by Kimberly Rivers

krivers@timespublications.com

A cluster of earthquakes, called a swarm, rumbled through the city of Ventura last week, causing locals to wonder if a big one could be coming.

“Ventura[‘s] activity is an earthquake swarm. That means no one quake is so much bigger than the others that it can be called the main shock,” tweeted Dr. Lucy Jones on Nov. 8 in the midst of the swarm. Jones is a seismologist who has served as a research associate at the Seismological Laboratory of CalTech since 1984. “Swarms are common . . . [and] are not more likely to be followed by a big quake.”

On Nov. 8 at about 1 p.m., Jones reported that there were 70 “events” in the swarm with five over magnitude 3.2. The largest was M3.6.

On the VCReporter’s Facebook comment thread, Teresa Archer reported feeling the

USGS map showing the swarm of quakes in Ventura, primarily on Nov. 8, 2019.

quakes as far away as Santa Paula, noting she especially felt “the [M]3.6 at 5:29 a.m.” on Nov. 8. Out in Saticoy, John Murchison felt a 3.0 shaker.

Ultimately the swarm included 82 shakers in the area shown on the map image, between Nov. 3 and Nov. 9. The swarm started at about 4 p.m. on Wednesday, Nov. 6, and lasted through Nov. 8. Thirteen of the quakes were over M2.5, and they were at depths of 10.1-14.6 kilometers.

Via email to the VCReporter, Jones said, “the swarm is definitely not occurring on the Ventura Fault. That fault runs east-west but these earthquakes are forming a north-south line. As there is no mapped north-south fault in that location, we think that they are occurring on some minor structure, too small to be seen at the surface.”  

Responding to a question about whether nearby activity in the Ventura Oil Field could be contributing to the swam, Jones responded, “without much more information it is impossible to be 100 percent certain of the impact of any human activity on the earthquakes. But it seems highly unlikely.” She said induced earthquakes, like the kind caused by oil wastewater injection, “occur where the water is being injected. These earthquakes are five miles deep and I have never heard of water being injected that deep.”

Ready and Resilient

During the cluster of quakes, the city of Oxnard sent out a notice reminding the community about important steps to take to get prepared for a significant earthquake.

“While we are not overly concerned, these earthquake swarms should remind all of us that we need to know what to do during an earthquake and we need to have an emergency kit and supplies ready to go at all times,” said Darwin Base, Oxnard Fire Chief.

The statement reminds folks to not run outside during the quake, but instead to find something sturdy to take shelter under or near until the shaking stops. Emergency services like police and fire may be less available and many residents may have to address issues like turning off the gas and water mains, and dealing with no electricity. Emergency kits should have enough supplies for the entire family and pets for up to seven days.

Learn more about planning for disaster and download a copy of the Oxnard Preparedness Guide at www.oxnard.org/disaster-preparedness

Track latest earthquakes at earthquake.usgs.gov/earthquakes/map