Complete Pp-Waveform Modelling For Determining Crust and Upper Mantle Structure

Abstract

SUMMARY We analyse the PP phase at upper mantle distances (25 to 60) to quantify its propagation characteristics and potential for determining crust and upper mantle velocity structure. Upper mantle distance PP waveforms are very complex, involving interference between a variety of arrivals traversing different depth ranges in the Earth. Even for laterally homogeneous structures, complete synthetic seismograms such as provided by reflectivity calculations are required to adequately model the full suite of arrivals that contribute to upper mantle PP phases. The complexity of PP waveforms can be exploited to extract crust and upper mantle structural information for relatively homogeneous paths. While PP-P differential traveltimes and PPlP amplitude ratios provide constraints on average upper mantle velocities above the transition zone, early PP coda, comprized of crustal reflections and conversions, and precursors to the PP phase, comprized of multiple Moho underside reflections, can provide information on crustal thickness and velocity gradients in the uppermost mantle. Positive velocity gradients below the crust generate large amplitude Moho underside reflections that are particularly visible for short-period and broad-band seismograms at distances from 25 to 38. We demonstrate the potential use of the PP phase for determining upper mantle structure by modelling long-period signals traversing relatively 'pure' paths across North America. A continuum of P-wave velocity models with varying lid structure is appropriate for the upper mantle beneath North America. The highest velocities are found beneath the northern Canadian shield, with the lid becoming progressively slower beneath the southern shield, continental platform and tectonically active regions, respectively. Most of the heterogeneity in P-velocity structure is concentrated above a depth of 250 km, although some data are consistent with models that are relatively slow at greater depths.