San Andreas Fault
California's Big Fault
by Joe Zentner
In May of 1975 more than 37 years ago I drove from Malibu, California, to San Francisco. Shortly after pulling onto Interstate 5, I found myself on the Tejon Pass in Grapevine Canyon going through the Tehachapi Mountains. This was not the first highway pass I ever drove through. I think of northern New Mexico’s Raton Pass, along I-25, and Colorado’s Berthoud Pass, along U. S. 40. But the Tejon Pass was eerily different. I remember it clearly because this place seemed to me, even as a untrained scientist (“political science” doesn’t count), to exude a sense of ancient and modern seismic activity. Something about the fractured landscape felt weird, even surreal. This is, after all, part of the fabled San Andreas Fault
What Is It?
The San Andreas is the principal feature of an intricate network of faults that extends more than 600 miles, from northwest California to the Gulf of California. The faults mark the boundary between the Pacific and North American tectonic platestwo of the seven major shard-like fragments of the earth’s crust. The San Andreas Fault penetrates 20 miles deep, from the surface of the crust to the viscous (like melting plastic) rock material of the outer mantle. The coastal cities of San Diego and Santa Barbara actually lie on the Pacific Plate, not on the North American Plate, with most of the rest of the United States.
Total displacement the movement of one side of the fault line relative to the other side along the San Andreas since its formation more than 30 million years ago has been about 350 miles. Currently, the Pacific Plate is moving northwest relative to the North American Plate at approximately the same rate that your fingernails grow. If you never cut your fingernails and you lived to be 100 years old, your fingernails would grow to approximately ten and a half feet in length.
The Origin of the Name
A noted 19th century geologist, Andrew Lawson, gave the San Andreas (an-DRAY-us) Fault its name. In 1893, he had taken a steamer trip from San Diego to San Francisco, stopping off to examine coastal development at several places. He noted that geological features in San Benito and Santa Clara counties, as well as the San Francisco Bay valleys, were remarkably arranged in a line, and that the general uplift of the continental margin indicated geological movements “that may yet be active.”
Lawson named the fault in 1895, when he noted that the fault features were typified by a straight valley on the San Francisco Peninsula that was occupied in part by San Andreas Lake (named by Father Francisco Palou, a Spanish missionary, on Saint Andrew’s Day, November 30, 1774). The correct Spanish name would be San Andrés, but the American corruption, “San Andreas,” has become rooted in the literature. San Andreas Lake today is occupied by one of two reservoirs that are major water storage units for the city of San Francisco.
The Three Major Segments
The southern segment one of three major segments begins near the Salton Sea and runs northward before it begins a slow bend to the west, where it meets the San Bernardino Mountains. It runs along the southern base of those mountains, crosses the Tejon Pass, and continues northwest along the northern base of the San Gabriel Mountains. These mountains have been uplifted by the movement along the San Andreas Fault. They are known as the Transverse Range. After crossing through Frazier Park, California, this segment begins to bend northeast. Scientists suspect that it is this area where the fault “locks up” in Southern California as the tectonic plates strain mightily to move past one another. Northwest of Frazier Park, the fault runs through the Carrizo Plain, a long treeless region. Through here much of the fault is visible. The southern segment is the most analyzed geological feature in the world.
The central segment of the fault runs in a northwest direction from Parkfield to Hollister. While the southern section of the fault and the parts around Parkfield do experience earthquakes, the rest of the central section exhibits a phenomenon called “aseismic creep.” This geological process results in a fault being able to move laterally without the need for earthquakes to release seismic pressure.
The northern segment of the San Andreas Fault runs from Hollister through the San Francisco Peninsula, where it briefly goes offshore, then follows the coast of California before making a sharp turn west and going offshore near Eureka.
Over much of its length, a linear trough, or long, narrow depression, reveals the presence of the fault. Viewed from the air, the linear arrangement of lakes, bays and valleys in this trough is pronounced. Viewed from the ground, however, the land features are subtler. For example, many people driving by Crystal Springs Reservoir, near San Francisco, or along Tomales Bay may not even realize they are within the San Andreas Fault zone.
On the ground, the fault can be recognized by carefully inspecting the landscape. The fault zone is marked by distinctive landforms which include escarpments (steep slopes or long cliffs that separate two relatively level areas of differing elevations), narrow ridges, and small, undrained ponds. Many stream channels jog sharply to the right where they cross the San Andreas Fault.
Why Earthquakes Occur
Faulting and the consequent earthquakes arise from shifting in the planet’s structure. The earth is separated into three layers: the crust, the mantle and the core. The crust is a jigsaw puzzle of 60-mile-thick plates of solid rock floating on the earth’s upper mantle. Continents sit on major plates that continue to slowly drift, collide and reshape the face of the globe. The grinding of one plate against another forms a fault zone.
These plates are continually being rearranged by stresses occurring deep within the earth. The ground first bends, then, upon reaching a certain limit, breaks and “snaps” into a new position. In the process of breaking, that is, faulting, the earth “quakes,” or vibrates.
The vibrations are of two types: compression waves and transverse or shear waves. Since compression waves, called “P” waves, travel faster through the earth, they arrive first at a distant point. The transverse or shear waves, called “S” waves, travel more slowly and therefore arrive later.
When an earthquake occurs, people may first notice a sharp thud, or blast-like shock; this marks the arrival of the P wave. A few seconds later, they may feel a swaying or rolling motion that marks the arrival of the S waves.
Earthquakes along the San Andreas Fault
In 1836 and 1838, violent earthquakes occurred in the San Francisco Bay area. The latter is described as having opened a great crack many miles in length. References to its location would fit fairly well for a part of the San Andreas Fault.
In 1857, the massive Fort Tejon earthquake rocked the ground violently and ruptured the San Andreas Fault for 225 miles, from northwest of Parkfield, California, to San Bernardino. Because the region was virtually uninhabited at the time, it killed only two people and destroyed only the Tejon Army post, located midway along the affected section of the fault.
On October 21, 1868, another earthquake caused extensive damage in San Francisco, particularly on filled ground. It caused the most destruction in the vicinity of San Jose and Hayward, directly on the fault line. It was locally referred to as “the great earthquake,” at least, until 1906.
April 18, 1906, 5:12 a.m.
It came on April 18, 1906, at precisely 5:12 in the morning. What would become known as the California Earthquake, with the epicenter near San Francisco, would rank as one of the most destructive earthquakes in recorded history. The P wave hit with sufficient force to be widely felt throughout the San Francisco Bay area. All hell broke loose 22 seconds later. Violent shocks punctuated strong shaking, which lasted for approximately 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada.
Most of San Francisco’s population was asleep at the time. People were awakened by the most violent shaking imaginable “like a terrier shaking a rat” and by the roar caused by the writhing and collapsing of buildings. Heard above everything else was the clanging of church bells as belfries swayed.
Some people, however, never heard the clanging. They died almost immediately in the collapse of their own homes, some crushed in bed by chimneys crashing through roofs.
Some 400 miles of the San Andreas Fault slipped, from San Juan Bautista to Cape Mendocino. Starting near the Golden Gate, the fault unzipped the earth’s crust at a rate of 500 miles per hour. The northern part of the fault slipped much more than the southern part. Land surfaces on either side of the fault were displaced horizontally up to 21 feet.
Figures at the time put the death toll at 700 to 800 people, although later estimates put the figure closer to 3000 deaths. Where the earth actually split open, in what is today Point Reyes National Seashore, there was only one casualty a cow named Matilda who, according to the official earthquake commission report at the time, “fell headfirst into the fault crack just before the earth closed in on her, so that only her tail remained visible.”
The 1906 earthquake is perhaps best remembered for the fire it spawned in San Francisco, giving the total phenomenon the somewhat misleading appellation of the “San Francisco Earthquake.” Shaking damage, however, was incredibly severe in many other places along the fault rupture, inflicting destruction apart from the ensuring fires.
Another disastrous event that resulted from seismic activity along the San Andreas Fault was the Los Angeles earthquake of 1994, in which 51 people died.
When will the next large earthquake strike along the San Andreas Fault? No one can say for sure. Geologic studies indicate that over the past 1500 years, large earthquakes have occurred at 150-year intervals on the southern San Andreas Fault. Since the last large earthquake in the southern region occurred in 1857, that section of the fault is considered a likely location for a major earthquake in the near future. The San Francisco Bay area has a slightly lower potential for a great earthquake, since “only” 100 years have passed since the great 1906 quake. However, moderate-sized, potentially lethal earthquakes could occur here at any time.
Since the North American and Pacific plates continue to grind against each other each year, as they have for millions of years, major earthquakes are inevitable. Energy is continually being stored under our feet, and we should be prepared for the results of its inevitable release. Long-term research on the San Andreas Fault indicates that parts of the fault may trigger a major earthquake sooner than previously believed, including the section near Palm Springs and the San Bernardino-Riverside areas.
What To See
You will discover that the Los Trancos Open Space Preserve in the Point Reyes National Seashore is an ideal spot to learn about earthquake geology. The San Andreas Fault splits the preserve. The preserve was built to inform visitors, with the hope that the more people know about their geologic environment, the better they will be able to protect themselves and their property against damage from future earthquakes.
Visitors to southern California who are curious to see where much of the shaking and shifting comes from will enjoy a three-hour coach tour to places where the San Andreas Fault is starkly visible on the desert floor.
The San Andreas Fault is one of the most fascinating geological features in the world. It has been and still is a major concern for inhabitants who live anywhere near it. It has gained widespread attention, among laypersons as well as scientists. For seismologists around the world, few geologic features on earth have greater significance. To observe visible signs of the San Andreas Fault along the desert floor is an educational, emotional, enlightening and unsettling experience.
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