Attenuation of broad‐band (50–1500 Hz) seismic waves in granitic rocks near the Earth’ surface

Abstract

We report Q‐estimates from borehole‐to‐tunnel seismic experiments at Nagra's “Grimsel Test Site” (GTS). The probed granitic rocks are cut by brittle faults but do not show any signs of pervasive weathering. Q‐values estimated from P‐wave first arrivals by the spectral ratio and rise time methods range from about 20 to 60 with a median value of 35 and a standard error of 20% or less. The surprisingly low Q‐values inferred indicate that even outside the weathering zone, seismic attenuation in some parts of the uppermost crystalline crust may be very high.