Multipathing Analysis of Mid-Pacific Anomaly Using S and Secondary S*Arrivals

Abstract:

Seismic data from earthquakes originating in the Fiji-Tonga region exhibit waveform complexity which appears to be caused by anomalous structure at the core-mantle boundary. The data, recorded by the dense USArray network, exhibits variation in multipathing, that is, the presence of secondary arrivals (called the S* phase) following the S phase at diffracted distances (S_d). The amplitude of S* is usually smaller than S_d and is lagged up to 10 seconds with distinct patterns. Here, we examine these patterns with a multipathing detector to establish characteristic length scales and the relationships of amplitudes with travel times. Generally, delayed S_d arrivals have lower amplitudes which are associated with delayed S* phases, indicative of 3D structures. Multipathing from neighboring earthquake events suggests that the waveform complexity is due to structures at the edge of the mid-Pacific LLSVP rather than isolated structures or plumes because of the presence of secondary arrivals at large distances. Using earth flattening transformations and 2D finite difference methods, numerical simulations of dome-like structures of low velocity material can produce such features. Preliminary results from a multipathing analysis of these models indicate that a variety of shapes, some as large as 6º in radius, along with a variety of velocity reductions, can produce the observed waveform complexity. Such shapes appear to be compatible with recently reported iron-rich oxide mixtures.