On Monday, July 29th at 1:00 PM thousands of people felt the 4.9 earthquake that originated roughly 16 miles northeast of Barstow – shaking was reportedly felt from Los Angeles to Las Vegas with reports here in the hi-desert as well. A 4.9 magnitude earthquake makes news because you can actually feel it – it’s big enough to swing lamps and knock over books and vases, but only the weakest structures would receive damage from a quake of that magnitude.
According to the Southern California Earthquake Data Center out of Caltech – Barstow sits right on top of a cluster of northwest-trending right-lateral strike-slip faults – known as the Eastern California Shear Zone. Picture dozens of diagonal lines scratched over a map with Barstow right in middle. That shear zone is estimated to accommodate between 9% and 23% of the relative motion between the Pacific and North American tectonic plates.
Image: Southern California Earthquake Data Center
The hi-desert has dozens of faults as well, with two faults that mirror its most popular highways. The Pinto Mountain Fault runs east-west adjacent to Highway 62, while the Johnson Valley faults run up Old Woman Springs Road to you guessed it – Johnson Valley and beyond.
All of those faults and plates are why Southern California is so seismically active – but we aren’t feeling most of the motion under our feet because most earthquakes just aren’t large enough to shake anything up top. It’s when they reach a 5.0 magnitude and above that people begin to take notice.
Image Credit: Wikipedia
According to the California Department of Conservation, there have only been about 17 earthquakes in the hi-desert since 1889, with the most recent and famous being the Landers Quake. That one occurred 32 years ago on June 28th, 1992… it was a 7.5 but did much more damage than the Barstow 4.9 trembler. That’s because the “moment of magnitude scale” – which is what they use to measure earthquakes – increases logarithmically. I’m not going to attempt to explain the math, but I can tell you that according to the United States Geological Service’s magnitude calculator – the Landers quake was nearly 400 times bigger, and nearly 8000 times stronger in energy release than the quake some of us felt on Monday.
A quake of Monday’s magnitude lets us off easy, acting as a reminder to do some basic emergency and earthquake prep before something larger than a 4.9 inevitably shakes the Morongo Basin.
Links and sources:
The Great American Shakeout – Earthquake Preparedness at SBCfire.gov
The Landers – Big Bear Quake at California Department of Conservation
4.9 Quake NE of Barstow – at USGS
Faults of Southern California – at Southern California Earthquake Data Center
