Subsurface characterization and liquefaction potential assessment of the Spirit Lake debris blockage at Mount St. Helens, Washington using multi-method geophysics

Abstract

In assessing geologic hazards, geophysical methods are commonly used in the development of a robust geologic model of the subsurface. To study subsurface soil characterization and assess liquefaction potential of the debris-avalanche trigged by the eruption of Mount St. Helens on May 18th, 1980, I used ground penetrating radar and active seismic methods. Characterization of the subsurface is particularly important at this study site because the debris-avalanche now serves as a natural dam to the regional basin lake, Spirit Lake, with potentially catastrophic results if the dam fails. The main failure mode for the dam is overtopping followed by rapid, episodic erosion and seepage erosion. Since the region around Mount St. Helen is prone to earthquakes, liquefaction also poses a serious geologic hazard. The boundary between the debris-avalanche and overlying tephra was imaged using ground penetrating radar. Water table depths and deposits were identified from seismic refraction data. Depth to the water table depth varies between 20 and 55 m. I use seismic surface wave data for delineating the boundary between the debris-avalanche and overlying tephra and modeling liquefaction potential. The southern end of the geophysical survey area shows thick regions of low-strength pyroclastic material overlying the debris-avalanche which can be significantly impacted by erosion. Liquefaction modeling revealed there is not a significant risk of liquefaction with moderate seismic activity.The geophysical subsurface investigation identified the boundary between the debris-avalanche and overlying pyroclastic, blast and ash cloud deposits. A deep pocket of ~15 m thick low shear wave velocity material along the dam crest was identified as highly erodible material. P-wave velocity from seismic refraction data successfully located the water table which at a depth varying between 20 and 55 m below the surface. Liquefaction potential is relatively low at the water table elevation during this study; 37% of the saturated soils are liquefiable for a moderate earthquake of magnitude 7.0 with a peak ground acceleration of 0.27. Liquefaction increases when more of the subsurface is saturated by a higher water table and when the magnitude and peak ground acceleration increase.