The History Of The San Andreas Fault Zone

Earthquakes like the San Francisco temblor of 1906 cause economic and personal hardship.

Understanding the history of the San Andreas Fault zone is vital to creating a clear picture of the geological history of California. By studying the fault zone, seismologists can determine earthquake risks and engineers and urban planners can implement safety planning for future earthquakes. Builders can reduce fatalities by creating and constructing buildings that will better withstand earthquakes.

San Andreas Fault Is Old and Long

The San Andreas Fault runs approximately 800 miles through California. Twenty million years ago, changes in plate configuration between the North American and Pacific plates produced the San Andreas Fault. In 1895, geologist A.C. Lawson, from the University of California at Berkeley, named the fault for San Andreas Lake, which is now one of the two reservoirs that provide water for San Francisco, according to the U.S. Geological Survey. After the 1906 San Francisco earthquake, Lawson discovered that the San Andreas Fault stretched into Southern California.

Newspapers Record Earthquakes

Early records that describe the San Andreas Fault are scarce. Before the 1848 gold rush, Franciscan missions that operated from 1769 until the 1830s provided scattered accounts of earthquake records. Documented earthquake sources from the Mexican period, the early 1830s to 1846, are also scarce. The gold rush eventually brought newspapers to California, and earthquake events began to be recorded. Newspaper accounts still play an important part in tracing San Andreas Fault history.

Fort Tejon Shakes

Two major earthquakes have occurred along the San Andreas Fault in recorded history, the 1857 Fort Tejon earthquake and the 1906 San Francisco earthquake. At Fort Tejon, located about 75 miles northwest of Bakersfield, Calif., buildings were damaged and only a few people were injured. Buildings in Los Angeles were cracked but sustained no major damage.The Southern California Earthquake Research Center reports that there were few fatalities because the area was sparsely populated.

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San Francisco Trembles

The San Francisco earthquake of 1906 rocked the coast of Northern California on Wednesday, April 18, 1906, measuring a magnitude of 7.8 on the Richter Scale of 10. Residents of Los Angeles and people as far away as Oregon and Nevada felt the temblor. The United States Geological Survey puts the death toll at 700. However, Gladys Hansen, now the curator at the Museum of the City of San Francisco, says the actual death toll from the quake and subsequent fire is closer to 3,000, making it one of the deadliest natural disasters in U.S. history.

John Milne Introduces Seismographs

In 1896, John Milne introduced instruments capable of systematically detecting California and Nevada earthquakes. Seismographs from these instruments and their successors provided earthquake information. Seismographic instruments began to be used extensively at the beginning of the 20th century. In 1926, the Wood-Anderson seismograph appeared, and in 1935 Charles F. Richter of the California Institute of Technology invented the Richter mathematical scale to measure earthquake magnitude.

Seismologists Predict Earthquakes

In recent years, seismologists have studied the San Andres Fault with the goal of discovering the history of the earth’s movement along the fault. They have documented major earthquakes other than the Fort Tejon and San Francisco temblors by examining soil layers using radiometric dating. Estimating the frequency and magnitude of these quakes helps seismologists define when the next earthquake will occur, which they say is already overdue.