Bay Area Earthquakes Struck On ‘Creeping’ Section Of San Andreas

A sign in Parkfield, Calif., advises motorists they are crossing over the San Andreas fault.

The magnitude 4.7 earthquake Tuesday east of Monterey Bay hit on a unique section of the San Andreas fault that has long generated interest from scientists.

The quake was felt across a wide area and caused no major damage. It was followed Wednesday morning with a 3.4 magnitude aftershock. But it offered a reminder of the San Andreas’ presence as a dangerous seismic presence in California.

How small quakes led to a big magnitude 7.8 earthquake on the San Andreas fault in California

Moderate earthquakes on the morning of Jan. 9, 1857 preceded a magnitude 7.8 earthquake on the southern San Andreas fault, rupturing 225 miles between Parkfield in Monterey County and Wrightwood in San Bernardino County.
(Raoul Rañoa / Los Angeles Times)

The ‘creeping’ section

The San Andreas fault has produced devastating quakes in Northern and Southern California.

But Tuesday’s quake occurred along a section that is notable for not having had dramatically large earthquakes in the modern historical record. Keith Knudsen, USGS geologist and deputy director of the agency’s Earthquake Science Center, called Tuesday’s quake “a garden variety San Andreas event” in this section.

“This is the 10th earthquake larger than magnitude 4 in the last 20 years in this area” within a radius of about six miles from Tuesday’s epicenter, Knudsen said.

A view of the San Andreas fault in the Carrizo Plain. A valley is deeply eroded along the fault.

A view of the San Andreas fault in the Carrizo Plain. A valley is deeply eroded along the fault.
(U.S. Geological Survey)

Power to the north and south

The stretches of San Andreas north and south of the creeping section have acted very differently in the modern historical period, rupturing in the state’s largest catastrophic quakes on record.

About 300 miles of the northern San Andreas fault, between San Juan Bautista in San Benito County and Cape Mendocino, ruptured in the great 1906 earthquake that destroyed much of San Francisco in shaking and fire.

Oct. 15, 2019

A 225-mile stretch of the southern San Andreas fault south of this creeping section, from Parkfield in Monterey County to Wrightwood in San Bernardino County, ruptured in the great 1857 earthquake, the most powerful event in modern Southern California history.

Both quakes are estimated to have been magnitude 7.8.

The 90-mile creeping section between San Juan Bautista and Parkfield hasn’t seen a big earthquake in the modern record. Scientists are continuing to study whether big earthquakes can continue to rupture through the creeping section of the San Andreas.

This simulation shows the shaking that could be felt during a magnitude 8 earthquake on the San Andreas fault.

This simulation shows the shaking that could be felt during a magnitude 8 earthquake on the San Andreas fault.
(Southern California Earthquake Center)

A lurking danger

The U.S. Geological Survey published a hypothetical scenario of a magnitude 7.8 earthquake on a 185-mile stretch of the southern San Andreas fault, from Imperial County to Los Angeles County. In this scenario, the quake would kill 1,800 people, injure 5,000, displace some 500,000 to 1 million people from their homes and hobble the region economically for a generation. It also would send strong shaking into Los Angeles, Orange, Riverside, San Bernardino, Kern and Ventura counties within minutes.

Here’s a list of strong quakes on the San Andreas fault over the past 200 years, according to the USGS and California Geological Survey:

1812, southern San Andreas and parts of San Jacinto fault, magnitude 7.5: Destroyed the Great Stone Church at Mission San Juan Capistrano, killing more than 40 people attending Mass.

1838, northern San Andreas fault, magnitude 7.4: Damage was reported from San Francisco to Santa Clara, fault likely ruptured between Woodside on the peninsula south of San Francisco to Watsonville.

1857, southern San Andreas fault, magnitude 7.8: Ruptured fault from southernmost Monterey County to Cajon Pass in San Bernardino County; damage was reported from Monterey County to Los Angeles County.

1890, northern San Andreas fault, magnitude 6.8: East of Watsonville; damage was reported in San Benito and Santa Cruz counties.

1906, northern San Andreas fault, magnitude 7.8: Great San Francisco earthquake; the San Andreas produced shaking waves from a long stretch of the fault, from Mendocino County to San Benito County. Experts estimate more than 3,000 deaths were caused, directly or indirectly, by the earthquake.

Original Article: https://www.latimes.com/california/story/2019-10-16/earthquake-struck-on-unusual-section-of-san-andreas-fault-known-for-creeping

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