Sharp variation of low velocity zone atop the 410 seismic discontinuity beneath the edge of western Basin and Range

Song et al (nature, 2004) model reginal S wave triplication in the northwestern US and find low velocity zone atop the 410 seismic discontinuity. Here we present detail modeling of waveforms travelling along the westernmost tectonic North America and recorded by the TriNet broadband seismic array. We adopt regional tomographic model to constrain the shallow upper mantle. 2-D finite difference method interfacing Kirchhoff difffraction operator (DiFD) is implemented to account for the response off the great circle path. Strong azimuthal variations in the interferences of S wave triplication are clearly observed over less than 100 km. To reconcile such rapid change in waveform interferences, we model this array data with DiFD and find a low velocity zone with a sharp western edge atop the 410 can explain the whole dataset quite well. Though the geometry of the LVZ is not unique due to limited data analyzed, the sharp edge of the LVZ is more robustly constrained with the array data and DiFD modeling. Low velocity zone with a sharp edge atop the 410 has strong implications on its origin. It is unlikely that a thermal anomaly could explain such a sharp edge. However, a neutrally buoyant melt layer due to dehydration melting across the 410 might be able to explain the sharp edge.