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A  formation of over 700 earthquakes called "swarm" hit one general area near the city of Westmorland in Southern California over the past several days. The incident puts scientists on high alert. 

The Southern California Seismic Network (SCSN) released a report regarding the Westmorland Swarm indicating the following:

• There were 96 events during the 3 days prior to the M4.9 (within a 10 km radius). As of 01 Oct 2020, 01:37 PM PDT, there have been 632 events recorded after the M4.9.
• The SCSN has so far recorded over 700 events in the swarm, the smallest M1.1. Many of the smaller events are still being analyzed and will be gradually added to the catalogue.
• More activity is expected in the next few days. There is a small chance (about 5%) that a larger quake could occur, with the likelihood decreasing over time.

The Westmorland area where the earthquake swarm is located in an active seismic region and is known for its swarms.

This particular swarm happened in the Brawley Seismic Zone of the Imperial Fault, which connects to the large San Andreas fault. San Andreas is perhaps one of the most dangerous and active zones.

Seismologists have noticed that many earthquakes focused in the same area can sometimes lead to a bigger one. 

On the other hand,  a report from the U.S. Geological Survey (USGS) stated, "The clustered earthquakes “are located in an area of diffuse seismic activity between the San Andreas fault in the north and the Imperial fault to the south.” 

This summer, California has been fighting raging wildfires. Now the earthquakes are making news. The Southern California Seismic Network (SCSN) has identified what appears to be the epicenter of the activity. 

About earthquake swarm

Some of the most studied previous swarm phenomena took place in 1991 and 2012.

Recently scientists were able to determine that oil and gas wastewater disposal wells or the underground injection of fluids can cause these earthquake swarms. This was documented in 2009, especially in parts of Oklahoma.

They also discovered that an earthquake swarm migrates along faults, like volcanos or oil and gas geothermal operations. The swarms move faster or slower in differing environments, possibly because of the variations of underground fluid production rates. 

According to the USGS, earthquake swarms have been known to last up to 20 days. But they typically last from 7 to 10 days.

Earthquake activity measured

On Thursday Caltech and the USGS published a report specific to the Westmorland earthquake swarm. 

They measured the earthquakes' intensities and found that the smallest magnitude was 1.1, and the highest was 4.9. Most of the tremors were below 3.0 magnitude, but several earthquakes registered over 4.5 magnitudes. 

Magnitude measures energy released at the epicenter or source of the earthquake, the U.S. Geological Survey explained. It is more accurate than the old Richter scale measurements.

The impacted area can be viewed on an interactive map.

Seismologist, Dr. Lucy Jones said that more than 45 earthquakes took place in the area within a two and a half-hour period on Wednesday. Jones described the shaking phenomenon as "one of the largest swarms we have had in the Imperial Valley."

The Imperial Valley has been identified as one of the most active seismic regions in California. The USGS reported on Friday that a dozen quakes hit the area since midnight.

The earthquake swarm continues to rattle the Westmoreland-Salton Sea area. Hundreds of smaller earthquakes are expected, many are too small to be felt by residents.

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