TKE TELESEISMIC MANIFESTATION OF pP: PROBLEMS AND PARADOXES

Abstract. Energy radiated upward from underground nuclear explosions has a coniplex interaction with thc free surface that strongly influences the seismic wavefields rccorded at teleseismic and regional distances. This interaction, differing from that for earthquakes primarily due to the much higher strains and strain rates involved, is essential to understand for both explosion yield estimation and for discriminating earthquakes from explosions. Reflection of explosion P wave energy from the free surface, which produces the pP phase, involves frequency-dependent. non-lincar processes that are intimately linked to surface spallation. Attempts to characterize the teleseismic DP arrival using a variety of time series analysis procedures have yielded seemingly inconsistent results, which can be attributed lo a combination of limited bandwidth, neglected frequency dependence, and unresolved trade-offs with source time function, receiver and attenuation effects. Recovery of broadband ground displaccmcnt, now viable with modern inslrumcntation, is resulting in more robust characterizations of the pP and spallation arrivals; however, the intrinsic trade-offs with source parameters and attenuation remain. Numerical procedures to account for the non-linear interactions, surhce topographic effects, and shallow crusval heterogeneity are enabling a more complctc modeling of the free surface interaction