Most people who have lived in Southern California their entire lives would be quick to tell you that earthquakes there are nothing out of the ordinary.
However, with the arrival of the Fourth of July holiday, residents north of Los Angeles had more than just fireworks to shake things up, as a swarm of earthquakes rattled desert communities throughout the region well into the following week.
NBC Los Angeles reported that as of Monday morning, “nearly 3,000 aftershocks [were] recorded in the area since Friday’s magnitude-7.1 main shock and magnitude-6.4 Independence Day quake, the strongest earthquakes to hit Southern California in the last 20 years.” The earthquakes were centered just outside the town of Ridgecrest in the Mojave Desert, approximately 150 miles northbound from Los Angeles.
Although earthquakes aren’t uncommon in the region, that isn’t to say that last week’s wave of holiday tremblors was anything typical, either.
USGS map depicting mainshock and aftershock locations shortly after the M7.1 event, which displays a distinctive “T” pattern, created by the two perpendicular fault zones. (USGS/Public domain).
Geologists note that there is only around a 5% likelihood that a significant earthquake will be followed by an even larger one; however, this was only one way that initial pair of earthquakes that struck the region seemingly defied the odds. Live Science reported that the Independence Day earthquake occurred on an unmapped fault region along the Little Lake fault zone near Ridgecrest, and involved right angle ruptures, which may be more easily detected today than they were only years ago.
“This is very interesting geologically,” geoscientist Michele Cooke told Live Science. “We don’t have a lot of earthquakes in our record that show simultaneous slip on two perpendicular faults,” she said.
“Many of us are wondering if these complications are actually the norm and that our instruments 10+ years ago were not sensitive enough to pick up these complications,” Cooke also said, suggesting that these “rare” seismic events may not be so unusual after all.
However, geologists weren’t the only ones noticing odd things coinciding with the recent California earthquakes. According to the National Association for Amateur Radio (ARRL), disturbances in HF propagation on the west coast appeared to coincide with the July 4 earthquake as well, resulting in a “massive short-wave radio blackout” over the holiday weekend, a phenomenon that was observed by a number of HAM radio operators in the region.
Alex Schwarz, an amateur radio operator out of British Columbia, Canada who operates the RF Seismograph, reported that there were correlations “between earthquake activity and HF band conditions, said the radio disruption began at around 1600 UTC on July 4, and continued into July 5.”
According to the ARRL, “[Schwarz] said that on July 4, the blackout was total except for 20 meters, where conditions were ‘severely attenuated’… The RF Seismograph also detected the magnitude 7.1 earthquake on July 6 in the same vicinity.”
Finally, NASA was also monitoring the quakes, with a little help from the Japanese Space Agency (JAXA) and one of their satellites, which resulted in colorful imagery of the areas affected by the earthquakes:
Image depicting earthquake elevational displacement following the July 4th seismic events (Credit: JPL/Caltech/NASA).
According to NASA’s Jet Propulsion Laboratory, the space agency’s Advanced Rapid Imaging and Analysis (ARIA) team borrowed data collected by synthetic aperture radar, which was then colorized to reveal minor changes that were visible in the area of the earthquakes.
“Each colour… represents 12 centimetres (about 4.8 inches) of lifting or sinking in the landscape, with evidence of cracks in the lines cutting through the fringes of some colours,” Science Alert reported.
Despite the significant strength of last week’s earthquakes, few injuries were reported since the epicenter of the quakes had been located in a remote area. It is also possible that more earthquakes and aftershocks will continue over the next several weeks, although the unique data being collected in their aftermath may help reshape our understanding of similar seismic phenomena for decades to come.
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