Hundreds of small earthquakes continue to rattle southern California, sending light shaking into San Diego County, as experts raise the odds that a much larger tremor could strike in the coming days.
The U.S. Geological Survey (USGS) reported that hundreds of earthquakes had been reported by Wednesday night in the Salton Trough near Westmorland and more than a dozen more sizeable earthquakes on Thursday.
The swarm has registered only a few 4.5 magnitude earthquakes with many of the tremors falling below 3.0 magnitude. The largest so far was a magnitude 4.9.
The USGS has now listed the possibly of a magnitude 7 or higher earthquake hitting in the next few days as one in 300 as a result of the recent activity.
Hundreds of earthquakes swarm southern California raising fears that a larger one is coming. The map above shows the large number of quakes that have hit the area in the past day
The U.S. Geological Survey shared a map showing the location of previous swarms
Tectonic plates are seen north of the Salton Sea where the current swarm is hitting
'In just 2.5hr Westmorland swarm has had 45 quakes of M≥3.0. One of the largest swarms we have had in the Imperial Valley - and it is historically the most active swarms in SoCal,' seismologist Dr Lucy Jones wrote on Twitter on Wednesday night.
The agency had recorded at least 240 quakes by that point.
Jones warned, however, that there was no reason to panic about a larger quake caused by the string of smaller ones as there are no large fault lines near the Imperial Valley line.
'Quakes make other quakes more likely, but only nearby. Today's M4.9 fault is ~2 miles across, so it can affect quakes within ~5 miles,' she wrote.
'The only faults nearby are small. In other words, there is no scientific reason to predict a big quake in another location today.
'There is an increased risk at the same location that has already had quakes. More M3s tonight are very likely. A M5 is possible. But only at the same location. A M5 in Imperial County has no impact in San Diego, Los Angeles, or San Francisco. Swarms in Imperial Co are normal,' Jones added.
The swarm of earthquakes began on September 30 in the Brawley seismic zone. The area is described as one of diffuse seismic activity between the San Andreas fault in the north and the Imperial fault to the south.
There were previous swarms seen there, including the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake.
The first swarm of 30 earthquakes rattled the area for one hour starting at 4pm Wednesday.
USGS data shows that the first quake, a 3.0, hit near Westmorland, and that another 27 followed in the same area.
The cluster of quakes can be seen to the bottom right beneath the Salton Sea
Seismologist Dr Lucy Jones said there is no reason to fear a larger quake on the way
More than half were magnitude 3.0 or greater, the agency said. Michigan Tech reports that between 2.5 and 5.4 magnitude are often felt but rarely cause much damage.
The largest felt was a 4.9 magnitude quake which struck at 5.31pm.
According to the USGS, there is a 90 percent change that the rate of earthquakes in the swarm will decrease over the next 7 days.
Localized damage could be caused to weak structures if they reach as high as magnitude 5.4 but many will range around magnitude 3.0, which may be felt by people close to the epicenters.
The second most likely scenario is that a larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.
The agency gives a ten percent chance of this