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The Really Big One

Northridge was mild compared to the Ft. Tejon earthquake of 1857. To understand what might come, look at then.

January 07, 2007|Susan E. Hough | Susan E. Hough is Scientist-in-Charge of the U.S. Geological Survey Pasadena office and author of "Richter's Scale: Measure of an Earthquake, Measure of a Man."

There are two kinds of people in Southern California: those who think they've experienced a major earthquake, and those of us who know we haven't.

If you were here Jan. 17, 1994, you probably put yourself in the first group. Many of us were jolted out of bed at 4:31 a.m. that day, and some of us experienced earthquake shaking that was about as bad as it gets. All of us recall the images: apartment buildings pancaked, freeway overpasses torn apart. The Northridge quake, an abrupt lurch across a patch of a fault some 10 miles square, caused 57 deaths. Those eight seconds were an emotional seismogram etched indelibly onto our central nervous systems.

So it offends sensibility that seismologists consider Northridge to be only a strong temblor, two steps below the dreaded magnitude 8 Big One. But here's the reality: A Big One on the southern San Andreas fault will be the result of a lurch along a swath of a fault. A long swath. What seismologists call the earthquake rupture will extend 10 to 15 miles deep and 200 to 250 miles long. Maybe more. It will be the size of 20 Northridge earthquakes laid end to end.

It will feel like the Ft. Tejon earthquake that struck 150 years ago, the Big One none of us remember.

On Jan. 9, 1857, the San Andreas fault unzipped from near the central California town of Parkfield down to Cajon Pass, a distance of nearly 250 miles. Stream channels crossing the fault reveal that, at least in some places, fault motion reached a staggering 30 feet, with an average sideways movement of about 15 feet. It blasted down the Carrizo Plain, past the modern mountain communities of Frazier Park and Gorman, along the edge of Palmdale, running out of steam only miles from San Bernardino. The earthquake appears to have proceeded from north to south, a freight train barreling down on San Bernardino at 6,000 mph. (Only in bad TV movies do earthquakes chase trains.) It rivaled the great 1906 San Francisco earthquake in magnitude, power and extent--every aspect that matters to a seismologist. But for Angelenos, it's a history that precedes us, a moment in time that does not define us.

Those in proximity to the fault 150 years ago recalled effects not seen in this part of the world in recent times. A letter to the editor of the Los Angeles Star described "immense trees . . . snapped off close to the ground" and of "every building between Fort Tejon and Lake Elizabeth leveled with the ground." The waters of Tulare Lake--at the time more than 500 square miles about 40 miles south of Fresno--were forced upward, leaving fish stranded on its shores. Estimates of the paroxysm's durationranged from one to several minutes. In the village of Los Angeles, one eyewitness said people "fled into the streets; many could not stand and in terror fell to their knees and cried out, 'Lord have mercy.' " Houses creaked and cracked; at least two homes collapsed in the San Fernando Valley, and many more were damaged.

One might be reasssured by the accounts. They seem to suggest that, although towns close to the San Andreas fault will be hard hit, the Greater Los Angeles region would be largely unscathed when the next huge quake strikes on the central San Andreas or on the more distant southern San Andreas, from Cajon Pass to the Salton Sea.

Seismologists aren't so sure. For one thing, 15 or 30 feet of lateral motion across the San Andreas will likely wrench apart many of the lifelines that Angelenos of 1857 couldn't have imagined but on which we depend, including power lines, aqueducts and major freeways.

And scattered among the accounts from 1857 are some that give a seismologist real pause: "The river was thrown out of its bed over the banks, and receded" . . . "only rarely do earthquakes last so long and have such strange motions" . . . "the motion of the earth resembled the long swell of the sea." With remarkable prescience one observer concluded: "The motions were long and lateral, instead of sudden, violent and vertical."

In 1857 seismology barely existed. Scientists didn't understand faults or their association with earthquakes; they didn't understand the nature of earthquake waves. Today there is still a lot we don't know, but some things we understand pretty well, including how the Los Angeles region will shake, rattle and roll when the next Big One hits the San Andreas. We know that the broad, flat expanse of Los Angeles is like the smooth top of the proverbial bowl of Jell-O, chilled and set over the ages, and that the bowl itself is made of the rock of surrounding mountains.

In 1985 the world saw what happened when waves from a distant earthquake reached another such bowl, the former lake-bed zone that underlies Mexico City. Armed with sophisticated theories about earthquake waves and powerful computers, seismologists can now set any bowl into motion with simulations. We can start to understand the nature of the long and lateral motions, and to find out what they will do to the types of buildings that weren't around in 1857.

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