Fears Growing over A BIG ONE in San Andreas Fault, Here’s what You Need to Know

On September 19th Los Angeles was rocked by an earthquake. Although it was a small quake of magnitude 3.6, it was enough to provoke some alarmed tweets and freak-out reaction GIFs. That same week, a devastating 7.1 magnitude quake struck Mexico, causing widespread damage and loss of life including in the densely-populated capital, Mexico City. Since then fears are growing of a BIG ONE in the San Andreas Fault, like the one in 1906 which destroyed about 80% of the city of San Francisco and killed around 3,000 people. Here’s a short overview and current situation of San Andreas Fault.

San Andrea Fault

The San Andreas Fault is a 1200 km continental transform fault located throughout California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizontal). The San Andreas Fault is divided into 3 segments; each of which has different characteristics and different degrees of earthquake probabilities.

The most significant segment is the southern one that passes within approximately 56 kilometers of Los Angeles. The slip rate along the fault ranges from 20 to 35 mm (0.79 to 1.38 in) per year.

Discovery and Naming:

The San Andrea Fault was first identified in 1895 by Professor Andrew Lawson of UC Berkeley, who discovered the northern zone. It is said to be named after the San Andreas Lake, which is a small body of water formed in a valley between the two tectonic plates. Although some of Lawson’s reports made between 1895 and 1908 suggest that he actually named the fault after the San Andreas Valley that surrounded it.

San Andrea Fault

San Andreas Fault Location:

Until 1906, the San Andrea Fault was thought to be only as big as its northern zone, but after the 1906 San Francisco earthquake, Lawson confirmed that the fault spread as far as all the way to southern California.

Catastrophes It Caused:

The earliest recorded earthquake caused by the San Andreas Fault occurred on January 9, 1857, at around 8:20 am with a magnitude of 7.9. But due to precautions and sheer luck, only 2 people got killed.

Then again on April 18, 1906, at around 5:12 am local time there was a massive earthquake in San Francisco with a magnitude of 7.8 at its core. The earthquake destroyed about 80% of the city of San Francisco and killed around 3,000 people. The reason behind this earthquake was movements in the tectonic plates of the San Andreas Fault.

San Andrea Fault

Current Situation:

The San Andreas Fault has been silent ever since, which is a good news. But it may also be a bad one too. Why? Because according to experts, these fault lines can remain inactive for centuries on end but still create havoc all of a sudden. And the San Andreas Fault has recorded hundreds of miles of lateral movement as discovered by geologist Thomas Dibblee in 1953.

San Andrea Fault

Robert Graves, a Research Geophysicist at the U.S. Geological Survey (USGS) said in an email interview with Newsweek. “Studies that have dated previous major offsets along the fault trace show that there have been about 10 major quakes over the past 1,000-2,000 years,” Graves continues. “The average time between these quakes is about 100-150 years.”

The bad girl San Andreas is silent for now, but who knows when she might wake up?

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