Article,
Comparison of location procedures: The Kara Sea Event of 16 August 1997
Bulletin of the Seismological Society of America, 97 (2): 389--400 (Apr 1, 2007)
DOI: 10.1785/0120040017
Abstract
Various location procedures and velocity models are compared for the
ML 3.3 Kara Sea event of 16 August 1997. This event has been the
subject of considerable discussion because of its geographical position
and the difficulty in obtaining a reliable focal-depth estimate.
A comprehensive data set was extracted by (re-)reading the records
from all available stations. These readings have then been used in
a sequence of location experiments to examine the effect of using
different velocity models to describe the travel times of the phases,
and also to compare the use of a fully nonlinear scheme (shakeNA;
Sambridge and Kennett, 2001) and a linearized location algorithm
(HYPOSAT; Schweitzer, 2001, 2002). A standard least-squares misfit
criterion has been used for direct comparisons between the two methods.
The results confirm both the importance of S-wave information in
assessing the depth of regional events, and the need to apply a reliable
velocity model to place the strongest constraints on the location
of the event. Even with only a limited data set, but an adequate
velocity model, it is possible to find the position of the Kara Sea
event close to the most probable locations; however, there is then
no depth resolution. Reported error ellipses from standard data centers
tend to have relatively small error ellipses. With the commonly made
assumption that the reading errors and the a posteriori residuals
have an unbiased normal distribution, such inversion results may
indicate an unreasonably high resolution and accuracy of the solution.The
epicenter estimates for the whole data set using the range of different
techniques agree quite well, with some overlap of the estimated confidence
regions. The observed seismic source was most likely an earthquake
in the middle or lower crust at about 10-30 km depth. 10.1785/0120040017
