why do we have earthquakes in california

We know that the San Andreas fault produces large earthquakes and that many other faults are also hazardous. However, it is often difficult to understand how to incorporate this information into our lives. Should we care only if we live near the San Andreas fault? Is every place just as dangerous? This section describes where earthquakes have occurred in the past and where they may likely occur in the future, how the ground will shake when they do, and what may happen in a plausible "big one" on the San Andreas. The earthquakes of California are caused by the movement of huge blocks of the earth's crust- the Pacific and North American plates. The Pacific plate is moving northwest, scraping horizontally past North America at a rate of about 50 millimeters (2 inches) per year. About two-thirds of this movement occurs on the San Andreas fault and some parallel faults- -- the San Jacinto, Elsinore, and Imperial faults (see map). Over time, these faults produce about half of the significant earthquakes of our region, as well as many minor earthquakes. The last significant earthquake on the Southern California stretch of the San Andreas fault was in
1857, and there has not been a rupture of the fault along its southern end from San Bernardino to the
Salton Sea since 1690.


It is still storing energy for some future earthquake. But we don't need to wait for a "big one" to experience earthquakes. Southern California has
thousands of smaller earthquakes every year. A few may cause damage, but most are not even felt. And most of these are not on the major faults listed above. The earthquake map above shows
that earthquakes can occur almost everywhere in the region, on more than 300 additional faults that
can cause damaging earthquakes, and countless other small faults. This is mostly due to the "big bend" of the San Andreas fault, from the southern end of the San
Joaquin Valley to the eastern end of the San Bernardino mountains (see diagram at right). Where the
fault bends, the Pacific and North American plates push into each other, compressing the earth's crust
into the mountains of Southern California and creating hundreds of additional faults (many more than shown in the fault map). These faults produce thousands of small earthquakes each year, and the
other half of our significant earthquakes. Examples include the 1994 Northridge and 1987 Whittier Narrows earthquakes.
Categories: October 3, 2008 In the same way as earthquakes are neither evenly nor randomly distributed throughout the world, California also has a few earthquake zones as well as vast areas which are essentially void of all Earth's tremors.


There are actually distinct bands and clusters of seismicity in our state which can be clearly spotted on a map of earthquakes. The most famous zone of all is, of course, the San Andreas Fault. It snakes almost all the way through the Golden State, from the Salton Sea in the Imperial Valley in the south, to Cape Mendocino in the north. At a first glance it looks as if earthquake foci line up along this zone like pearls on a string. But looking a little bit more closely one finds that the San Andreas Fault is not just one clear thin line but a zone of tectonic movement, which can be up to several dozen miles wide. Sometimes the zone consists of several faults, which parallel each other. The picture gets murkier in the Los Angeles Basin. The reason is that the crust under LA is cracking along dozens of short fault segments, many of them not yet even named by seismologists. The ultimate cause for the earthquakes along the San Andreas system and in the LA basin is the sliding of the Pacific Plate against the North American Plate with a velocity of about 2. 5 inches per year.


Where the San Andreas Fault ends in the north, the seismicity fans out into the Pacific Ocean like an elephant's trunk to the west of Cape Mendocino. There the "Mendocino Fracture Zone" takes over the steady slide of the tectonic plates. Other clear bands of seismicity occur along the Garlock Fault in Southern California's Transverse Ranges, along its continuation through the Owens Valley, and further north along the steep eastern flank of the Sierra Nevada. The earthquakes there are not caused only by the sliding of the plates, but are also a consequence of the slow lifting of the Sierra Nevada, which has occured during the last several million years. And last but not least, there is a third class of earthquakes in California. These temblors can be found in clusters around the Geysers along the Sonoma and Lake County border, around Mammoth Lakes east of Yosemite, and to a smaller extent around the Coso Field near Ridgecrest. The causes of these mostly small quakes are remnants of volcanic and geothermal activity, like the restless Long Valley caldera next to Mammoth, which blew up in a gigantic volcanic eruption 760,000 years ago. (hra007)

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