Rash of Seismic Activity Also Rattling Nerves in Northern California

A series of minor earthquakes has left residents of the San Francisco Bay Area feeling uneasy over the last month. How big have these quakes been, and what does it signal about what may lie ahead along a major fault line? Read on for all of the details.

Surge of Seismic Activity in the Bay Area Creates Unsettled Feeling

Dozens of small earthquake events underneath the East Bay suburb of San Ramon, California, have rattled the major population center over the last few weeks. This community is located near the Calaveras fault, sparking concerns that a major earthquake could be looming on the horizon.

Last Monday saw at least 19 earthquakes measuring magnitude 2 or higher. The largest was a magnitude 3.6 quake that hit at 9:07 am as people were starting their work week. There were also at least six earthquakes confirmed in the 5 am hour, some that were strong enough to jolt people out of bed.

These seismic clusters started on November 9, a day that notched 13 earthquakes. Two more quakes shook the region the next day, with one each day recorded on November 15, 17, 18, and 20.

The area above San Ramon is not the only part of the Bay Area to see seismic activity as of late. An additional magnitude 2.9 earthquake struck the weary region at 2:55 pm on Monday. This epicenter was located near the Hayward fault in Oakland's Montclair neighborhood.

According to the U.S. Geological Survey (USGS), San Ramon has historically been a hotbed for earthquake swarms. The major city in Contra Costa County also experienced seismic clusters in 2002, 2003, and 2015. Nearby Danville saw similar activity in 2018. The promising news is that none of these clusters was a harbinger for a major quake on the Calaveras fault. In fact, a destructive quake has never been recorded in the Tri-Valley area along this fault line.

Experts say that there is little predictive value when looking at past earthquake swarms. In addition, this part of the country is a common area to see these surges of activity. The longest period of seismic activity in the Bay Area lasted for 42 days.

Residents can take solace in knowing that there is only a 5% chance that any one earthquake will lead to an even bigger one nearby over the following week. You have to go back decades to find the last time that a major quake followed a more moderate event. While they are unsettling, most earthquake swarms do not translate to major events.

About the Major Fault Lines in Northern California

Earthquake experts believe that the Calaveras fault has the ability to generate a magnitude 6.7 or greater earthquake. This major fault runs from the unincorporated portion of Contra Costa County near Alamo to the southeast along Interstate 680 and into Danville, Dublin, and Pleasanton. The Calaveras fault then meets up with the infamous Hayward fault in Santa Clara County. The Calaveras fault juts off to the southeast, taking it past the community of Gilroy before it merges with the San Andreas fault in San Benito County.

The most recent significant earthquake on the Calaveras fault happened in 1984. This magnitude 6.2 earthquake resulted in over 20 injuries and millions of dollars in economic damage. An even bigger estimated 6.5 earthquake hit in 1911 near this fault line. More recently, a magnitude 5.1 tremor spun up on the Calaveras fault east of San Jose in 2022.

While it is not likely, some scientists warn that it is possible that an earthquake with origins along the San Andreas fault could continue to the northwest up the Calaveras fault. This path of rupture could also take it into the Hayward fault, causing a large swath of damage.

What keeps many residents of the Bay Area awake at night is a projection by the USGS, estimating that there is a 72% probability that a major quake of magnitude 6.7 or greater rattles this part of Northern California by 2043. The most concerning zone is around the Hayward fault, a line that traverses through Richmond, Berkeley, Oakland, Fremont, and San Jose. Models suggest that a magnitude 7 quake in this region could kill at least 800 people and cause 18,000 injuries.

The Hayward fault experienced its last major earthquake in 1868. Imperfect records dating back to this time put this quake at a magnitude of 6.8 and 7.0. The 1868 event is distinguished as one of the most destructive in state history.

Although experts say that this is not the time to push the panic button, the increase in seismic activity should serve as a reminder to prepare for a potentially larger event. Securing objects such as bookcases and televisions is a good start in making sure that you will not be hit by a projectile.

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