Source specific station corrections for regional phases at Fennoscandian stations

IASPEI91 global travel-time curves are used as the default for event locati on at the Prototype International Data Center (PIDC). In order to improve e vent location, a 1-D Baltic travel-time model was implemented at the PIDC i n 1997 for locating events using regional phases from Fennoscandian station s. Where a single model is insufficient for characterizing the regional geo logy, path-dependent corrections, or Sourer Specific Station Corrections (S SSCs), are mote appropriate for event locations. We have developed SSSCs fo r regional phases at the Fennoscandian stations by interpolating travel tim es through different I-B models. SSSCs for stations NRIS and SPITS are also derived, given the fact that paths from both stations to high latitude eve nts are within the Fennoscandia regionalization as Baltic. Validation testing of the SSSCs demonstrates that using SSSCs in event loca tion is superior to not using SSSCs, and, in most cases, to using the I-D m odel directly when locating events. For a ground-truth data set which inclu des events in the Baltic Shield with location accuracy better than 2 km, th e average improvement in location due to SSSCs is 9 km, and the median cove rage ellipse is reduced by 2710 km(2) (from 3830 to 1120 km(2)). These resu lts are similar to those obtained using the I-D Baltic model. For a CEB (Ca libration Event Bulletin) data set which includes events along the North At lantic oceanic ridge and in central:southern Europe, using SSSCs the ridge events move closer to the ridge axis, and the European events move closer t o CEB locations than I-B Baltic locations. For a constrained JHD (Joint Hyp ocenter Determination) data set of events In the Novaya Zemlya region, when using SSSCs or the I-D Baltic model, relative to the JHD locations misloca tions are less or similar to those without SSSCs. All coverage ellipses are smaller but still contain the JHD solutions. Our SSSCs are strongly dependent on the 1-D regional models and regionaliza tion. Future development in I-D velocity models and travel-time curves shou ld improve such SSSCs, event locations, and uncertainties. It is hoped that the implementation and demonstration of SSSCs in the PIDC software will en courage these further developments. These SSSCs were implemented at the PID C for Reviewed Event Bulletin (REB) location in April 1999.