HR: 08:00h
AN: NS21A-01 INVITED    [Abstracts]
TI: Shallow Geophysical Characterization of the Mount St. Helens Edifice
AU: * Thelen, W A
EM: wethelen@ess.washington.edu
AF: Earth and Space Sciences, University of Washington Box 351310, Seattle, WA 98195,
AU: Pullammanappallil, S
EM: satish@optimsoftware.com
AF: Optim, University of Nevada - MS 174 Seismology Lab 1664 N. Virginia St., Reno, NV 89557,
AU: Louie, J N
EM: louie@seismo.unr.edu
AF: Nevada Seismological Laboratory, MS 174 1664 N. Virginia St., Reno, NV 89557,
AB: Multiple Refraction Microtremor (ReMi) transects were completed on Mount St. Helens, Washington to obtain shear-velocity soundings valuable to the generation of a shallow velocity model for earthquake location. Prior to the 2004 eruption of Mount St. Helens, the 1-D velocity model used to locate earthquakes used a 4.6 km/s velocity to depths of 2.2 km. Given the volume of pyroclastic flows and altered dacite domes exposed in the crater walls, the velocity model was overestimating the shallow velocities. The result was reduced, or no depth constraint for shallow earthquakes preceding the 2004 eruption, which greatly hindered early interpretations. Surface-wave sources included off-end, shallow (<1 km) volcanic earthquakes and rockfalls from the over-steepened horseshoe crater. Transects with 15-20 channels and transect lengths of 300-400m constrained shear velocities in excess of 100 m. These transects were used on erosional scarps and in areas with known geology to test the precision of the ReMi technique in a volcanic setting. Our transects sampled andesite, basalt, pyroclastic flows, landslide deposits, and dacitic domes. The correlation between the mechanical strength of the observed geology on erosional scarps, and modeled shear-velocity structure was generally good. A 36 channel, 4 km long transect was conducted, with a combination of 1 Hz and 4.5 Hz sensors, to determine a suitable velocity model for locating shallow earthquakes. Results show that the transect constrained shear velocities to depths below 2 km depth. The averaging quality of surface-wave techniques is advantageous for the construction of 1-D velocity models for earthquake location since a velocity model averaged over the entire edifice reduces the use of large static station corrections at any one station. Indeed, when the surface-wave derived velocity model is applied to shallow earthquakes at Mount St. Helens, the station corrections decrease and the depth errors are improved. This application of surface-wave dispersion with passive sources is potentially applicable on volcanoes worldwide, where accurate velocity models are lacking. An additional utility of this technique may be in the use of landslide hazard studies, where potentially weak rock is obscured below the surface. ReMi data collected on a hazardous volcano could be combined with geologic observations and geotechnical testing to provide a more thorough analysis of edifice stability.
DE: 0935 Seismic methods (3025, 7294)
DE: 7255 Surface waves and free oscillations
DE: 7280 Volcano seismology (8419)
DE: 8488 Volcanic hazards and risks
DE: 8494 Instruments and techniques
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting