Zusammenfassung
The IASPEI91 global travel-time curves are used as the default for
event location at the Prototype International Data Center (PIDC).
In order to improve event location, a 1-D Baltic travel-time model
was implemented at the PIDC in 1997 for locating events using regional
phases from Fennoscandian stations. Where a single model is insufficient
for characterizing the regional geology, path-dependent corrections,
or Source Specific Station Corrections (SSSCs), are more appropriate
for event locations. We have developed SSSCs for regional phases
at the Fennoscandian stations by interpolating travel times through
different 1-D models. SSSCs for stations NRIS and SPITS are also
derived, given the fact that paths from both stations to high latitude
events are within the Fennoscandia regionalization as Baltic. Validation
testing of the SSSCs demonstrates that using SSSCs in event location
is superior to not using SSSCs, a nd, in most cases, to using the
1-D model directly when locating events. For a ground-truth data
set which includes events in the Baltic Shield with location accuracy
better than 2 km, the average improvement in location due to SSSCs
is 9 km, and the median coverage ellipse is reduced by 2710 km2 (from
3830 to 1120 km2). These results are similar to those obtained using
the 1-D Baltic model. For a CEB (Calibration Event Bulletin) data
set which includes events along the North Atlantic oceanic ridge
and in central/southern Europe, using SSSCs the ridge events move
closer to the ridge axis, and the European events move closer to
CEB locations than 1-D Baltic locations. For a constrained JHD (Joint
Hypocenter Determination) data set of events in the Novaya Zemlya
region, when using SSSCs or the 1-D Baltic model, relative to the
JHD locations mislocations 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 regionalization.
Future development in 1-D velocity models and travel-time curves
should improve such SSSCs, event locations, and uncertainties. It
is hoped that the implementation and demonstration of SSSCs in the
PIDC software will encourage these further developments. These SSSCs
were implemented at the PIDC for Reviewed Event Bulletin (REB) location
in April 1999.
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