SHALLOW GEOPHYSICAL CHARACTERIZATION OF THE MT. ST. HELENS EDIFICE

We performed multiple surface wave dispersion measurements on the volcanic edifice of Mount Saint Helens, Washington to obtain a velocity sounding applicable to landslide hazard studies. We analyzed multiple sites, both on the crater rim and in the crater itself, studying andesitic lava flows, block and ash flows, pyroclastic flows, landslide deposits and dacitic domes. Mount Saint Helens provides a unique opportunity to correlate velocity structure with the exposed geology, as the landslide and subsequent eruption have carved out an avalanche caldera 1000 m high. We have found surface wave dispersion to be an accurate method of determining velocities to 100 m depth, using 20 sensors and a line length of up to 600 m. The correlation between the geology as seen on the caldera wall and the modeled velocity structure is generally good. Reftek RT-125 instruments (Texans) provide a relatively easy way to perform geophysical surveys in previously inaccessible regions. With no active seismic source needed and only 0.5-1.0 hour of recording time per station, large areas can be characterized rapidly. The combination of surface wave dispersion and highly portable instrumentation is very effective for volcanological studies. Volcanic edifices can now be characterized to 100 m depth for their structural stability. This is important in the case of glaciated volcanoes where the geology is often obscured. Such data can be combined with geologic data and geotechnical testing to provide a more complete analysis of edifice stability in the Cascades and abroad.