Was the 16 August 1997 Seismic Disturbance near Novaya Zemlya an Earthquake?

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 D ‫ס‬ 1280 km, azimuth ‫ס‬ 317Њ) is reported to be weak. Similar low attenuation is suggested from high-frequency observations at KEV (Kevo, Finland, D ‫ס‬ 1130 km, azimuth ‫ס‬ 268Њ). 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 freesurface corrected three-component seismograms), combined with simple P n 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 r ‫ס‬ 255Њ, d ‫ס‬ 115Њ, w ‫ס‬ 120Њ (similar to that reported for the 1 August 1986 Kara Sea earthquake). The synthetic P n seismograms at HFS and NORES and the relative SV/SH amplitudes of the synthetic S n seismograms at KEV and SPITS match those observed reasonably well. Our model also predicts (1) weak P n at NRI (Norilsk, Russia), suggesting that the emergent P n 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, D ‫ס‬ 360 km, azimuth ‫ס‬ 152Њ). 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).

The detection of low magnitude seismic events using array-based waveform correlation

“…Further evidence that the two events were co-located has been presented by Gibbons & Ringdal (2005b) who showed, by expanding upon the waveform correlation procedure in the present paper, that the time delay between P-phase onsets for the two events was the same (to within 0.1 seconds) for each of the three stations SPITS, NORSAR and Amderma. Bowers (2002) found that the main event produced signals consistent with a double-couple source. A source mechanism for the aftershock has not been determined due to the small number of observations and the low SNR of the signals where they were observed.…”

“…The event has been the subject of many publications (e.g. Richards & Kim 1997;Hartse 1998;Ringdal et al 1997;Asming et al 1998;Ringdal et al 2002;Bowers et al 2001;Kremenetskaya et al 2001a;Bowers 2002;Schweitzer & Kennett 2002) and the generally accepted conclusion, based upon location, spectral characteristics and other observations, is that the event was in fact an offshore earthquake.…”

Improvements to Seismic Monitoring of the European Arctic Using Three-Component Array Processing at SPITS

“…In the current pipeline detection process, f-k calculations on the In this paper, we have considered only the capability to detect and locate seismic events and have not speculated on the nature of any of the events listed in tables 1 and 2. Event D in Table 1, the Kara Sea event of 16 August 1997, has received the greatest amount of attention and it is widely believed that the event was an earthquake, both due to the nature of the signals generated [Bowers, 2002], the occurrence of a small aftershock some four hours later [Richards and Kim, 1997], and a probable depth between 10 km and 30 km in the crust [Schweitzer and Kennett, 2007]. Ringdal et al [2002] note that observations from events in the region demand that caution is necessary when attempting to apply classical discriminants, particularly at lower magnitudes.…”

Regional P-Coda for Stable Estimates of Body Wave Magnitude: Application to Novaya Zemlya and Nevada Test Site Events

“…By scaling narrowband envelopes between our reference event and the other explosions and earthquakes, we were able to tabulate relative coda amplitudes. Figure 3 below shows coda envelope amplitude residuals (y-axis) relative to the maximum likelihood magnitude m b (ML) for explosions (red squares) and earthquakes (blue triangles) (Lilwall and Marshall, 1986;Marshall et al, 1989;Bowers, 2002). This regression was done using roughly 100 seconds of P-coda in the 2-3-Hz band.…”

“…By scaling narrowband envelopes between our reference event and the other explosions and earthquakes, we were able to tabulate relative amplitude estimates and hence, m b estimates. Figure 2b shows P-coda-derived m b estimates (y-axis) relative to the maximum likelihood magnitude m b (ML) for explosions (red squares) and earthquakes (blue triangles) [Lilwall and Marshall, 1986;Marshall et al, 1989;Bowers, 2002]. This regression was done using roughly 120 seconds of P-coda in the 2-3 Hz band ( Figure 2a Paths from NZ to NORSAR are still at regional distance, and one might expect the Pwave and its coda to be comprised of waves that sample the crust and upper mantle over a range of take-off angles from the source.…”