THE NATURE OF PARTICLE MOTION IN REGIONAL SEISMOGRAMS AND ITS UTILIZATION FOR PHASE IDENTIFICATION

The particle motion of regional arrivals is frequently treated in auto matic phase-recognition schemes as that appropriate to simple P or S w aves incident on an elastic, laterally homogeneous half-space. This mo del implies that the motion in 'P-type' phases can be described in ter ms of a single, generalized signal process and 'S-type' phases in term s of two independent processes (SV and SH) and thus, all regional arri vals could be fully characterized by three components of motion. In th is paper, we present anlyses of the particle-motion patterns of variou s regional arrivals recorded at the ARCESS array from closely spaced e vents in the Kola Peninsula. We have found that only Pn-particle motio n, described in terms of two independent signal processes, can be reli ably characterized by three-component recordings. On the other hand, t he various regional arrivals following Pn, such as Pg, Sn and Lg, can only be poorly characterized on the basis of three-component recording s alone. The reason is that these arrivals must be described in terms of more than two independent generalized signal processes, at least th ree for Pg and Sn, and possibly up to five for Lg. Recognition of thes e phases will thus require the use of more sensors than signal process es in the observing sensor configuration, such as three-component sens ors combined with a small tripartite array. We have investigated the f easibility of adaptive, automatic recognition of regional arrivials by a wavefield extrapolation scheme utilizing such a mini-array. The pro cess, which appear to be promising, adaptively learns the particle-mot ion patterns of individual arrivals, including complex site-response f unctions, from examples of closely located regional events.