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Teleseismic receiver function analysis of the crust and upper mantle of southwestern British Columbia Cassidy, John Francis


The northern Cascadia subduction zone has been the site of numerous geophysical studies during the past two decades. However, little is known of the deep structure (> 40 km) or S-velocities throughout this region. In this study, locally generated P-to-S conversions (Ps) contained in ~100 teleseismic P-wave coda have been analysed to determine the S-velocity structure to upper mantle depths. Prior to the analysis, the applications and limitations of this technique as applied to a dipping layer environment have been examined. It is concluded that strict stacking bounds (≤ 10° in ∆ and BAZ) should be applied. It is demonstrated that dipping boundaries which could not be detected using this technique (e.g. ∆Vs = 0.08 km/s), may significantly alter the amplitude and arrival time of reverberations from deeper interfaces. Therefore, such phases should not be quantitatively modelled. As reverberations are an important constituent of receiver functions, formal inversion of these waveforms is not justified in this environment. Only arrivals which exhibit the amplitude and arrival time characteristics of primary P-to-S conversions are considered in this study. Finally, most studies have normalised receiver functions to unit amplitude prior to modelling. However, synthetic data demonstrate that undetected dipping boundaries may alter Ps/P ratios and lead to inaccurate earth models. A recent modification to this technique (Ammon, 1991) which provides 'absolute' amplitudes is examined. In addition to providing information on the near-surface velocity structure and on dipping layers, this modification provides for a more accurate image of the earth structure. Three 3-component broadband event triggered seismic stations were deployed in a 90 km long linear array oriented perpendicular to the continental margin of southwestern British Columbia. Between December 1987 and October 1989 approximately 100 teleseisms covering a wide azimuthal and distance range were recorded and analysed. The two largest phases observed in data from the westernmost station ALB-B reveal a prominent low-velocity zone extending from 37-41 km depth beneath central Vancouver Island. This feature correlates well with the reflective 'E' zone, a region which also exhibits high electrical conductivity. Combining the S-velocity estimates with refraction P-velocities yields a high Poisson's ratio for this layer. The low P- and S-velocities and high Poisson's ratio and electrical conductivity are supportive of the recent interpretation of this feature as a fluid-saturated shear zone above the subducting Juan de Fuca (JdF) plate. Analysis of data at the mid-array and easternmost sites, LAS and EGM respectively, permits this zone to be mapped northeastward to a depth of 54 km beneath the British Columbia mainland, approximately 250 km from the locus of subduction. The subducting oceanic crust is imaged at 47-53 km depth dipping 15°±5° in the direction N30°E±20° beneath central Vancouver Island. The dip angle increases to 22° ±5° at a depth of 60-65 km beneath the Strait of Georgia. The results of this analysis provide the first definitive evidence for the location of the subducting plate in this region and indicates that the seismicity at depth occurs within the oceanic crust. Further, the dip direction of N30°E supports the theory (Rogers, 1983) that the JdF plate is arched upwards as it subducts in this region. Finally, the continental Moho is imaged at 36 km depth beneath LAS, and there is evidence at both this site and EGM for a low-velocity zone in the lower crust. A similar feature is imaged beneath Vancouver Island and coincides with the reflective ‘C’ zone. The depth estimated to the top of this layer denotes the lower limit of shallow seismicity suggesting a significant structural or compositional change at a depth of 20-26 km.

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