Upper crustal seismic velocity structure and microearthquake depths at the Endeavour Segment, Juan de Fuca Ridge

We present the results of a study to invert microearthquake and explosive shot data from the Endeavour segment of the intermediate-spreading Juan de Fuca Ridge. The average isotropic P wave velocity structure, derived from the shot data, in the uppermost 1.5 km of the oceanic crust is characterized by an increase with age of similar to8% from the axis to at least 0.5 Ma, that is attributed to the sealing of layer 2A porosity by hydrothermal processes. Superimposed on this variation are axis-parallel, 2-km-wide, alternating bands of high and low velocity with a peak-to-peak variation of 5-12%. High and low velocities away from the axis correspond to bathymetric trenches and ridges, respectively and are likely due to variations in layer 2A thickness. P wave azimuthal anisotropy is present in the data that is best fit with a model of 9% anisotropy at 750 m depth, decreasing to 1% at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along-axis direction. Anisotropy and velocity heterogeneity are coupled; anisotropy alone may explain the form but not the magnitude of the axis-parallel bands. There are strong trade-offs between the hypocentral depths of microearthquakes and the P and S wave velocity structures. Changing the mean hypocentral depth by up to 0.5 km leads to only modest increases in the travel time RMS but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower.