degree angle.

During Friday’s earthquake, one large block slid up the fault, relieving the pressure and raising the height of the San Gabriel Mountains by about 2 inches.