It's apparently earthquake season! Following big earthquakes in southeastern California over the July 4th holiday and aftershocks continuing through yesterday, as well as a small earthquake that rattled Morgan Hill Monday, there's now been a 4.3-magnitude quake east of Blackhawk in the East Bay, and a second 3.5 quake near Martinez.
Some East Bay residents reported feeling the first jolt, as ABC 7 reports, and initial reports from the USGS called it a 4.4. It was just a wee shaker that hit at 1:11 p.m., but all this quaking has California a little on edge!
USGS has reported a 4.4 earthquake east of Blackhawk, CA. There are no reports of damages, but we will continue to assess community damages. At this time there are no protective actions needed.— Alameda County Office of Emergency Services (@AlamedaCoAlert) July 16, 2019
And as the Chronicle reports, a second 3.5M quake struck 13 minutes later, at 1:24.
These temblors was not on the likely-soon-to-rupture Hayward Fault, but on the lesser known Clayton-Marsh Creek-Greenville Fault, which is part of a series of parallel fault lines to the San Andreas, running between Alameda and Contra Costa Counties.
The latest swarm of earthquakes seems to back up a 2017 study that suggested a connection between heavy winter rains and earthquakes. The study found that winter snow and rain may acts as "weights" on mountains, pressing into the earth and impacting pressure on faults. Scientists cautioned that "earthquake season" really isn't a thing, but the study nonetheless found a correlation in the aftermath of particularly wet winters in California.
While the Hayward Fault is the most overdo for a big earthquake, and has been for decades, a 2014 study found that three other local faults also have high likelihoods of rupture in the San Andreas Fault Zone: the Calaveras Fault, which runs from about Hollister to Danville; the Rodgers Creek Fault which runs from about Petaluma to Healdsburg (and is basically an extension of the Hayward Fault); and the Green Valley Fault in Solano County, which the study suggested "is likely to have a larger earthquake than people previously thought."
This post has been updated to reflect the second quake at 1:24 p.m.