What is the San Andreas Fault and why are people so afraid of it?

The people of Southern California are always on the alert. This is because geologists have expressed concern about the imminent danger of another major earthquake along the San Andreas Fault.

What is the San Andreas Fault?

The San Andreas Fault is a fault line that runs for approximately 800 miles (1,287 kilometers) through California in the United States.

The fault line marks the transform boundary between the Pacific Plate and the North American Plate.

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The lateral slip rate of the fault’s central segment is around 25 millimeters per year, while in other, more distant segments, it reaches 30 millimeters per year, which could indicate an accumulation of elastic strain in the area of ​​the fault.

The Baja California peninsula is believed to have been formed by the activity of this fault.

This same process is moving the city of Los Angeles toward San Francisco Bay, at a speed of around 4.6 centimeters per year.

This movement is so slow that it can’t be perceived on a human scale, but it has caused considerable damage to engineering works such as aqueducts, roads, and ranches.

The Big One returns

The Big One, also known as “El Grande,” is an earthquake that scientists expect to occur in California along the San Andreas Fault.

Seismologists from the United States Geological Survey (USGS) have simulated the effects of a major earthquake in California for a study program.

One of their computer models assumes that the next major event on the San Andreas fault will be a magnitude of 7.8, initiating a rupture in Southern California near the Salton Sea and then shooting north along the fault line to hit Los Angeles.

An earthquake in the southern segment of the San Andreas fault would have a direct impact on Los Angeles, the second most populated city in the United States.

San Andreas Fault

(Source: USGS)

The most conservative estimates suggest that, if an earthquake of such magnitude were to occur in that segment, around 2,000 people would die and more than 50,000 would be injured.

Around 1% of the buildings in an area of ​​10 million people would collapse, and around half the buildings in the area would have to be abandoned.

The damage is estimated at 200 billion dollars.

New data

Scientists have found new data to understand structural factors in seismicity evolution, offering potential prospects for improving aftershock forecasts.

A geophysical research paper, published on April 9 in Science Advances, explains that large earthquakes often lead to transient deformation and enhanced seismic activity, with their fastest evolution occurring at the early, ephemeral post-rupture period.

Using geophysical observations from the 2004 moment magnitude 6.0 Parkfield, California, earthquake, the authors of the paper imaged continuously evolving afterslip, along with aftershocks, on the San Andreas fault over a minutes-to-days postseismic time span.

“Our results reveal a multistage scenario, including immediate onset of afterslip following tens-of-seconds-long coseismic shaking, short-lived slip reversals within minutes, expanding afterslip within hours, and slip migration between subparallel fault strands within days.”

They found that the movement of the fault after the earthquake, or early afterslip, and associated stress changes appear synchronized with local aftershock rates, with increasing afterslip often preceding larger aftershocks, suggesting the control of afterslip on fine-scale aftershock behavior.

Large earthquakes often lead to transient deformation and enhanced seismic activity, with their fastest evolution occurring at the early, ephemeral post-rupture period.

“It is well known that many large earthquakes are followed by vibrant aftershock sequences and transient, longer-term deformation, both of which release and redistribute stresses over a rapidly evolving post-rupture period.”

Although the researchers’ experience of characterizing the aftermaths of major seismic events continues to grow, their knowledge of how earthquake ruptures transition to post-rupture processes remains limited.

The Parkfield section of the San Andreas fault (SAF) in central California has long provided a natural laboratory for studies of seismic and aseismic fault zone processes, owing to the dense, continuous regional monitoring network.


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