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SCV: The San Andreas Fault is the most studied earthquake fault in the world and it’s our neighbor

Posted: February 6, 2011 1:55 a.m.
Updated: February 6, 2011 1:55 a.m.

A sign indicates a geologic transition point near Parkfield.

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Editor’s note: The Northridge Earthquake of Jan. 17, 1994, devastated portions of the Santa Clarita Valley. The 6.7 earthquake was caused by a previously unknown fault. How many of us in the Santa Clarita Valley are really aware of the sleeping giant that lies nearly beneath our feet? The San Andreas Fault runs nearly through the SCV. The area known as the “Big Bend” — near Fraizer Park — has caused earthquakes with a magnitude of 6.5 or greater at intervals between 45 and 144 years. Have you updated your earthquake kit?

The Santa Clarita Valley exists in  a dynamically changing planet, though from the perspective of a human lifespan, the geological world around us seems relatively static. Despite appearances, we are surrounded by geological features that are in a constant state of change.

The Earth’s crust is fractured into a series of massive “plates” that have been moving slowly over the Earth’s surface for millions of years.

The Pacific Plate is the largest of these tectonic plates. Volcanism in an area under the ocean called the East Pacific Rise is pushing the Pacific Plate slowly northward. In conveyor belt fashion, the other side of the Pacific Plate is diving under the North American Plate at the Aleutian Trench south of Alaska.

Here in California, the Pacific Plate is slowly sliding northwestward as it grinds past the continental North American Plate. At the same time, the continental North American Plate is moving southwest.

Over the course of geological time, the movement and interaction of these massive tectonic plates produces compressive forces, which cause a variety of geological features, including the uplifting of the coastal ranges.

A fault is a break or fracture in the Earth’s crust caused by the movement of land masses on either side of it.

In California, an especially long fault known as the San Andreas Fault is the boundary between the Pacific Plate and the North American Plate.

The San Andreas is called a transform fault because the tectonic plates slide past each other horizontally, rather than the alternative, where one plate dives under the other.

The San Andreas Fault is one of the longest transform-fault boundaries in the world.

The fault was named in 1895 by geologist A.C. Lawson. He named it after San Andreas Lake, which lies on the fault about 20 miles south of San Francisco.

It was originally thought to be a smallish, local fault, but after the historic 1906 San Francisco earthquake, it was realized that the fault really extended for hundreds of miles in both directions.

The San Andreas Fault lies entirely within California. It is approximately 810 miles long, extending northwest from Imperial County in southern California (Salton Sea) to a place near Cape Mendocino where it bends offshore under the Pacific Ocean.

This creeping motion of the Pacific Plate and the North America Plate can be observed in sections of the San Andreas Fault near the small town of Parkfield in central California. There is an area 8 miles north of Parkfield that routinely experiences extremely regular, yet small earthquakes every few weeks or so (M3 or less).

The rupture points causing these small earthquakes are relatively close to the surface at a depth of 2 to 3 miles. As a result of this activity and its close proximity to the surface, Parkfield has become one of the most popular places in the world to study earthquakes.

The San Andreas Fault is currently the most studied fault in the world.

In 2004, work began on the San Andreas Fault Observatory at Depth (SAFOD). The project is funded by the National Science Foundation in conjunction with the U.S. Geological Survey and NASA. Completed in 2007, they were successful in drilling holes into the San Andreas Fault, the deepest reaching nearly 2 miles below the surface. Arrays of sensors were installed to capture and record earthquakes that occur in this extremely active region.

Seismologists also observed that this section of the fault near Parkfield consistently produced larger magnitude 6.0 earthquakes about every 22 years. Following earthquakes recorded in 1857, 1881, 1901, 1922, 1934 and 1966, scientists predicted an earthquake would hit the Parkfield area in 1993.

This quake eventually struck much later — in 2004 — proving earthquakes are nearly impossible to predict with current technology.

This creeping motion of one plate past the other does not occur everywhere, as it does at Parkfield.

In other sections of the San Andreas Fault the rocks can become locked together. Since the massive plates do not stop moving, this means the stresses build up until the jammed rocks eventually break, releasing the accumulated energy very quickly. The result is what we experience as an earthquake.

Two major earthquakes have occurred along the San Andreas Fault in recorded history — the 1857 Fort Tejon earthquake, and the slightly smaller, although better known, San Francisco earthquake in 1906.

It has been 154 years since the last major earthquake on the southern section of the San Andreas Fault in southern California.

The occurrence of large earthquakes before records were kept can be determined by a detailed examination of rock and soil layers along the fault, in combination with various dating techniques.

A recent study by geologists at the University of California Irvine and Arizona State University concluded that the interval between large earthquakes occurring along the San Andreas Fault may be more frequent than researchers had previously thought.

This study, published in August 2010, shows that large earthquakes of magnitude 6.5 or greater have occurred on the “Big Bend” segment of the San Andreas Fault at intervals between 45 and 144 years.

Their conclusions were based on finding evidence of six earthquakes impacting the Carrizo Plain area since 1360.
Prior to this study, the interval was thought to be between 130 and 185 years.

The Big Bend is an approximately 100-mile section of the fault about 35 miles to the north of the Santa Clarita Valley.

The Big Bend is so named because the San Andreas Fault changes direction by over 30 degrees. It starts to bend near Frazier Park and curves towards the southern Carrizo Plain area where it straightens and continues northwest towards San Francisco.

The Big Bend is thought to hinder the movement of one plate sliding past the other, so the rocks become strongly locked together.

This causes the buildup of ever increasing stresses in the earth’s crust. When these stresses eventually reach a breaking point, they are likely to result in relatively large-scale earthquakes (M6.5-M8).

The largest earthquakes on the San Andreas Fault are usually caused by very deep ruptures, perhaps 8 to 15 miles below the surface.

The location of the epicenter of the massive 1857 earthquake (M7.9) is debated by experts. Many think the earthquake originated in the Big Bend area, deep beneath the Carrizo Plain. If this is true, then The Big Bend has produced the largest earthquake in California in recorded history (records started in 1769 with the Spanish).

The rate of slippage along the San Andreas Fault averages approximately 33 to 37 millimeters (1.3 to 1.5 inches) every year.

As a point of comparison, this rate closely matches the average rate of growth of a human fingernail. One way to determine the rate of movement along the fault is by measuring the offset of “tie points”. Tie points are distinctive geologic features with a clearly defined geological age that have subsequently been offset by movement on either side of the fault.

For example, there are volcanic intrusions that can be seen next to Old Post Road that parallels the I-5 near Gorman.

These are known collectively as the Neenach Volcanic Formations, dated as approximately 23.5 million years old.

The San Andreas Fault began to form about 22 million years ago, shortly after the Neenach volcanics erupted. When the San Andreas Fault cut through this area, the western part of the Neenach Volcanics was carried north-westward on the Pacific Plate. Now the corresponding part of these formations can be found in the Pinnacles National Monument 195 miles away.

Assuming the current plate movement continues unchanged, the landmass west of the San Andreas Fault, including Los Angeles, will eventually slide past San Francisco over a period of perhaps 16 million years. Maybe even more quickly than this, as the rate of plate movement appears to be increasing over geological time. Today, the average movement of about 1.4 inches a year is almost twice as fast as 10 million years ago.

Peter C. Gray is a freelance writer and amateur historian living in Agua Dulce.

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