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Unpacking CalVO's new seismic monitoring boxes

If you've noticed any changes to the earthquake counts released in CalVO's weekly updates, don't worry - some behind-the-scenes improvements to our monitoring system have been implemented which allow us to focus on unrest specifically related to our volcanoes and volcanic regions.

Scientists who monitor volcanoes often rely heavily on automatic detections of earthquakes as one of the first warning signs that a volcano is acting up. We set up computer scripts that count up the number of earthquakes and send us alerts that wake us up in the middle of the night. Obviously, we don’t want to be woken up for earthquakes that are happening far away from our volcanoes, so we create alert boxes around the areas we’re confident have a chance of causing earthquakes that might be a sign of unrest. 

In some places, it’s easy to just draw a simple box or circle around a volcano and be pretty sure that whatever earthquake is happening there might be because of the volcano. For many of California’s volcanoes, it isn’t so simple. There can also be nearby fault zones and geothermal areas that produce a lot of seismicity, but are not good indicators of unrest. Therefore, we have to draw more complicated shapes (polygons). 

When we first started releasing monthly earthquake counts, polygons were drawn around each of California’s volcanoes and nearby tectonic and geothermal areas. Earlier this year, CalVO seismologists decided, for our internal monitoring purposes, that the polygons needed to be updated to better reflect the areas where we would typically expect volcanic unrest. This month we decided to start using the new polygons for our weekly updates as well. 

In the following animated GIFs, the blue box is the initial area where earthquakes are queried from several earthquake catalogs. It's mostly useful for the seismologists to check on what’s going on around—but not at—the volcano. The black or red boxes are the area where earthquakes are actually counted, and are chosen based on a combination of geological information, such as the footprint of a volcanic field and decades of historical seismicity patterns. The red dots are earthquakes from 2019, and give an idea of where recent seismicity is clustered. 

Animated GIF of Clear Lake volcanic field monitoring extents, cycling between two maps with polygons enclosing earthquake zones.
The Clear Lake polygon was extended to include pockets of deep seismicity on the north and northwest sides of the lake. The box for the Geysers area is also slightly larger to reflect the most recent extent of earthquakes in that area.
Animated GIF of Lassen Volcanic Center monitoring extents, cycling between two maps with polygons enclosing earthquake zones.
The new Lassen monitoring extent was enlarged to include deep seismicity to the west of the volcano. The blue query area was also greatly extended to the north to monitor the Hat Creek region. 
Animated GIF of Mount Shasta monitoring extents, cycling between two maps with polygons enclosing earthquake zones.
At Mount Shasta, a separate polygon was added to include the Tennant area, and the polygons for Shasta and Medicine Lake were reshaped to fit. 

The two biggest changes are to the Long Valley Volcanic Center and Coso Volcanic Field: 

Animated GIF of Long Valley volcanic region monitoring extents cycling between 2 maps with polygons enclosing earthquake zones.
Animated GIF of Coso volcanic field monitoring extents, cycling between two maps with polygons enclosing earthquake zones.

At Long Valley, we enlarged the area around Mammoth Mountain to capture earthquakes that were previously missed on the north flank, near Red Cones, and deeper pockets of seismicity to the west. We also closed the gaps between the Mammoth, Long Valley, and Mono-Inyo polygons.  

For Coso, the original polygon extended far to the south because this is where some of our monitoring equipment is. After the 2019 Ridgecrest earthquake the old polygon contained a huge number of tectonic aftershocks unrelated to the volcanic field itself. The new, smaller polygon now more closely matches the actual footprint of the volcanic field. Earthquake counts for Coso will now contain fewer aftershocks of the Ridgecrest earthquake to the south, though some will still be counted from the northern end of the Ridgecrest region. 

The result of these changes is that the number of earthquakes we report each week for these volcanic areas will be slightly different than they were before, but more reflective of unrest specifically related to at each of our volcanoes. 

For questions about monitoring, specific earthquakes in volcanic areas, or how CalVO deals with alerts, contact

Update written by Alicia Hotovec-Ellis, USGS CalVO

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