T-phase Observations in Northern California: Acoustic to Seismic Coupling at a Weakly Elastic Boundary

Abstract

Plans for a hydroacoustic network intended to monitor compliance with the CTBT call for the inclusion of five T-phase stations situated at optimal locations for the detection of seismic phases converted from ocean-borne T phases. We examine factors affecting the sensitivity of land-based stations to the seismic T phase. The acoustic to seismic coupling phenomenon is described by upslope propagation of an acoustic ray impinging at a sloping elastic wedge. We examine acoustic to seismic coupling characteristics for two cases; the first in which the shear velocity of the bottom is greater than the compressional velocity of the fluid (i.e., vp > vs > vw), the second is a weakly elastic solid in which vs ≪ vw vw we show that acoustic energy couples primarily to shear wave energy, except at very high slope angles. We show that the weakly elastic solid (i.e., vs ≪ vw) behaves nearly like a fluid bottom, with acoustic energy coupling to both P and S waves even at low slope angles.