The Mercury News: A 3.9 magnitude quake jolted the South Bay this morning, in an area notorious for seismic restlessness, according to the U.S. Geological Survey.
Around 9:40 a.m., just after San Jose residents experienced hail, torrential rain and thunderous dark clouds, the earth beneath them suddenly shifted. The 3.9 quake was centered about nine miles northeast of downtown San Jose, in the Alum Rock region. Tremors were felt in Milpitas, Fremont and Santa Clara, the USGS reported. Office towers shook in downtown San Jose.
“I’m only seeing one earthquake in that area above a magnitude 2.5 for the last week,” said Paul Caruso, a USGS geophysicist.
“Rain, hail, wind, thunder, lightning, AND an earthquake? Mother Nature ain’t having it today,” tweeted Carl Cortez. “Ya’ll better pray,” joked Tiffany B on Twitter.
One hour later, it was bright and sunny again.
On the USGS web page “Did you feel it?” 4,828 people filled out reports.
This earthquake had a depth of 8.3 km or 5.1 miles deep.
Any earthquake has a 5% chance of being a forshock to a larger earthquake.
— Looking4Truth (@Lifesgoodtimes) April 16, 2018
Woke up to thunder and lightning followed by an earth quake. GOOD MORNING CALIFORNIA!
— Seasonal (@SecretSeasonal) April 16, 2018
The cause is well known — and for this jolt, it’s the Calaveras Fault, which routinely releases stress caused by the earth’s shifting continental plates. Monday’s rattle is the latest in a series of sporadic tremors triggered by this fault, which extends from just south of Walnut Creek to south of Hollister, where it joins with the San Andreas Fault.
Last December, another 3.9 magnitude quake struck the Alum rock region, following a 4.1 October temblor in the same spot. According to the USGS, it is not unusual for the Calaveras Fault to have magnitude 4 quakes, which are disconcerting but unlikely to cause structural damage to buildings.
Before that, a magnitude 5.4 temblor hit near Alum Rock in 2007.
A flash of close lightning, a loud peal of thunder and an earthquake coinciding is a helluva coincidence. I don’t think this is a “bad” omen so much as a “foreboding” one. I feel safer knowing the heavens, sea and earth are as concerned about the world as I.
— High Yeen? Aah! (@hyenagirl64) April 16, 2018
Activity along the Calaveras Fault gets close attention because it has a high degree of rupturing in a large magnitude quake, exceeding magnitude 6.7, over the next 30 years. There’s a 26 percent chance of that event, trailed only by the nearby Hayward-Rogers Creek Fault, which runs through the heart of East Bay cities and has a 31 percent chance of rupturing. 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.
Anderson Dam, a dam which has been identified as being seismically at risk, sits right next to the Calaveras Fault. Construction on a $200 million project to drain and repair the dam, increasing its safety, is slated to start mid-2020.
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.
its been thundering, it hailed, it sleeted in the bay area and there was an earthquake in San Jose. WHO FORGOT TO SACRIFICE THE GOAT?! THE GODS ARE MAD NOW!
— Fletch @ Going to bed on time (@AmazingFletch) April 16, 2018
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.
Mary Greeley News