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Earthquake depths and the eruption of March 8th 2005 The earthquakes at Mount St. Helens might seem like they're getting deeper since the March 8th, 2005 eruption. Please be advised that this apparent change may only come from greater errors in earthquake locations. The errors are due to the loss of several seismic stations in the crater. They do not necessarily reflect a change in true earthquake depths. A bit more detail: The PNSN lost three seismic stations to the eruption of March 8th 2005. All three stations were located in the crater, very close to where the earthquakes were occuring. Because these stations were the closest to the recent Mount St. Helens earthquakes, their data allowed us to constrain earthquake depths more accurately. Without them, earthquake depths aren't as accurate, and some of the new earthquakes seem to be slightly deeper. The reader might ask why we don't also see shallower earthquakes, if the changes in depths are due to increasing errors. That is a very a good question. The answer is that the Mount St. Helens earthquakes were already practically at the surface -- so shallow, in fact, that our programs could not locate them any higher, or they would be in mid-air! Fortunately, our programs have specific controls to keep earthquake locations under the ground, where they belong. So, even as the location errors increase, the earthquakes cannot be shallower.

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