SHALLOW SHEAR WAVE VELOCITIES IN CHARLESTON, SOUTH CAROLINA: COMPARISON OF AMBIENT NOISE AND BOREHOLE TECHNIQUES

We have determined shallow shear wave velocities at 25 sites in the greater Charleston, South Carolina region using an ambient noise (microtremor) technique. All of these sites also have shear wave velocities determined using the seismic cone penetration test (SCPT) technique. The ambient noise technique employed is the refraction microtremor (ReMi) method, where microtremor ground motion is recorded using a standard refraction seismic system and a linear array of 24 geophones. In addition, P-wave refraction and at some sites S-wave refraction data was collected. At all sites we can resolved shear wave velocities down to 30 meters using the ReMi technique; SCPT boreholes do not go to 30 meters at all sites, likely due to an erosional surface between the Quaternary and Tertiary. We find the ReMi and SCPT methods result in a similar V_S30 estimate, when it can be calculated using both methods. We find it is necessary to know the shallow P-wave structure, in particular the depth of the water table where V_P > 1500 m/sec, to construct an accurate shear wave velocity model using the ReMi method. ReMi shear wave velocity models without a P-wave velocity constraint consistently overestimate V_S30.