Region-Specific Pn Geometric-Spreading Models for Eurasia

Abstract:

Pn is one of the most important seismic phases used for regional-event location, discrimination and yield estimation, yet its amplitude is the most variable among all regional phases. This is due to its acute sensitivity to small-scale uppermost-mantle structure and material-property heterogeneities including the lateral variation of Pn velocity gradient. In order to better model Pn amplitudes to reduce the discrimination uncertainty and yield-estimation error, we are developing region-specific Pn geometric-spreading models for Eurasia based on observed Pn travel times and amplitudes. We first estimate path-specific Pn velocity gradient from Pn travel times and the Regional Seismic Travel Time model (RSTT). We assume that these path-specific gradient values are the average of laterally varying Pn velocity gradient along the paths, and invert the values for a 2D Pn velocity-gradient map through tomography. The gradient map is used to divide Eurasia into sub-regions of different dominant gradients. For each region, we correct observed Pn amplitudes for spreading using a model based on the dominant Pn velocity gradient of that region. The amplitude residuals are then used to estimate an average Pn attenuation for that region. After removing the estimated attenuation effect from the original Pn amplitudes, we fit a region-specific Pn geometric-spreading model to the corrected amplitudes to capture the small-scale Pn velocity-gradient variations. We will use the new region-specific Pn geometric-spreading models for spreading correction in the development of 2D Pn attenuation models, and test the performance of the new models in reducing Pn amplitude variations.