ALBERT

Description: Characterizing the seismicity of Novaya Zemlya and the surrounding Arctic seas requires accurate event-location estimates. Low-magnitude events in this region are currently observed only by a small number of stations in the European Arctic, with a large azimuthal gap, making the accuracy of regional velocity models all the more important. Regional travel-time calibration is difficult given the scarcity of sufficiently well-constrained events. On 11 October 2010, a magnitude 4.5 event occurred close to the northern tip of Novaya Zemlya. This event is significant in that it is the first event in this region to have been recorded both on the relatively recent regional networks and arrays, and also teleseismically with good azimuthal coverage. We examine how well we can constrain the location and origin time using only teleseismic phases. Using only first teleseismic P arrivals, we constrain the epicenter to approximately 76.25° N and 64.75° E but with no depth resolution. Clear depth phases, notably on stations in the southern United States, indicate a depth between 9 and 15 km. This independent hypocenter and origin time estimate allow evaluation of regional phase travel-time prediction using different models. The predicted Sn travel time appears to cause the greatest variability in regional location estimates. The 3D Regional Seismic Travel Times models provide excellent Pn travel-time estimates for Barents Sea paths but may slightly overestimate Sn travel times from this source region. A modified regional 1D velocity model is defined, which best predicts Pn and Sn observations at multiple stations up to 15°. The significance of the regional travel-time models for estimating location is demonstrated for a low-magnitude event on or close to the northern island of Novaya Zemlya in March 2014, recorded with a satisfactory signal-to-noise ratio at only four stations.

Description: Aftershock sequences following very large earthquakes present enormous challenges to near-real-time generation of seismic bulletins. The increase in analyst resources needed to relocate an inflated number of events is compounded by failures of phase-association algorithms and a significant deterioration in the quality of underlying, fully automatic event bulletins. Current processing pipelines were designed a generation ago, and, due to computational limitations of the time, are usually limited to single passes over the raw data. With current processing capability, multiple passes over the data are feasible. Processing the raw data at each station currently generates parametric data streams that are then scanned by a phase-association algorithm to form event hypotheses. We consider the scenario in which a large earthquake has occurred and propose to define a region of likely aftershock activity in which events are detected and accurately located, using a separate specially targeted semiautomatic process. This effort may focus on so-called pattern detectors, but here we demonstrate a more general grid-search algorithm that may cover wider source regions without requiring waveform similarity. Given many well-located aftershocks within our source region, we may remove all associated phases from the original detection lists prior to a new iteration of the phase-association algorithm. We provide a proof-of-concept example for the 2015 Gorkha sequence, Nepal, recorded on seismic arrays of the International Monitoring System. Even with very conservative conditions for defining event hypotheses within the aftershock source region, we can automatically remove about half of the original detections that could have been generated by Nepal earthquakes and reduce the likelihood of false associations and spurious event hypotheses. Further reductions in the number of detections in the parametric data streams are likely, using correlation and subspace detectors and/or empirical matched field processing.

Description: 〈span〉〈div〉ABSTRACT〈/div〉On 3 September 2017, the Democratic People’s Republic of Korea (DPRK) carried out its sixth declared underground nuclear test (NK6) at the Punggye‐ri test site. With body‐wave magnitude 6.1, this explosion was significantly larger than any of the previous five explosions, and it has been followed by numerous smaller seismic events. The explosion generated seismic waves dominated by significantly lower frequencies than the earlier tests, which makes accurate measurement of relative time delays using cross‐correlation challenging. Finding a frequency band at which one observes common features in the NK6 signals and the corresponding signals from an earlier event can result in reduced signal‐to‐noise ratio (SNR). Classical double‐difference location estimates for NK6 show a significant spread, depending on the set of measurements used. We treat the first five declared DPRK explosions as a source array and demonstrate, using a geometrical argument about the relative time shifts visible between the signals on pairs of stations, that NK6 was very close to the 9 September 2016 explosion (NK5), assumed to be close to the maximal overburden beneath the summit of Mount Mantap. In addition to the magnitude 4.1 presumed collapse event 8 min after NK6, numerous other small events have been observed at or close to the test site since September 2017. We demonstrate how the test site is monitored to magnitudes below two using multichannel correlation templates from all existing observations. Processing all available historical data from the KSRS and USRK arrays reveals a few small events in 2013, 2014, and 2016 that are similar in nature to those observed in late 2017. This suggests that the more recent low‐magnitude events are not simply a direct result of NK6. We urge caution in the interpretation of the correlation functions between the signals from different events at or close to the test site because the signals are a function of both the source term and of near‐source structure, with the effects of topography likely to be significant.〈/span〉

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.