A finite element computation of seismic diffraction about the rise axis magma chamber

Abstract

In a previous study, models of the crust and magma chamber at 12°50′N on the East Pacific Rise were derived with a two‐dimensional ray‐tracing analysis of rise‐normal refraction lines. The best model featured a 4 km wide axial magma chamber, but the data contained diffraction effects that were not modeled by geometric ray theory. This raised questions as to whether this model would reproduce the waveform data and whether diffraction might be obscuring a much larger magma chamber. We have tested the model with a finite element calculation of synthetic seismograms. This method incorporates full wave effects, including diffraction. The synthetics accurately reproduce the data and exhibit diffraction phenomena similar to those in the data. The previous travel time analysis was largely successful and was only slightly compromised by diffraction. Diffraction in rise‐normal refraction experiments is, therefore, not obscuring a large, low velocity magma chamber.