SHEAR-WAVE VELOCITY FROM TWO 150 M BOREHOLES IN SEATTLE, WA

Over the last half century, the Seattle metropolitan area has experienced damaging ground shaking effects from three earthquakes. Ground motion and site amplification effects are strongly influenced by the physical properties of the surficial geologic units through which earthquake energy passes. As part of a continuing program to study earthquake hazards in the Seattle region the U. S. Geologic Survey (USGS) drilled and logged two 150+ m (500 ft.) boreholes in 2002. Locations were selected with the intent of sampling representative glacial and interglacial lithologic materials in two of the larger north-south trending topographic highs in Seattle.

Volunteer Park (VPD) and Beacon Hill Park (BHP) boreholes have some similarities; both have thick outwash near the ground surface, and both have thick glaciolacustrine clay near the middle depths of the boreholes. The surficial unit, Vashon advance outwash (Qva-"Esperance Sand"), varies in thickness from 51 m at VPD to 26 m at BHP, 5 km to the south. Comparisons made between lithology, standard borehole geophysical logs and a series of borehole Vs models reveal that the Qva unit can be sub-divided into an upper, clean, well graded, interbedded, gravel-rich sand layer and a lower, dense, silty to clayey sand layer. Shear-wave velocity models at both sites show an increase in average velocity of 30 to 33% respectively between the layers (415 to 580 m/s at VPD and 450 to 675 m/s at BHP).

Interpretation of borehole cuttings and regional geology indicate that the Beacon Hill borehole penetrates a more complete sequence of Vashon Drift, deposits from the Olympia nonglacial interval (Qob), glaciolacustrine deposits from the Possession glaciation, and even older glacial diamict deposits. In general, shear-wave velocity of glaciolacustrine deposits are slower with an average of 500 m/s (480 to 520 m/s) at VPD and 450 m/s (390 to 520 m/s) at BHP. Older glacial units at BHP (125 m to bottom of hole) have the highest shear (930 m/s) and compressional-wave (2255 m/s) velocities measured at either site.