Improved Phase Characterization of Far-Regional Body Wave Arrivals in Central Asia

摘要

The early body-wave coda of far-regional events (14' - 29' degrees) contains triplicated arrivals from upper-mantle discontinuities that, when properly identified, can improve seismic monitoring functions. However, far- regional seismograms are typically under-utilized because along-path heterogeneity and phase interactions make the arrival suite difficult to interpret accurately. We have developed a set of techniques for small aperture arrays that improve phase characterization and identification at these distances. We have focused our efforts on two arrays in Kazakhstan (MKAR and KKAR), which record far-regional events throughout South-Central Asia. Our techniques include improved array processing methods (e.g., phase-weight semblance stacking) to characterize arrival time, back-azimuth and slowness of individual phases within the P-coda arrival suite, and methods to more accurately identify the measured arrivals (e.g., tau-p transformation and clustering analysis to determine wavefield templates). The most direct means of phase identification is through matching arrival time and slowness estimates to theoretical predictions. In complex tectonic regions such as South-Central Asia, global reference models perform poorly at far-regional distances, and more specific knowledge of the along-path and near-array earth structure is required for confident primary and secondary phase identification. To account for near- array effects on the array measurements, we use receiver functions to image below the arrays. Receiver functions allow a direct estimate of the structure below the array (e.g. Moho dip), independent of the array measurements and without bias to other along-path effects. We then use array measurements (slowness and arrival times) to empirically derive regionalized velocity-depth profiles that more accurately predict the far-regional phase succession.