san jose quake april 2018
Hello, Monday! A 3.8 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.

"It was snowing and hailing, with thunder, then there was an earthquake. It felt like a big jerk, not rolling," said Angel Barlow, park services attendant at Joseph D. Grant County Park, in the hills east of San Jose. "It was a landslide of weather!"'

hail san jose
© Laura A. Oda/Bay Area News Group
Students from Montara Middle School play in the hail after their bus was grounded due to the hail storm on Redwood Road in Oakland, Calif., on Monday, April 16, 2018. The hail snarled traffic on Highway 13 due to cars sliding on icy roads and surrounding streets and delayed the start of the school day for many schools in the area.
Chairs rolled at the ranger station at Ed R. Levin County Park in Milpitas, about four miles from the epicenter.

The drama unfolded all over Twitter. "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.

The weather is expected to stay unsettled all day, with intermittent sun and showers.

There is no such thing as "earthquake weather," according to Malcolm Johnston of the UGSG's Earthquake Hazards Program in Menlo Park. Statistically, there is about an equal distribution of earthquakes in any weather - rainy, dry, hot or cold. That theory had its origins in the 4th Century B.C., when Aristotle proposed that earthquakes were caused by winds trapped in subterranean caves, but that notion has long since been disproved.

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, a 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.

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.

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.