Mapping the ongoing Mt. St. Helens Earthquake Swarm
Using the techniques outlined in the article, I have plotted the earthquake swarm that Mt. St. Helens has been experiencing in recent days.
Earlier today, the USGS Cascades Volcano Observatory released a press announcement, including a warning of possible explosions. The press release can be found here:
http://www.pnsn.org/NEWS/PRESS_RELEA...H_09_2004.html
The latest earthquakes were taken from:
http://www.geophys.washington.edu/re...Helens_eqs.htm
This is updated regularly. The maps below were produced with data taken on Mon Sep 27 19:00:10 PDT 2004, and includes earthquakes back to 14th September.
My overview map of central Washington State is too large to display here, but can be found at the following URL:
http://www.winwaed.com/public/mapima...ens_270904.gif
This plots earthquake hazard (red/orange = relatively low; through to blues and purples = relatively high). Washington experiences a lot of earthquakes. Although some are volcanic, most are related to subduction along the west coast. Hence most of Washington's large earthquakes occur near the coast or in the Puget Sound area.
Zooming in on the Mt. St. Helens area, gives the following map:
Each earthquake is represented by a circle. Circle size represents the magnitude (energy) of the earthquake, and ranges from 1 (the smallest) to 3 (the largest). Notice that virtually all of the earthquakes are located under the lava dome. This is shown better in the following map:
Mt. St. Helens has a lava dome that has developed in the main crater that was created by the 1980 eruption. The present summit is on the southern rim of the crater. The above map clearly shows that virtually all of the earthquakes are occurring beneath the lava dome and not the summit.
The earthquake data also includes depth data for the earthquakes, and this is plotted in the following map:
Depths range from 26km (darkest) to 0.1km (white). The following map zooms in on Mt St Helens:
Comparing the two maps, notice that the main earthquake swarm under the lava dome consists of very shallow earthquakes.
The earthquakes are volcanic in origin. We can see this by their close correlation with the lava dome. They are also very shallow. Tectonic earthquakes (eg. from plate activity) tend to have a deeper mean depth. Also, the warning from the Cascade's Observatory describes some of the earthquakes as being of a type that probably involves fluids (pressurised water and steam, or magma). These earthquakes are probably showing a net volume change - something that should not occur with a pure tectonic/fault earthquake.
I will post further maps of Mt St Helens (or elsewhere) according to events.
Note: the hazard background map in the above maps, is plotted at a resolution of 0.008 degrees although the USGS data is 0.5 degrees. Interpolation and plotting was performed using the Grid Imp Add-In (
http://www.winwaed.com/products/mappoint/gridimp.shtml ).