A Shaky Situation

Scripps seismologists gain valuable data on the phenomenon of earthquake swarms

A swarm of earthquakes rattled residents near the U.S.-Mexico border throughout February and were felt as far away as Los Angeles County. For researchers at Scripps Institution of Oceanography at UC San Diego, they were a learning opportunity.

The motion along the Cerro Prieto fault woke up border residents with a magnitude 5.4 earthquake on Feb. 8 and continued to rattle nerves with a magnitude 5.0 on Feb 19. In total, about 225 small- to medium- sized earthquakes centered near Mexicali, Baja California, were detected during the month of February by the seismic networks operated by Scripps Oceanography.

According to seismologists, this level of activity, although unusual in the last 10 years, is not uncommon. Scientists hope data collected from the February swarm will help them better understand why these sudden bursts occur.

The Cerro Prieto Fault, known as the "Mexicali Seismic Zone" is a very seismically active region, which typically generates small- and medium-sized earthquakes. Larger magnitude 6.0 events are known to occur in this region every 20 years on average.

Seismologists have determined that earthquakes over magnitude 5.0 will usually be followed by a series of aftershocks, the largest of which will typically be one magnitude smaller than the main tremor.

Earthquake swarms are different.

"Earthquake swarms are interesting because they are exceptions to the common view that earthquake occurrence is essentially random," said Scripps geophysicist Peter Shearer. "Swarms are a clustering of earthquakes that cannot be explained as a mainshock-aftershock sequence, and which are likely caused by some underlying process, such as a slow creep event or fluid migration.

Swarms are clusters of earthquakes occurring close together but without a well-defined mainshock. Research suggests they are caused by the slow creeping on nearby faults driven by the long-term movement of the tectonic plates, or from the migration of pressurized fluids, such as water, which can serve to lubricate the faults and promote their failure.

Debi Kilb, a Scripps geophysicist and science director of the Scripps Visualization Center, produces publicly accessible 3-D visualizations of the seismic data recorded by Scripps's networks.

"These magnitude 5.0 events near Calexico and the following aftershocks were unusual in that there were four relatively large earthquakes within a fairly short time span," said Kilb. "For scientists, this represents a fresh opportunity to better understand earthquake source physics."

Scripps operates several seismic networks, including the Southern California ANZA and the USArray Transportable Array networks. Data from these networks are sent in near-real time for use by the scientific community and also provided to the general public through the website.

Seismologists are currently able to identify regions where large magnitude earthquakes are expected to occur but are not yet able to pinpoint exactly when or where an earthquake will occur. The San Andreas Fault is known to produce large earthquakes every 300 years. According to scientists, the fault could produce a large quake in the next 50 years or so.

Annie Reisewitz April 2008