San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]

Faults and Earthquakes in San Diego County
Thomas A. Deméré, Ph.D.; Curator of Paleontology

Like the rest of southern California, San Diego County has a number of active earthquake faults. These faults generally run in a northwest-southeast direction and are the product of crustal stresses associated with movement of the Pacific and North American lithospheric plates.

From east to west the major active faults consist of the San Jacinto, Elsinore, La Nacion, and Rose Canyon faults onshore and the Coronado Bank, San Diego Trough, and San Clemente faults offshore. Often the traces of these faults are marked by river valleys and canyons such as in the Lake Henshaw area where the Elsinore Fault passes along the northeast shore of the lake, or in Balboa Park where the small Florida Canyon Fault passes along the western slope of the canyon and beneath the parking lot of the Naval Hospital.

Since 1984 earthquake activity in San Diego County has doubled over that of the preceding 50 years. In modern times the strongest recorded quake (seismographs were not developed until 1934) in coastal San Diego County was the M5.3 temblor that occurred on 13 July 1986 on the Coronado Bank Fault, 25 miles offshore of Solana Beach.

Historic documents record that a very strong earthquake struck San Diego on 27 May 1862, damaging buildings in Old Town and opening up cracks in the earth near the San Diego River mouth. This destructive temblor was centered on either the Rose Canyon or Coronado Bank faults and descriptions of damage suggest that it had a magnitude of about 6.0.

In recent years there have been several earthquakes recorded within the Rose Canyon Fault Zone as it passes beneath the city. Three temblors shook the city on 17 June 1985 (M3.9, 4.0, 3.9) and a stronger quake occurred on 28 October 1986 (M4.7).

Ongoing field and laboratory studies suggest the following maximum likely magnitudes for local faults: San Jacinto (M6.4 to 7.3), Elsinore (M6.5 to 7.3), Rose Canyon (M6.2 to 7.0), La Nacion (M6.2 to 6.6), Coronado Bank (M6.0 to 7.7), San Diego Trough (M6.1 to 7.7), San Clemente (M6.6 to 7.7).

The Richter Scale - What do the Numbers Mean?

Individual earthquakes differ in strength. The Richter Scale was devised as a means of rating earthquake strength and is an indirect measure of seismic energy released. The scale is logarithmic with each one point increase corresponding to a 10 fold increase in the amplitude of the seismic shock waves generated by the earthquake. In terms of actual energy released, however, each one point increase on the Richter Scale corresponds to about a 32 fold increase in energy released. So a magnitude (M) 7 earthquake is 100 times (10 X 10) more powerful than a M5 earthquake and releases 1,024 times (32 X 32) the energy.

Seismologists (scientists who study earthquakes) use seismographs to determine the magnitude of earthquakes. The seismograph itself is a complex device that electronically amplifies seismic shock waves arriving from the earthquake event and records them on a seismogram.

An earthquake generates different types of seismic shock waves that travel outward from the focus or point of rupture on a fault. Since the focus is actually deep within the crust seismologists more often refer to the epicenter, which is the point on the earth's surface directly above the focus. Seismic waves that travel through the earth's crust are called body waves and are divided into primary (P) and secondary (S) waves. Because P waves move faster (1.7 times) than S waves they arrive at the seismograph first. By measuring the time delay between arrival of the P and S waves and knowing the distance to the epicenter, seismologists can compute the Richter Scale magnitude for the earthquake.

The Modified Mercalli Scale is another means for rating earthquakes, but one that attempts to quantify intensity of ground shaking. Intensity under this scale is a function of distance from the epicenter (the closer to the epicenter the greater the intensity), ground acceleration, duration of ground shaking, and degree of structural damage.

Plate Tectonics

The earth's crust is divided into seven major lithospheric plates. Powered by forces operating in the earth's molten interior, these plates are in constant slow motion. As they move they carry with them the continents and ocean basins. The edges or boundaries of the plates are where most tectonic action occurs.

New crust is generated along spreading boundaries such as the East Pacific Rise and the Mid-Atlantic Ridge. Crust is consumed at convergent boundaries such as occur along the west coast of South America or along the Pacific margin of the Aleutian Islands. The final type of plate boundary is called a transform boundary and occurs here in California as the San Andreas Fault. At transform boundaries the plates slide past one another.

Because of the huge scale and awesome forces of plate tectonics it is not surprising that most earthquake activity is concentrated along plate boundaries. This is especially true for convergent and transform boundaries.

Slippage Along Faults

Large or small, most earthquakes are caused by the slippage of masses of crustal rock along earth fractures called faults. Faults that have horizontal movement are called Strike-slip faults. Faults that primarily have vertical movement are called Dip-slip faults and come in two primary types. A normal dip-slip fault is caused by extensional (pull apart) tectonic forces and a reverse dip-slip fault is caused by compressional (pushing) tectonic forces.

The Northridge quake in January of 1994 was caused by a reverse dip-slip fault. Strike-slip faults are caused by horizontal shearing tectonic forces and result in the rocks on either side of the fault moving in opposite directions.

The infamous San Andreas Fault is a very large scale strike-slip fault. Most north-south faults in southern California are of this type, including the most active faults in San Diego County -- the San Jacinto, Elsinore, Rose Canyon, and Coronado Banks faults.

What Causes Earthquakes

Ultimately, all earthquakes are caused by movement of the earth's lithospheric plates and the plutonic forces that drive it. This movements cause seismic stress and strain to build up in the crust. Frictional forces constrain this stress and strain until a critical failure point is reached, beyond which the rocks rupture. This rupture releases the stress and strain as seismic energy which we feel as an earthquake.

Earthquakes can also be caused by volcanic eruptions as evidenced by the seismic activity that surrounded the Mount St. Helens eruption of 1981.

We humans can also cause earthquakes by underground detonation of nuclear bombs, injection of fluids into deep water wells, and over-extraction of water from aquifers.

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