ALBERT

Description: The detectability of low magnitude seismic events in the European Arctic is determined primarily by the small-aperture International Monitoring System arrays ARCES and SPITS. In August 2004, the SPITS array was upgraded to a broadband array with an increase in the sampling rate from 40 to 80 Hz. Most important, however, for the detection and location of small-magnitude seismic events was the deployment of three-component instruments at six of the nine sites. Detection and correct classification of secondary phases are of paramount importance for events observed by only a small number of stations at regional distances; and, in the absence of the strong Lg phases typically observed for continental propagation paths, multiple three-component stations were deemed necessary to exploit the higher S-phase amplitudes anticipated on the horizontal sensors. We demonstrate improved signal-to-noise ratios (SNRs) for S phases on horizontal beams for several events close to Novaya Zemlya. Horizontal component f-k analysis improves direction estimates and phase classification for low-SNR signals. We demonstrate secondary phases that are misidentified by vertical-only f-k analysis but which are correctly classified by three-component array processing. A significant problem with array processing at SPITS is the overlap in slowness space of regional P and S phases. Phase identification is improved greatly by comparing the coherence between vertical traces with the coherence between horizontal traces. Considerations in the routine array processing of SPITS data are reviewed, including the need for elevation corrections in slowness estimation and the need to take into account azimuth-dependent variation of apparent velocity estimates for regional phases.

Description: We have investigated the Reviewed Event Bulletin (REB) of the International Data Center (IDC) for the time period 1 January 2001 to 31 December 2011 in order to quantify the event detection capability of individual seismic stations of the International Monitoring System (IMS). In order to obtain regionalized detection thresholds, we divide the events into a binned global grid system and investigate three estimation algorithms applied to each specific target area. Our preferred algorithm is to consider the ensemble of REB reported events in the area, and downscale each event magnitude with the observed signal-to-noise ratio (SNR) at the station. In this process, it is necessary to take into account events not detected by the station, in order to avoid a bias in the threshold estimate. We address this problem by using a maximum-likelihood estimation procedure whenever information on nondetections is available in the REB and correct for an estimated bias in other cases. A major result of this study is quantification and ranking of the IMS primary and auxiliary seismic stations based on their capability to detect events within regional and teleseismic distance ranges. We note that for each station, source regions with noticeable signal amplitude focusing effects (bright spots) and defocusing effects can be identified and quantified. We apply the results of this study to calculate updated global detection capability maps for the IMS primary seismic network.

Description: In the monitoring of earthquakes and nuclear explosions using a sparse worldwide network of seismic stations, it is frequently necessary to make reliable location estimates using a single seismic array. It is also desirable to screen out routine industrial explosions automatically in order that analyst resources are not wasted upon detections which can, with a high level of confidence, be associated with such a source. The Kovdor mine on the Kola Peninsula of NW Russia is the site of frequent industrial blasts which are well recorded by the ARCES regional seismic array at a distance of approximately 300 km. We describe here an automatic procedure for identifying signals which are likely to result from blasts at the Kovdor mine and, wherever possible, for obtaining single array locations for such events. Carefully calibrated processing parameters were chosen using measurements from confirmed events at the mine over a one-year period for which the operators supplied Ground Truth information. Phase arrival times are estimated using an autoregressive method and slowness and azimuth are estimated using broadband f-k analysis in fixed frequency bands and time-windows fixed relative to the initial P-onset time. We demonstrate the improvement to slowness estimates resulting from the use of fixed frequency bands. Events can be located using a single array if, in addition to the P-phase, at least one secondary phase is found with both an acceptable slowness estimate and valid onset-time estimate. We evaluate the on-line system over a twelve month period; every event known to have occured at the mine is detected by the process and 32 out of 53 confirmed events were located automatically. The remaining events were classified as "very likely" Kovdor events and were subsequently located by an analyst. The false alarm rate is low; only 84 very likely Kovdor events were identified during the whole of 2003 and none of these were subsequently located at a large distance from the mine. The location accuracy achieved automatically by the single-array process is remarkably good, and is comparable to that obtained interactively by an experienced analyst using two-array observations. The greatest problem encountered in the single array location procedure is the difficulty in determining arrival times for secondary phases, given the weak Sn phase and the complexity of the P-coda. The method described here could be applied to a wide range of locations and sources for which the monitoring of seismic activity is desirable. The effectiveness will depend upon the distance between source and receiver, the nature of the seismic sources and the level of regional seismicity. © Springer Science + Business Media, Inc. 2005.

Description: The detectability of low magnitude seismic events in the European Arctic is determined primarily by the small-aperture International Monitoring System arrays ARCES and SPITS. In August 2004, the SPITS array was upgraded to a broadband array with an increase in the sampling rate from 40 to 80 Hz. Most important, however, for the detection and location of small-magnitude seismic events was the deployment of three-component instruments at six of the nine sites. Detection and correct classification of secondary phases are of paramount importance for events observed by only a small number of stations at regional distances; and, in the absence of the strong Lg phases typically observed for continental propagation paths, multiple three-component stations were deemed necessary to exploit the higher S-phase amplitudes anticipated on the horizontal sensors. We demonstrate improved signal-to-noise ratios (SNRs) for S phases on horizontal beams for several events close to Novaya Zemlya. Horizontal component f-k analysis improves direction estimates and phase classification for low-SNR signals. We demonstrate secondary phases that are misidentified by vertical-only f-k analysis but which are correctly classified by three-component array processing. A significant problem with array processing at SPITS is the overlap in slowness space of regional P and S phases. Phase identification is improved greatly by comparing the coherence between vertical traces with the coherence between horizontal traces. Considerations in the routine array processing of SPITS data are reviewed, including the need for elevation corrections in slowness estimation and the need to take into account azimuth-dependent variation of apparent velocity estimates for regional phases.

Description: We have investigated the Reviewed Event Bulletin (REB) of the International Data Center (IDC) for the time period 1 January 2001 to 31 December 2011 in order to quantify the event detection capability of individual seismic stations of the International Monitoring System (IMS). In order to obtain regionalized detection thresholds, we divide the events into a binned global grid system and investigate three estimation algorithms applied to each specific target area. Our preferred algorithm is to consider the ensemble of REB reported events in the area, and downscale each event magnitude with the observed signal-to-noise ratio (SNR) at the station. In this process, it is necessary to take into account events not detected by the station, in order to avoid a bias in the threshold estimate. We address this problem by using a maximum-likelihood estimation procedure whenever information on nondetections is available in the REB and correct for an estimated bias in other cases. A major result of this study is quantification and ranking of the IMS primary and auxiliary seismic stations based on their capability to detect events within regional and teleseismic distance ranges. We note that for each station, source regions with noticeable signal amplitude focusing effects (bright spots) and defocusing effects can be identified and quantified. We apply the results of this study to calculate updated global detection capability maps for the IMS primary seismic network.