California Dolphin: statewide California news

Earthquake rattles South Bay

A 3.9 magnitude jolted the South Bay Monday morning, in an area notorious for seismic restlessness, according to the U.S. Geological Survey.
The earthquake struck at 9:39 a.m. and was centered about 11 miles northeast of downtown San Jose, in the Alum Rock region.
The jolt was the latest in a series of sporadic tremors triggered by the Calaveras Fault, which routinely releases stress caused by the Earth’s shifting continental plates.
Last December, another 3.9 magnitude quake struck the region, following a 4.1 October temblor in the same region. According to the USGS, it is not unusual for the Calaveras Fault to have magnitude 4 quakes, which are unlikely to cause structural damage to buildings.
The USGS estimates a 7.4 percent likelihood of a 6.7 magnitude or greater earthquake on the Calaveras Fault in the next 30 years, and a 31 percent chance on the Hayward – Rogers Creek Fault, which runs through the heart of East Bay cities. The Hayward section of the fault runs from near Mount Misery, east of San Jose, north to San Pablo Bay. The Rodgers Creek portion picks from there and runs north to Santa Rosa.
The Calaveras and Hayward faults are believed to be linked, which means that both could someday rupture together, resulting in a significantly more destructive earthquake than previously thought.
The last major earthquake in the Bay Area was the 1989 Loma Prieta earthquake — a magnitude 6.9 quake that struck on Oct. 17, 1989, in the Santa Cruz Mountains. It killed 62 people and caused $6 billion in damages. The largest earthquake since Loma Prieta shook Napa in August 2014 with a magnitude of 6.0. No one was killed in the quake but hundreds of buildings were damaged and dozens were red-tagged. Before that, a magnitude 5.4 temblor hit near Alum Rock in 2007.
California straddles the boundary between two of the Earth’s tectonic plates — as a result, it is broken by numerous earthquake faults. Literally thousands of small earthquakes occur in California each year, providing scientists with clear indications of places where faults cut the Earth’s crust.
In 2007, a panel of experts estimated there is a 63 percent chance that in the next 30 years the San Francisco Bay Area will experience a catastrophic earthquake at least as powerful as the magnitude 6.7 Northridge earthquake that rocked Southern California in 1994. There is a far greater chance — 99 percent — that an earthquake that size will strike somewhere in the state during that time.
Of the seven major fault systems in the region, the San Andreas is also judged to be dangerous. Researchers estimated a 21 percent chance of a damaging quake along the northern San Andreas Fault, which includes the Peninsula.
The scale of earthquake measurement is logarithmic: a recording of 7, for example, signifies a disturbance with ground motion 10 times as large as a recording of 6. An earthquake of magnitude 2 is the smallest size normally felt by humans. Earthquakes measuring 5 or higher are potentially damaging.
 
The San Francisco earthquake of 1906 has been estimated at a magnitude 8.3. The earthquake that leveled the capital of Haiti measured 7.
 
As with any large earthquake, there is a possibility of damaging aftershocks. If an aftershock occurs, the USGS recommends that people who are indoors stay there, taking shelter under a piece of furniture, in a hallway or against an inside wall, away from windows, fireplaces and heavy objects.
 
If you are outdoors, get into the open away from buildings, power lines and other things that could fall. If driving, stop carefully and move out of traffic. Avoid bridges, trees and other falling objects. Stay in your car until the shaking stops.

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