In Brief
  • More than 140 instances of earthquake swarms were reported last Monday.
  • The USGS cautions that there isn't necessarily cause for alarm just yet.

Quake swarms

Concerns of a massive earthquake along the San Andreas Fault shook the news last Friday, as the latest data from the United States Geological Survey’s (USGS) monitoring of undersea “earthquake swarms” near Bombay Beach, California had some seismologists worried.

According to the USGS, the swarms — rapid successions of short earthquakes — started rumbling under the Salton Sea Monday, September 26. More than 140 occurrences were recorded, at depths of 4 to 9 km (2.5 to 5.5 miles) with a magnitude range of 1.4 to 4.3. The earthquake sensors in the area were installed in 1932, and since then, similar earthquake swarms were recorded in 2001 and 2009, with last Monday’s registering more shakes.

Credits: USGS
Sept 26 swarms. Credits: USGS

The fault

Understandably, seismologists were alarmed. And alarmed seismologists make alarming calculations: the chances of a magnitude 7 or greater earthquake since last Monday until today (October 4) along the San Andreas Fault was as high as 1 in 100 and as low as 1 in 3,000, said the USGS. The average chance for such an earthquake striking on any given week is 1 in 6,000.

Then, last Saturday, October 1, the agency updated its calculations to more modest scales: less than 1 in 10,000 to 1 in 500, for the next few days up to October 7. These chances go down as days pass. “Swarm-like activity in this region has occurred in the past, so this week’s activity, in and of itself, is not necessarily cause for alarm,” cautions the USGS.

While it’s true that the San Andreas Fault seems to be overdue for a big earthquake, with the last one occurring in 1680, this weekend’s shaky (pun intended) calculations demonstrate how the science of earthquake predictions is not an exact one.

The USGS says that much. Predicting earthquakes is not an exact science, and “it is unlikely that [scientists] will ever be able to predict them.” Estimates are estimates. The USGS gives these out not to cause unnecessary panic, so much as to keep us on our toes.