The plates are slowly moving past one another at a couple of inches a year - about the same rate that your fingernails grow. But this is not a steady motion, it is the average motion. For years the plates will be locked with no movement at all as they push against one another. Suddenly the built-up strain breaks the rock along the fault, and the plates slip a few feet all at once. The breaking rock sends out waves in all directions, and it is the waves that we feel as earthquakes.
In San Bernardino and Los Angeles Counties, many of the roads along the fault cut through great mountains of gouge, the powdery, crumbled rock that has been pulverized by the moving plates. The hallmark of the San Andreas Fault is the different rocks on either side of it.
Being about 28 million years old, rocks from great distances have been juxtaposed against rocks from very different locations and origins. The Salinian block of granite in central and northern California originated in Southern California, and some even say northern Mexico. Pinnacles National Monument in Monterey County is only half of a volcanic complex, the other part being miles southeast in Los Angeles County and known as the Neenach Volcanics.
Offset Drainage: Aerial photo of the San Andreas Fault showing drainage that is offset by movement of the fault. There are many myths and legends about the San Andreas Fault, the biggest being that it will one day crack and California will slide into the sea. The San Andreas Fault is more accessible than any other fault in the world. The Pacific Plate on the west moves northwestward relative to the North American Plate on the east , causing earthquakes along the fault.
The San Andreas is the "master" fault of an intricate fault network that cuts through rocks of the California coastal region. The entire San Andreas fault system is more than miles long and extends to depths of at least 10 miles within the Earth. In detail, the fault is a complex zone of crushed and broken rock from a few hundred feet to a mile wide.
Many smaller faults branch from and join the San Andreas fault zone. Almost any road cut in the zone shows a myriad of small fractures, fault gouge pulverized rock , and a few solid pieces of rock.
Where Is It? The San Andreas fault forms a continuous narrow break in the Earth's crust that extends from northern California southward to Cajon Pass near San Bernardino. Southeastward from Cajon Pass several branching faults, including the San Jacinto and Banning faults, share the movement of the crustal plates. In this stretch of the fault zone, the name "San Andreas" generally is applied to the northeastern most branch.
What Surface Features Characterize It? Over much of its length, a linear trough reveals the presence of the San Andreas fault; from the air, the linear arrangement of lakes, bays, and valleys in this trough is striking. Viewed from the ground, however, the features are more subtle. On the ground, the fault can be recognized by carefully inspecting the landscape. The fault zone is marked by distinctive landforms that include long straight escarpments, narrow ridges, and small undrained ponds formed by the settling of small blocks within the zone.
Many stream channels characteristically jog sharply to the right where they cross the fault. Blocks on opposite sides of the San Andreas fault move horizontally. If a person stood on one side of the fault and looked across it, the block on the opposite side would appear to have moved to the right. Geologists refer to this type fault displacement as right-lateral strike-slip. During the earthquake in the San Francisco region, roads, fences, and rows of trees and bushes that crossed the fault were offset several yards, and the road across the head of Tomales Bay was offset almost 21 feet, the maximum offset recorded.
In each case, the ground west of the fault moved relatively northward. Sudden offset that initiates a great earthquake occurs on only one section of the fault at a time.
Total offset accumulates through time in an uneven fashion, primarily by movement on first one, and then another section of the fault. The sections that produce great earthquakes remain "locked" and quiet over a hundred or more years while strain builds up; then, in great lurches, the strain is released, producing great earthquakes. Other stretches of the fault, however, apparently accommodate movement more by constant creep than by sudden offsets that generate great earthquakes.
In historical times, these creeping sections have not generated earthquakes of the magnitude seen on the "locked" sections. Geologists believe that the total accumulated displacement from earthquakes and creep is at least miles along the San Andreas fault since it came into being about million years ago. On the west side of the fault sits most of California's population, riding the Pacific Plate northwest while the rest of North America inches south.
The Pacific Plate is moving to the northwest at 3 inches 8 centimeters each year, and the North American Plate is heading south at about 1 inch 2. Before then, another oceanic plate, the Farallon plate, was disappearing beneath North America at a subduction zone , another type of plate boundary.
The new configuration meant the two plates slid past one another instead of crashing into each other, a boundary called a strike-slip fault. Researchers have measured identical rocks offset by miles kilometers across either side of the fault. Geologists think the total amount of displacement along the fault is at least miles km since it formed. Geologists divide the fault into northern and southern segments, separated in the middle by a curiously quiet portion that "creeps.
The northern segment runs from Hollister north through the Point Reyes National Seashore, then eventually moves offshore. The southern segment stretches from Parkfield south through the Salton Sea. The central, creeping section includes everything from Parkfield to Hollister. In historical times, this creeping section has not generated powerful earthquakes similar to those on the "locked" sections. Post-earthquake studies by Gilbert again confirmed his reputation as a great geologist.
The San Francisco earthquake was the chief contributor to knowledge about the San Andreas, but now there are more questions than ever regarding the nature, geologic history, and significance of this important crustal structure. The present consensus about the role of the fault in local and global tectonics surely will be modified by revolutionary new conceptual models. Shibboleth Sign In. OpenAthens Sign In. Institutional Sign In. Sign In or Create an Account.
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