Seismic propagation across the East Pacific Rise: Finite difference experiments and implications for seismic tomography

Abstract

We use a full‐waveform acousto‐elastic finite difference technique to investigate seismic propagation across the East Pacific Rise at 9°30′N for a two‐dimensional velocity model based on that proposed by Vera and others (1990). The primary feature of the model is an upper crustal low‐velocity region, corresponding to the axial magma chamber, which includes a small magma body located 1.6 km beneath the seafloor at the rise axis. The high velocity gradients in this region result in a complex pattern of propagation which includes considerable scattering of energy above and below the magma chamber. A qualitative comparison of finite difference seismograms with data collected by receivers located 9 km and 20 km off axis during a tomography experiment at 9°30″N shows generally good agreement. For paths that cross the rise axis, the first arrival in the finite difference solutions diffracts above the magma chamber. This phase has a very low amplitude and at larger offsets falls below the ambient noise levels observed during the tomography experiment. In such cases, the first arrival with significant energy is a diffraction from below the magma chamber. A high‐amplitude Moho‐turning (PmP) phase which results from the large velocity change across the Moho beneath the rise axis is apparent in both the finite difference solutions and the observations. Ray‐theoretical calculations of the paths of the diffracted arrivals are very unstable, and for the diffractions above the magma chamber no solution can be found with a single‐precision algorithm. Synthetic delay‐time inversions using an approximate ray‐tracing algorithm demonstrate the importance of ensuring that picked arrival times are assigned to paths that pass to the correct side of the magma body. Synthetic inversions of spectral estimates of t* show that Q−1 models are compromised not only if the ray paths are incorrect but also if t* estimates include significant contributions from more than one phase. Deterministic scattering from the magma chamber may contribute noticeably to spectral estimates of t*, but the results of the finite difference experiments imply that high levels of attenuation observed for phases passing below the magma chamber are predominantly the result of intrinsic attenuation.