Abstract
The Reviewed Event Bulletin for 16 August 1997, produced by the prototype
International Data Center (pIDC), reported a small seismic disturbance
in the vicinity of the northern Russian test site at Novaya Zemlya
(at about 02:11 UTC). Initial reports suggested that this disturbance
was caused by a small nuclear explosion, whereas others identify
the source as an earthquake using vertical-component S/P ratios.
However, other authors show evidence that questions the validity
of the S/P method for this region and conclude that the disturbance
cannot be positively identified as an earthquake using seismological
evidence. The apparent attenuation from the 16 August 1997 disturbance
to SPITS (Spitsbergen, distance Delta = 1280 km, azimuth = 317 deg)
is reported to be weak. Similar low attenuation is suggested from
high-frequency observations at KEV (Kevo, Finland, Delta = 1130 km,
azimuth = 268 deg). Thus, high-frequency P and S waves propagate
efficiently across the Barents Sea, leading to the possibility that
the radiation pattern of the source may be inferred. We show, using
a grid-search method, that the high-frequency SV/SH amplitude ratios
observed at SPITS and KEV (measured from the complex envelopes of
the free-surface corrected three-component seismograms), combined
with simple Pn signals (with positive first motion) at HFS (Hagfors,
Sweden) and NORES (Norway), are consistent with a double-couple source,
with a range of orientations. We compute synthetic seismograms for
one such double-couple source oriented at sigma = 255 deg, delta
= 115 deg, psi = 120 deg (similar to that reported for the 1 August
1986 Kara Sea earthquake). The synthetic Pn seismograms at HFS and
NORES and the relative SV/SH amplitudes of the synthetic Sn seismograms
at KEV and SPITS match those observed reasonably well. Our model
also predicts (1) weak Pn at NRI (Norilsk, Russia), suggesting that
the emergent Pn onset and low signal-to-noise ratio observed is related
to the focal mechanism, and (2) the negative P first motion and large
SH/SV ratio observed at AMD (Amderma, Delta = 360 km, azimuth = 152
deg). Thus, we conclude that the 16 August 1997 seismic disturbance
was an earthquake beneath the Kara Sea and that identification is
possible using only data that would normally be available to the
pIDC (assuming KEV can be used as an alternative for the nearby station
ARCES, which was not working at the time of the disturbance). 10.1785/0120020012
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