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Exploring the beautiful nature of California


California Nature: Earthquakes


An earthquake is a trembling or shaking of the ground caused by the sudden release of energy stored in the rocks below the surface, radiating from a fault along which movement has just taken place. Earthquakes can occur at any time of the year and at any time of the day or night. Earthquakes occur under all weather conditions, sunny, wet, hot, or cold--without special tendency. The Earth’s outer shell is broken into large, brittle pieces called lithospheric plates. The thickness and composition of a lithospheric plate varies, depending upon whether the plate is made of oceanic or continental material. Forces in Earth, such as earthquakes and volcanoes, occur along the boundaries of these lithospheric plates. The movement of lithospheric plates causes forces that build up energy in rocks. Some of this energy is released as earthquakes. Earthquakes occur every day. Many are too small to be felt by humans, but each event tells scientists something more about Earth.

California’s landscape is different now compared to how it was in the past. Movements of plates have changed California dramatically. Seas have disappeared. While old mountains have eroded away, new mountains have been uplifted. Plate boundaries have formed and then disappeared. Today, a continental transform plate boundary cuts across the state. At the northern end of California, a convergent plate boundary sits offshore. This active plate tectonic setting produces earthquakes, volcanoes, and mountains.
ithospheric plates.are the cause of earthquakes in California Most of California is situated on the North American Plate. A small part of California, west of the San Andreas Fault, lies on the adjacent Pacific Plate. The Pacific Plate moves northwest, relative to the North American Plate, at a velocity of about 3.4 cm per year. The plate does not slide smoothly, but sticks much of the time and moves in jerks. Each time a jerky movement occurs, an earthquake happens.

The San Andreas Fault is a transform plate boundary that is located between the North American Plate and the Pacific Plate. Because the San Andreas Fault is a transform plate boundary, it is also a strike-slip fault, extending all the way from Cape Mendocino in the north to the Salton Sea in the south. The San Andreas Fault is not a straight line. Where there are bends in the fault, blocks of rock get pushed up or drop down, making mountains or basins. The Transverse Ranges and the Coastal Ranges of California have been pushed up as the Pacific Plate moves past the North American Plate. The Los Angeles Basin, the Venture Basin, and the San Francisco Bay are all blocks of rock that have dropped down.
Over time, these faults produce about half of the significant earthquakes of our region, as well as many minor earthquakes. 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. Earthquakes can occur almost everywhere in California, 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 below). 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. These faults produce thousands of small earthquakes each year, and the other half of our significant earthquakes.
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 The largest recorded earthquake in California was the 1857 Fort Tejon earthquake, with an estimated magnitude of 8.0. This earthquake ruptured the San Andreas Fault from Parkfield to Wrightwood, a distance of 225 miles

The most destructive earthquake to date was the 7.9 magnitude 1906 San Francisco earthquake, in which over 3000 people perished in the earthquake and the fires that followed. The 1906 quake ruptured the northern segment of the San Andreas Fault for 296 miles, from San Juan Bautista to near Cape Mendocino in the north.

More recently, the 1989 Loma Prieta earthquake, which affected the San Francisco bay area, and the 1994 Northridge earthquake which hit the Greater Los Angeles area, caused widespread damage and deaths in their respective regions.
Scientists can estimate the directions and speeds that the plates are moving. They can predict future plate boundary interactions. For example, the small part of California that is on the Pacific Plate, including Los Angeles, will continue moving northwest along the coast relative to the North American Plate. This means that Los Angeles and San Francisco are approaching each other about as fast as your fingernail grows.

Earthquakes produce seismic waves that pass through the planet. The speed and direction of the seismic waves change when the properties of the Earth materials they are traveling through change. The waves bounce off or bend as they approach a new layer. Scientists have learned about the details of Earth’s internal layering by analyzing the paths of these waves.
 
 
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