Earthquake Forecast for the Westmorland Swarm beginning Sept. 30, 2020

Release Date:

A swarm of earthquakes southeast of the Salton Sea, in the Brawley seismic zone, began on Sept. 30, 2020.

Forecast last updated October 7, 2020 at 8:00am PDT.

October 7, 2020

map of swarm area showing earthquakes as circles

Screenshot of Latest Earthquakes (as of October 7, 8:00am PDT) showing the Westmoreland Earthquake Swarm that began on the afternoon of Sep 30, 2020. Earthquakes are shown as circles with the most recent in red. (Public domain.)

We have been carefully monitoring a swarm of earthquakes southeast of the Salton Sea in the Brawley seismic zone that began on Sept. 30, 2020.  The swarm activity has stabilized at a low rate, with only two magnitude 3 or larger earthquakes recorded since the morning of Oct. 3rd. There have been over 60 magnitude 3 and larger earthquakes since the beginning of this swarm, most of which (46) occurred in the first burst of activity on the evening of Sept. 30th. The largest earthquake that has occurred, as of this release, is a magnitude 4.9 at 5:31 PM PDT on Sept. 30th. This earthquake swarm is located in an area of diffuse seismic activity between the San Andreas fault to the north and the Imperial fault to the south. This area has seen swarms in the past – notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake.  Past swarms have remained active for 1 to 20 days, with an average duration of about a week.

Since the swarm activity has stabilized, we will no longer be updating this forecast on a daily basis. However, this does not mean that the risk of a larger earthquake is gone. Both the San Andreas Fault to the north, and the Imperial Fault to the south are capable of rupturing in large magnitude earthquakes (magnitude 7+). The Imperial Fault hosted a magnitude 6.4 earthquake in 1979, and a magnitude 6.9 earthquake in 1940. The southern San Andreas Fault last ruptured in a M7+ earthquake about 300 years ago.

In a typical week, there is approximately a 1 in 3,000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. As noted below, the likelihood of a large earthquake has now returned to approximately this typical long-term level. This swarm did not decrease the potential for a larger earthquake in the future and Southern California continues to be one of the most seismically active areas in the nation.

While the swarm appears to have quieted down for the moment, this area may have future bursts of activity that will impact the probabilities discussed below and prompt additional forecast updates. We will update our forecast if swarm activity increases, or if larger earthquakes occur.

The following three scenarios describe possibilities of what could happen from 7 October to 14 October.

Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

1. Scenario One (Most likely: 99%):  Earthquakes occur as normal but none will be larger than magnitude 5.4 within the next 7 days.

The most likely scenario is that the rate of earthquakes in the swarm will remain low. Additional moderately sized earthquakes (M4.5 to 5.4) could occur, which could cause localized damage, particularly in weak structures. Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

2. Scenario Two (Less likely: <1%): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

A less likely scenario is that a somewhat larger earthquake could occur (up to a M6.9). 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

3. Scenario Three (Least Likely: approximately 1 in 3000):  A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

A much less likely scenario, compared with the previous two scenarios, is that 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). 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.

What people can do about earthquakes

The U.S. Geological Survey advises everyone to be aware of the possibility of future earthquakes, especially when in or around vulnerable structures such as unreinforced masonry buildings. This swarm may lead to larger and potentially damaging earthquakes in the future, so remember to: Drop, Cover, and Hold on if you feel shaking or receive an earthquake alert powered by the ShakeAlert earthquake early warning system. When there are more earthquakes, the chance of a large earthquake is greater, which means that the chance of damage is greater.

Please refer to preparedness information provided by your local and state emergency management offices.

About our earthquake forecasts

No one can predict the exact time or place of any earthquake, including aftershocks or events in swarms. Our earthquake forecasts give us an understanding of the chances of having more earthquakes within a given time period in the affected area. We calculate this earthquake forecast using a statistical analysis based on past earthquakes.

One uncertain aspect of this swarm is how long the elevated earthquake activity will last.  We include this uncertainty in swarm duration in our forecasts.  The chance of large earthquakes will remain elevated as long as the swarm continues.  About half of the swarms in this area are over within a week.  We will update our forecast as swarm activity increases or decreases, or if larger earthquakes occur.

We are carefully monitoring activity throughout the region and will continue to provide information to help people stay safe and care for themselves and each other.

Updated forecasts will be released on the USGS Earthquake Hazard Program website.
For media inquiries, please contact Ryan McClymont at rmcclymont@usgs.gov

For More Information

 

October 6, 2020

map showing swarm area with earthquakes as circles

Screenshot of Latest Earthquakes (as of October 6, 8:00am PDT) showing the Westmoreland Earthquake Swarm that began on the afternoon of Sep 30, 2020. Earthquakes are shown as circles with the most recent in red. (Public domain.)

A swarm of earthquakes southeast of the Salton Sea, in the Brawley seismic zone, began on Sept. 30, 2020.  The swarm continues to show a small amount of activity, with several magnitude 2 earthquakes, but no magnitude 3 or larger earthquakes recorded since Oct. 3rd. There have been over 60 magnitude 3 and larger earthquakes since the beginning of this swarm, most of which (46) occurred in the first burst of activity on the evening of Sept. 30th. The largest earthquake that has occurred, as of this release, is a magnitude 4.9 at 5:31 PM PDT on Sept. 30th. This earthquake and the associated swarm 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 area has also seen swarms in the past –notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake.  Past swarms have remained active for 1 to 20 days, with an average duration of about a week.  The current swarm is occurring about 20 kilometers (12 miles) to the south of the swarm that occurred near Bombay Beach in August 2020.

In a typical week, there is approximately a 1 in 3000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. During this earthquake swarm, the probability of larger earthquakes in this region is slightly greater than usual – about 1 in 2500.  The swarm continues to evolve, and we expect to update this forecast with more specific probability information as we collect more data.

The following three scenarios describe possibilities of what could happen from 6 October to 13 October.

Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

1. Scenario One (Most likely: 99%):  Earthquakes continue, possibly including earthquakes up to magnitude 5.4.

The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderately sized earthquakes (M4.5 to 5.4) may occur, which could cause localized damage, particularly in weak structures.  Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

2. Scenario Two (Less likely: 1%): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

A less likely scenario is that a somewhat larger earthquake could occur (up to a M6.9). 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

3. Scenario Three (Least Likely: approximately 1 in 2500):  A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

A much less likely scenario, compared with the previous two scenarios, is that 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). 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.

 

October 5, 2020

map of swarm area showing earthquakes as circles

Screenshot of Latest Earthquakes (as of October 5, 8:00am PDT) showing the Westmoreland Earthquake Swarm that began on the afternoon of Sep 30, 2020. Earthquakes are shown as circles with the most recent in red. (Public domain.)

A swarm of earthquakes southeast of the Salton Sea, in the Brawley seismic zone, began on Sept. 30, 2020.  The swarm continues to show a small amount of activity, with several magnitude 2 earthquakes, but no magnitude 3 or larger earthquakes recorded yesterday Oct. 4th. There have been 62 magnitude 3 and larger earthquakes since the beginning of this swarm, most of which (46) occurred in the first burst of activity on the evening of Sept. 30th. The largest earthquake that has occurred, as of this release, is a magnitude 4.9 at 5:31 PM PDT on Sept. 30th. This earthquake and the associated swarm 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 area has also seen swarms in the past – notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake.  Past swarms have remained active for 1 to 20 days, with an average duration of about a week.  The current swarm is occurring about 20 kilometers (12 miles) to the south of the swarm that occurred near Bombay Beach in August 2020.

In a typical week, there is approximately a 1 in 3000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual – about 1 in 1500.  The swarm continues to evolve, and we expect to update this forecast with more specific probability information as we collect more data.

The following three scenarios describe possibilities of what could happen from 5 October to 12 October.

Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

1. Scenario One (Most likely: 98%):  Earthquakes continue, possibly including earthquakes up to magnitude 5.4.

The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderately sized earthquakes (M4.5 to 5.4) may occur, which could cause localized damage, particularly in weak structures.  Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

2. Scenario Two (Less likely: 2%): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

A less likely scenario is that a somewhat larger earthquake could occur (up to a M6.9). 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

3. Scenario Three (Least Likely: approximately 1 in 1500):  A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

A much less likely scenario, compared with the previous two scenarios, is that 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). 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.

 

October 4, 2020

map of swarm area showing earthquakes a circles

Screenshot of Latest Earthquakes (as of October 4, 8:00am PDT) showing the Westmoreland Earthquake Swarm that began on the afternoon of Sep 30, 2020. Earthquakes are shown as circles with the most recent in red.  (Public domain.)

A swarm of earthquakes southeast of the Salton Sea, in the Brawley seismic zone, began on Sept. 30, 2020.  The swarm continues to show a small amount of activity, with two more magnitude 3 and larger earthquakes recorded yesterday Oct. 3rd. There have been 64 magnitude 3 and larger earthquakes since the beginning of this swarm, most of which (48) occurred in the first burst of activity on the evening of Sept. 30th. The largest earthquake that has occurred, as of this release, is a magnitude 4.9 at 5:31 PM PDT on Sept. 30th. This earthquake and the associated swarm 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 area has also seen swarms in the past – notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake.  Past swarms have remained active for 1 to 20 days, with an average duration of about a week.  The current swarm is occurring about 20 kilometers (12 miles) to the south of the swarm that occurred near Bombay Beach in August 2020.

In a typical week, there is approximately a 1 in 3000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual – about 1 in 1000.  The swarm continues to evolve, and we expect to update this forecast with more specific probability information as we collect more data.

The following three scenarios describe possibilities of what could happen from 4 October to 11 October.

Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

1. Scenario One (Most likely: 97%):  Earthquakes continue, possibly including earthquakes up to magnitude 5.4.

The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderately sized earthquakes (M4.5 to 5.4) may occur, which could cause localized damage, particularly in weak structures.  Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

2. Scenario Two (Less likely: 3%): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

A less likely scenario is that a somewhat larger earthquake could occur (up to a M6.9). 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

3. Scenario Three (Least Likely: approximately 1 in 1000):  A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

A much less likely scenario, compared with the previous two scenarios, is that 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). 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.

 

October 3, 2020

map of Westmoreland swarm showing earthquakes as circles

Screenshot of Latest Earthquakes (as of October 2, 8:00am PDT) showing the Westmoreland Earthquake Swarm that began on the afternoon of Sep 30, 2020. Earthquakes are shown as circles with the most recent in red. (Public domain.)

A swarm of earthquakes southeast of the Salton Sea, in the Brawley seismic zone, began on Sept. 30, 2020. The swarm continues to show a small amount of activity today. There was only one magnitude 3 or larger earthquake recorded yesterday Oct. 2nd, compared to 13 magnitude 3 earthquakes on Oct. 1st and 48 magnitude 3 and larger earthquakes on the evening of Sept. 30th. The largest earthquake that has occurred, as of this release, is a magnitude 4.9 at 5:31 PM PDT on Sept. 30th. This earthquake and the associated swarm 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 area has also seen swarms in the past –notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake. Past swarms have remained active for 1 to 20 days, with an average duration of about a week. The current swarm is occurring about 20 kilometers (12 miles) to the south of the swarm that occurred near Bombay Beach in August 2020.

In a typical week, there is approximately a 1 in 3000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual – about 1 in 1000. The swarm continues to evolve, and we expect to update this forecast with more specific probability information as we collect more data.

The following three scenarios describe possibilities of what could happen from 3 October to 10 October.

Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

1. Scenario One (Most likely: 97%): Earthquakes continue, possibly including earthquakes up to magnitude 5.4.

The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderately sized earthquakes (M4.5 to 5.4) may occur, which could cause localized damage, particularly in weak structures. Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

2. Scenario Two (Less likely: 3%): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

A less likely scenario is a somewhat larger earthquake could occur (up to a M6.9). 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

3. Scenario Three (Least Likely: approximately 1 in 1000): A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

A much less likely scenario, compared with the previous two scenarios, is that 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). 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.

 

October 2, 2020

A swarm of earthquakes southeast of the Salton Sea, in the Brawley seismic zone, began on Sept. 30, 2020.  The swarm continues to show activity today, with some evidence of slowing down. There were 13 magnitude 3 or larger earthquakes recorded yesterday Oct. 1st, compared to an initial burst of 48 magnitude 3 and larger earthquakes on the evening of Sept. 30th. The largest earthquake that has occurred, as of this release, is a magnitude 4.9 at 5:31 PM PDT on Sept. 30th. This earthquake and the associated swarm 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 area has also seen swarms in the past –notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake.  Past swarms have remained active for 1 to 20 days, with an average duration of about a week.  The current swarm is occurring about 20 kilometers (12 miles) to the south of the swarm that occurred near Bombay Beach in August 2020.

In a typical week, there is approximately a 1 in 3000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual – about 1 in 500.  The swarm continues to evolve, and we expect to update this forecast with more specific probability information as we collect more data.

The following three scenarios describe possibilities of what could happen from 2 October to 9 October.

Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

1. Scenario One (Most likely: 93%):  Earthquakes continue, possibly including earthquakes up to magnitude 5.4.

The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderately sized earthquakes (M4.5 to 5.4) may occur, which could cause localized damage, particularly in weak structures.  Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

2. Scenario Two (Less likely: 7%): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

A less likely scenario is a somewhat larger earthquake could occur (up to a M6.9). 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

3. Scenario Three (Least Likely: approximately 1 in 500):  A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

A much less likely scenario, compared with the previous two scenarios, is that 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). 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.

     

    October 1, 2020

    A swarm of earthquakes southeast of the Salton Sea, in the Brawley seismic zone, began on Sept. 30, 2020.  The swarm continues today at a lower rate, with 4 magnitude 3 earthquakes recorded in the first 7 hours of today, compared to 40 magnitude 3 and larger earthquakes recorded in the last 7 hours of Sept. 30th. The largest earthquake that has occurred, as of this release, is a magnitude 4.9 at 5:31 PM PDT on September 30. This earthquake and the associated swarm 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 area has also seen swarms in the past –notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake.  Past swarms have remained active for 1 to 20 days, with an average duration of about a week.  The current swarm is occurring about 20 kilometers (12 miles) to the south of the swarm that occurred near Bombay Beach in August 2020.

    In a typical week, there is approximately a 1 in 3000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual – about 1 in 300.  The swarm continues to evolve, and we expect to update this forecast with more specific probability information as we collect more data.

    The following three scenarios describe possibilities of what could happen from 1 October to 8 October.

    Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

    1. Scenario One (Most likely: 90%):  Earthquakes continue, possibly including earthquakes up to magnitude 5.4.

    The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderately sized earthquakes (M4.5 to 5.4) may occur, which could cause localized damage, particularly in weak structures.  Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

    2. Scenario Two (Less likely: 10%): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

    A less likely scenario is a somewhat larger earthquake could occur (up to a M6.9). 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

    3. Scenario Three (Least Likely: approximately 1 in 300):  A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

    A much less likely scenario, compared with the previous two scenarios, is that 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). 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.

     

    September 30, 2020

    map showing earthquakes as circles near Salton Sea

    Screenshot of Latest Earthquakes (as of the evening of September 30, 2020) showing the Westmoreland Earthquake Swarm beginning Sep 30, 2020. Earthquakes are shown as circles with the most recent in red. (Public domain.)

    The largest earthquake that has occurred, as of this release, is a magnitude 4.9 at 5:31 PM PDT. This earthquake and the associated swarm 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 area has also seen swarms in the past –notably the 1981 Westmorland swarm, which included a M5.8 earthquake, and the 2012 Brawley swarm, which included a M5.4 earthquake.  Past swarms have remained active for 1 to 20 days, with an average duration of about a week.  The current swarm is occurring about 40 kilometers (25 miles) to the south of the swarm that occurred near Bombay Beach in August 2020.

    In a typical week, there is approximately a 3 in 10,000 chance of a magnitude 7+ earthquake in the vicinity of this swarm. During this earthquake swarm, the probability of larger earthquakes in this region is significantly greater than usual.  Currently, the swarm is rapidly evolving, and we expect to update this forecast with more specific probability information as we collect more data.

    The following three scenarios describe possibilities of what could happen from 30 September to 7 October.

    Only one of these scenarios will occur within the next week. These scenarios include the possibility of earthquakes on and off the Imperial and San Andreas Faults.

    1. Scenario One (Most likely):  Earthquakes continue, possibly including earthquakes up to magnitude 5.4.

    The most likely scenario is that the rate of earthquakes in the swarm will decrease over the next 7 days. Some additional moderately sized earthquakes (M4.5 to 5.4) may occur, which could cause localized damage, particularly in weak structures.  Smaller magnitude earthquakes (M3.0+) may be felt by people close to the epicenters.

    2. Scenario Two (Less likely): A larger earthquake (magnitude 5.5 to 6.9) could occur within the next 7 days.

    A less likely scenario is a somewhat larger earthquake could occur (up to a M6.9). 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. This scenario occurred in a previous swarm in the area – in 1981, when a swarm in this region included a magnitude 5.8 earthquake.

    3. Scenario Three (Least Likely):  A much larger earthquake (magnitude 7 or higher) could occur within the next 7 days.

    A much less likely scenario, compared with the previous two scenarios, is that 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). 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.