Relocation of earthquakes recorded by SIL network (Iceland) using empirical travel-time tables

Abstract:

Three dimensional empirical travel time tables (ETT) are estimated for each station of the South Iceland Lowlands network (SIL) by interpolating travel-time observations mapped to the hypocenter of each event. ETT are used to relocate SIL database earthquakes using P- and S- arrival times. Results are compared with the initial locations from the SIL database and locations from local sub-networks in Iceland. The SIL database locations are estimated using 5 different 1-D velocity models for different regions. The arrival times were re-referenced to one single model (the SIL model). Minor discrepancies in archived travel-time reduction were identified and removed together with outliers. Travel times are decomposed into 1) the prediction of a 1-D reference velocity model; 2) deterministic variation of residuals from that 1-D model constructed by smoothing over a regional scale; and 3) a smaller-scale, stochastic component. The stochastic component is separated into a spatially incoherent part (noise) and a spatially coherent part assumed to correspond to remaining structural signal. The stochastic structural component is interpolated and summed to the other two travel time components in order to construct the ETTs. Relocation is computed with a non-linear grid-search algorithm. The resulting ETTs are useful for improving earthquake location where 1-D velocity models are frequently used and a fast secondary relocation process is required. Moreover, ETTs are “cleaned” travel times that can be used as input into 3D tomography, where the error estimation (incoherent component of residuals) serves as a useful guideline for data weighting.