Hundreds of earthquakes swarm southern California raising fears

Hundreds of small earthquakes swarm southern California raising fears that a much larger quake will be triggered as experts raise the odds one could hit in the next week

  • At least 240 earthquakes were recorded near Westmorland in southern California by Wednesday night
  • Dozens more sizeable earthquakes continued into Thursday
  • The swarm of quakes has sent light shaking into San Diego County
  • So far, many of the tremors have registered around 3.0 magnitude or lower
  • The largest was magnitude 4.9 reported on Wednesday evening 
  • The swarm has sparked fears that a significantly larger earthquake could be triggered in the area in the next seven days 

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 occurring. 

‘Earthquakes of this size could cause damage around the area close to the earthquakes that have already occurred and would be followed by aftershocks that would increase the number of smaller earthquakes per day,’ they said. 

‘This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.’

The USGS warned Thursday that the swarm of earthquakes had continued

The agency said, however, that the possibility of a larger quake was still low

They noted that swarms such as this have occurred before in the area without a larger quake

The least likely scenario is that a much larger earthquake (magnitude 7 or higher) could occur within the next 7 days. 

‘The ongoing swarm could trigger an earthquake significantly larger than the M4.9 that occurred on the 30 September (i.e., M7.0 and above),’ they warned. 

‘While this is a very small probability, if such an earthquake were to occur, it would have serious impacts on communities nearby and would be followed by aftershocks that would increase the number of smaller earthquakes per day.’ 

The agency added that in a typical week, there is approximately a 1 in 3000 chance of a magnitude 7+ earthquake in the vicinity of this swarm but the current probability of larger earthquakes in this region is significantly greater than usual – about 1 in 300. 

The USGS said they will continue to update as the data changes but the swarm already appears to be decreasing in activity. 

There were four magnitude 3 earthquakes recorded in the first seven hours of Thursday compared to 40 magnitude 3 and larger earthquakes recorded in the last 7 hours of Wednesday. 

Earlier in the week, three small tremors rattled Milpitas on Tuesday for the second time in three days. 

The USGS reported that a 2.7 magnitude earthquake with a depth of 4.5km hit Milpitas around 8.16am Tuesday morning. 

Ten minutes later, a 2.9 magnitude tremor with a depth of 4km hit the same area. 

The third quake struck around 9.02am with a magnitude of 3.3 and a depth of 4.3km. 

Dozens of people in Fremont, Milpitas and San Jose felt the quakes, which occurred on the Calaveras fault system. 

Last weekend, two small tremors also struck Milpitas. 

According to the USGS, a 3.4-magnitude quake with a depth of 5.3km hit around 3.54pm on Sunday. Hours later at 10.16pm, another 3.4-magnitude quake hit the area. 

Hundreds of earthquakes were registered this week in the Salton Trough, pictured

And on September 19, a magnitude 4.6 earthquake struck Southern California. 

The earthquake hit around 11.40pm, about 2 miles outside of South El Monte, near Los Angeles, the agency reported. Preliminary reports indicate it was about 11 miles deep.

There were also about 20 earthquakes reported in August alone. 

Meanwhile, California is also dealing with wildfires that have devoured parts of the state amid unseasonably hot and dry weather. 

High winds that spread new fires this week in the Napa and Sonoma wine country and in a far northern area of the state were reduced to breezes, but vegetation remained ripe for burning in high temperatures amid very low humidity.

The National Weather Service said the weather conditions would last for several days due to high pressure centered over the state. 

Heat advisories were in effect or pending along about three-quarters of the California coast and many areas had poor air quality due to smoke.

Numerous studies in recent years have linked bigger wildfires in America to global warming from the burning of coal, oil and gas, especially because climate change has made California much drier. A drier California means plants are more flammable.

About 70,000 people were under evacuation orders in the wine region north of San Francisco where the Glass Fire has incinerated dozens of homes along with winery installations and other buildings.

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