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New year, new California quakes

Alka Tripathy-Lang, Ph.D.

By Alka Tripathy-Lang, Ph.D. (@DrAlkaTrip)
 

On Wednesday night, two small earthquakes welcomed California into the next decade.

 

Citation: Alka Tripathy-Lang (2020), New year, new California quakes, Temblor, http://doi.org/10.32858/temblor.063
 

A view of Morgan Hill, the locus of a recent magnitude-3.9 earthquake felt by some northern California residents. Credit: activerain.com

 

California was lightly jostled into the new year with two unrelated earthquakes separated by about three hours and 450 kilometers (280 miles). A magnitude-3.9 temblor shook the area around Morgan Hill, in Northern California, on Wednesday, Jan. 1 at 11:16 PM local time. At 2:12 AM the next morning, a magnitude-4.0 shock rattled residents in Southern California near Port Hueneme. These quakes occurred in regions that experience quakes of this size several times a year.

 

A late-night reminder for NorCal

As of this writing, 3,991 Northern California residents reported feeling the magnitude-3.9 earthquake via the USGS “Did You Feel It?” citizen science initiative. Sara McBride, the Social Science Coordinator for ShakeAlert at the U.S. Geological Survey (USGS), told her Twitter followers that she initially “thought it might be a car door slamming.”

 


 

At a depth of 5.9 kilometers (3.7 miles) according to the USGS preliminary report, this earthquake was a shallow strike-slip event that was small enough to cause no damage or injuries, but large enough to surprise those near the epicenter with shaking.


According to Ross Stein, CEO of Temblor, the quake was on or near the Calaveras Fault, which is a major branch of the San Andreas Fault system. Its last large event was the 1984 magnitude-6.2 Morgan Hill earthquake, which caused serious damage and some injuries.

 

Map showing epicenter (green dot) of magnitude-3.9 earthquake near Morgan Hill. The quake probably struck on a subsidiary of the Calaveras Fault. Credit: Ross Stein, Temblor.

 

And an early morning jolt for SoCal

Not to be left out, 1,950 Southern California residents near Port Hueneme, which is between Santa Barbara and Long Beach, reported feeling the magnitude-4.0 earthquake, according to USGS.

Stein says that this quake was a thrust event near the Santa Cruz Islands Fault. Because the epicenter of this earthquake was about 25 kilometers (15 miles) offshore, shaking felt by the local population was much less than what was felt in Northern California.

 

Map showing epicenter of magnitude-4.0 earthquake off the coast of Port Hueneme. The quake is located south of the trace of the Santa Cruz Islands Fault. Notice that although geologists use different names, the Santa Cruz Island, Malibu, and Santa Monica Fault are likely continuous, and potentially capable of a M~7.5 shock, according to Stein. Given their thrust motion, a tsunami could be generated by such a rare but possible quake. Credit: Ross Stein, Temblor.

According to the USGS, the magnitude-4.0 quake was very shallow, at a depth of less than 0.1 km (330 feet). No tsunami warning was issued, though it was an offshore event.

 

Why didn’t I get a ShakeAlert?

ShakeAlert, California’s Earthquake Early Warning system, distributes alerts via MyShake, California’s official Earthquake Early Warning app. It will only issue alerts for earthquakes greater than magnitude-4.5, as estimated by ShakeAlert.

The Wireless Emergency Alerts system also sends notifications for earthquakes for ShakeAlert, but only when the initial estimated magnitude is greater than 5.0, McBride says.

Though felt, these quakes were simply not big enough to merit a warning pushed to people’s phones. They were, however, big enough to remind Californians that this is still earthquake country, and that if you feel an earthquake, drop, cover and hold on.