USGS Report Regarding Ongoing Swarm that Followed M5.3 Earthquake in Soda Springs, Idaho

Be sure to read the 3 scenarios further on in the article.

via USGS:

On September 2, 2017, there was a M5.3 earthquake east of Soda Springs, Idaho. It caused moderate shaking over a broad area of southeastern Idaho, northern Utah, and western Wyoming.

The earthquake has been followed by a sustained and highly active sequence of smaller earthquakes (aftershocks), more than we would typically observe for earthquakes of this size. Because of this, we are providing earthquake forecast scenarios, below, that we’ll update regularly as long as the sequence continues.

The cumulative moment release of the Soda Springs sequence as a function of time (as of September 16, 2017), with the most prominent earthquakes indicated by magnitude (M). The moment release is a measure of the amount of energy released from an earthquake.

Magnitudes of the Soda Springs sequence (as of September 16, 2017) as a function of time. The “beachballs” are focal mechanisms indicating the direction of slip during the earthquake rupture.

About the M5.3 Mainshock
The M5.3 earthquake occurred as the result of normal faulting within the shallow crust on a fault dipping at an intermediate angle either to the west or to the east. This faulting style is typical of earthquakes located in the Intermountain Seismic Belt, a prominent NS-trending zone of seismicity in the Intermountain West, and a region of moderate-to-high seismic hazard. This region is characterized by movement along north-trending, east- and west-dipping range-bounding normal faults that accommodate gradual horizontal extension of the Earth’s crust.

Earthquakes occur frequently in the Intermountain Seismic Belt, and it is unlikely that this sequence is related to the Yellowstone volcanic region which lies over 200 km to the northeast.

Past Earthquakes in This Area
South and Central Idaho have experienced at least 14 other M5+ earthquakes within 300 km of the September 2, 2017 earthquake over the preceding century. The largest was the October 28, 1983, M6.9 Borah Peak earthquake, which struck about 250 km to the northwest of the September 2, 2017 earthquake. The Borah Peak earthquake is the largest known to have occurred in Idaho, and resulted in two fatalities, two injuries, and considerable damage in Challis, Idaho.

Earthquake Forecast
The M5.3 Soda Springs Sequence is particularly active, producing more aftershocks on average than other earthquakes of this magnitude. In terms of future larger earthquakes in this area:

When there are more earthquakes, the chance of a large earthquake is greater, which means that the chance of damage is greater.
Idaho and Montana experience earthquakes. While earthquakes are not that common, it is also not surprising to see earthquakes in these areas.

This sequence may have damaging earthquakes in the future, so remember to: Drop, Cover, and Hold on if you feel the ground shaking.
Due to the active and ongoing nature of this sequence, we have developed an earthquake forecast for continuing seismicity. No one can predict the exact time or place of any earthquake, including aftershocks. What our earthquake forecasts do is give us an understanding of the chances of having more earthquakes within a given time period. We calculate this earthquake forecast using a statistical analysis based on past earthquakes in similar tectonic environments, as well as the aftershocks recorded to date for this sequence.

Our forecast changes as time passes due to the decay in the frequency of aftershocks, larger aftershocks that reinvigorate the sequence, and changes in forecast modeling based on the earthquake data collected. Similar to weather models, our models will also change with more data and information. The longer this sequence goes on, the more we learn about what it will do in the future.

We have taken a detailed forecast and developed three scenarios that we consider to be the most likely. These scenarios are for the following month, but earthquakes will continue to be possible at later times. The forecast was last updated on Sept. 28, 2017. We will continue to update the forecast as more information becomes available.

Here are the three scenarios or possibilities for the month starting Sept. 28, 2017, based on earthquake forecast models:

Scenario #1 (most likely: 90-95% chance):
The sequence will continue to decay over the next month, which means there will be fewer earthquakes. Earthquakes above M3 may be felt by those in the area, and occasional spikes in activity may be accompanied by additional M4 or larger earthquakes, but with none larger than the M5.3 mainshock. While all earthquake sequences decay over time, there are several other possible outcomes, which are listed next.

Scenario #2 (less likely than Scenario #1 but possible with 5-10% chance):
A similar sized or larger earthquake than the M5.3 mainshock may occur. This situation is often referred to as a “doublet” when a similar sized earthquake follows the original earthquake that kicked off the sequence. Doublets have occurred in places around the world, but they are not very common.

Scenario #3 (the least likely scenario but still possible with less than 1% chance):
A much larger earthquake than M5.3 could occur, up to and including the M7 range, in which case we would call what has happened prior to any larger earthquake a foreshock sequence. We have seen this happen in other places around the world, with the most notable being L’Aquila, Italy in 2009. It is important to understand that this is a highly unlikely scenario, but we cannot ignore the possibility of this occurring.

This forecast is for one month, but the sequence may continue past that timeframe. The probabilities will be updated periodically.

Here is a link to the USGS report in case you would like to see the full report.