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The four unidentified events are particularly important since they were the first recordings from Novaya Zemlya after the SPITS array was upgraded in 2004. One important feature of this upgrade was an increase in the sampling rate from 40 to 80 samples per seconds, which means that signals at much higher frequencies can be analyzed than was possible before the upgrade.
Spectrograms of the Spitsbergen B1 seismometer (vertical component) for
the Novaya Zemlya event on 5 March 2006. The waveform is plotted below
the spectrogram.
The most noticeable feature on the figure above is the high SNR of the P-phase for this small (mb=2.65) event. In fact, the SNR on the array beam is above 100, indicating that even an event at this site more than an order of magnitude smaller could have been detected. Furthermore, the amount of high-frequency energy is remarkable, taking into account the large epicentral distance (more than 1000 km). There is significant P-wave energy even above 20 Hz. This observation is important both from a scientific point of view and for the future possibilities of identifying small seismic events in the region.
The figure below further illustrates the same point. Here we show the signal filtered in various frequency bands. Again, we see the remarkable amount of signal energy even in the highest frequency band (20-36 Hz).
Spitsbergen recordings for the 5 March 2006 Novaya Zemlya event
2006-064:23.19.00.
On both figures, we notice that two distinct seismic phases are recorded. The first and strongest phase is the P-wave, while the second weaker phase is the S-wave which arrives about two minutes later. The time difference between the two phases indicates the distance between the event and the recording station, while the direction to the event can be determined through array waveform analysis. It is thereby possible to obtain a location estimate even if only one array has detected the event. However, generally a more accurate location can be found if a network of seismic stations and arrays is available.
Another feature of the SPITS upgrade has been the introduction of several three-component stations in the array. This has significantly improved the possibility of detecting S-phases, which is essential in order to monitor the smallest seismic events. This is illustrated in the figure belove.
Selected Spitsbergen array beams for the 5 March 2006 Novaya Zemlya event.
The top trace is a beam steered to the epicenter with a P-wave velocity, and using a typical detection filter (3-16 Hz). Note that the S-wave on this trace is fairly small, and would give a fairly marginal detection by the automatic process. The middle trace is an “optimum” beam designed to detect the S-wave. It represents the beams of the transverse components of the six three-component seismometers in the array, filtered in the band 2-4 Hz and steered to the epicenter with an S-phase velocity. Note the greatly improved gain in signal-to-noise ratio (SNR) on this trace. The bottom trace shows, for comparison, an S-beam of vertical sensors using the same (2-4 Hz) filter. Clearly, the detection of S-phases can be greatly improved by augmenting the beam deployment with several steered beams, rotated so as to provide transverse components, toward the grid points in the beam deployment system. This is one of the research results that have been implemented in the NORSAR regional processing system.
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