Essay: San Francisco 1906 Earthquake

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One of the greatest earthquakes happened in 20th century is in San Francisco in year 1906. It measured 7.8 degrees.  Many building were destructed and several hundred of people got killed.  The earthquake also started a fire, which destroyed the central business district.
 
The earthquake happened on the San Andreas Fault, which is the major fracture of the Earth’s crust.  It is trending northwestward through southern and northern California, U.S., for 650 miles (1,050 km) and passing seaward in the vicinity of San Francisco.  Movement along this transform fault is of the strike-slip type and is characterized by occasional large earthquakes originating near the surface along the path of the fault. The disastrous San Francisco quake of 1906 and the less serious earthquake of 1989 were both caused by movement along the fault.  According to the theory of plate tectonics, the San Andreas results from the abutment of two major plates of the Earth’s crust, the Northern Pacific and the North American.  Along the fault, the Northern Pacific plate is sliding past the North American plate in a northerly direction, at a relative movement of about 1 cm (0.4 inch) per year over geologic time, though the rate of movement has been 4 to 6 cm (1.6 to 2.4 inches) per year over the past century. Parts of the fault line moved as much as 6.4 m (21 feet) during the 1906 earthquake.  The San Francisco Bay Area Rapid Transit System (BART) bored a tunnel right through the fault zone, and various cities, towns, and housing developments lie on or near it.
 
At almost precisely 5:12 a.m., local time, a foreshock occurred with sufficient force to be felt widely throughout the San Francisco Bay area. The great earthquake broke loose some 20 to 25 seconds later, with an epicenter near San Francisco. Violent shocks punctuated the strong shaking which lasted some 45 to 60 seconds. The earthquake was felt from southern Oregon to south of Los Angeles and inland as far as central Nevada. The highest Modified Mercalli Intensities (MMI’s) of VII to IX paralleled the length of the rupture, extending as far as 80 kilometers inland from the fault trace. The earthquake was rupturing the northernmost 430 kilometers of the San Andreas Fault from northwest of San Juan Bautista to the triple junction at Cape Mendocino.  It formed a 290 miles long crack on the ground.  For comparison, the 1989 earthquake only had a rupture length of about 25 miles.  There is also about 20 feet offset along the crack.  The ground was moving at 2.7 km/s.
 
The result of the earthquake was tremendous.  More than 800 people died, 225,000 from population of 400,000 became homeless.  28,000 buildings were destroyed.  More than 400 million monetary losses.
 
The 1906 earthquake marked the dawn of modern scientific study of the San Andreas Fault system in California.  Before 1906, earthquake research in the U.S. had advanced slowly compared to efforts in Japan and Europe.  After the quake, nearly all scientists in California began to assemble observations of the earthquake and its effects.  The report (published in 1908) was an exhaustive compilation of detailed reports from more than twenty contributing scientists on the earthquake’s damage, the movement on the San Andreas Fault, the seismograph records of the earthquake from around the world, and the underlying geology in northern California.
 
The 1906 earthquake essentially turned off earthquakes of magnitude about 6 and larger for the next 73 years (with one exception in 1911). In the 70 years before, there had been at least 16 earthquakes of this magnitude, as shown in the diagram.  The likely explanation for this period of seismic quiet is that slip on the 1906 fault plane redistributed stress on other San Francisco Bay area faults.  The important lesson here is that for most of this century, central California has been experiencing a seismically quiet period caused by stress relaxation after 1906. The region may slowly be recovering from this “stress shadow” to a more normal state of seismicity as the tectonic plates continue to move, and the stresses on the major faults recover to the values that they had in 1905.
 
The “Great 1906 San Francisco Earthquake” is one of the strongest ever recorded on the North American continent. If a similar earthquake occurred in Northern California today, after many decades of rapid urban growth, thousands of people would likely be killed and economic losses might be in the hundreds of billions of dollars. Such an event would easily be the worst natural disaster in the Nation’s history.  How soon is such an event likely to happen? Recent research offers some answers by providing new insights into the 1906 quake and the San Andreas Fault system.  The scientists predict that another shock as powerful as the 1906 earthquake is not likely to strike Northern California soon, probably not for at least 100 years.
 
By studying quakes like the Great 1906 San Francisco Earthquake, scientists and engineers gain the knowledge and understanding necessary to assess the risk from future shocks and to reduce the vulnerability of buildings and other structures to damage in these inevitable and terrifying events. In this way they help to protect the lives and property of the people.

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