By DOUGLAS W. MOTLEY
Senior Writer
For the second time this month, the San Bernardino Valley, as well as the mountain communities and much of Southern California, was rattled by a magnitude 4.2 earthquake that struck a mile-and-a-half from downtown San Bernardino at 7:43 p.m. on Wednesday, Jan. 24.
The U.S. Geological Survey (USGS) reported that moderate shaking was felt across Adelanto, Hemet, Riverside, Fontana, Rialto, Rancho Cucamonga and Redlands. Weaker shaking was reportedly felt in Moreno Valley, located 12 miles from the epicenter, and Anaheim, 39 miles away.
According to Cal Tech seismologist Lucy Jones, the shaking occurred along the nearby San Jacinto Fault Zone and was a combined compression and shearing motion, which means that blocks pushed together went at a more diagonal motion with respect to one another.
“That part of the fault is generally locked; it had an M7 in the 19th century. We often see small quakes like this below locked segments,” Jones said in her post on X (formerly Twitter).
According to the USGS, the temblor – first-reported as magnitude 4.5 and later downgraded to 4.2 – may have been felt by as many as 25 million persons across three counties. Its shallow depth of 5.6 miles made it feel stronger than a deeper one, because it is closer to the surface. No significant damage was reported, other than some broken windows and objects falling from shelves.
The Jan. 24 event was the second magnitude 4.2 earthquake to strike in the San Bernardino area in 20 days. The first one struck near Lytle Creek, about 16 miles northwest of downtown San Bernardino, at 10:55 a.m. on Friday, Jan. 5. On that same day, California had 19 magnitude 1.5 or greater earthquakes in a 24-hour period, 107 earthquakes in the past seven days and 491 in the past 30 days.
Southern California areas that would be most affected in the event of a major earthquake. (Photo by Southern California Weather Force.)
A spokesperson from the Corona-based Southern California Weather Force has issued an earthquake watch effective through Feb. 8 for the chance of a stronger earthquake occurring on the San Andreas or San Jacinto Fault Zones. This area last shook with a foreshock, which is what this might be, in the 1970s. Noting that no actual aftershocks have been recorded, a geology major at Cal State University says this latest series of earthquakes could very well be a foreshock, or shock before the main shock.
“You can think of it as holding onto a ledge. A finger slips and you get a jolt. You still have the rest of your hand left before you go and slip completely. That is my analogy of a foreshock. If this was a foreshock, we have a short period of time to see the mainshock. A larger quake on the San Andreas or San Jacinto Fault will rip northwestward through the Inland Empire and High Desert area, Los Angeles, San Gabriel Valley, Antelope Valley and Kern County areas, along with Ventura. Should it break the entire fault southward, you have Big Bear, Banning, Coachella Valley and Imperial Valley.”
Mountain area residents should be aware of the likelihood that various viaducts and bridges on Highway 330 and Highway 18, such as the Bonnie Canyon and Crestline Bridge, may no longer be useable in the event of a major earthquake.
Keep in mind that cellphone towers will be jammed, and you won’t be able to get through to anyone, so you will need to be prepared in advance by having an emergency plan. Make a supply kit that includes enough food and water and clothing for several days, a flashlight, a fire extinguisher and a whistle to signal for help, if needed.
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