Frode Ringdal

Seismic Monitoring of the North Korea Nuclear Test Site Using a Multichannel Correlation Detector

North Korea announced a second nuclear test on 25 May 2009, the first having taken place on October 9, 2006. Both tests were detected by the global seismic network of the Comprehensive nuclear Test-Ban-Treaty Organisation. We apply a correlation detector using a 10-s signal template from the 2006 test on the MJAR array in Japan to: 1) assess the potential for automatically detecting subsequent explosions at or near the test site; and 2) monitor the associated false alarm rate. The 2009 signal is detected clearly with no false alarms in a three-year period. By detecting scaled-down copies of the explosion signals submerged into background noise, we argue that a significantly smaller explosion at the site would have been detected automatically, with a low false alarm rate. The performance of the correlator on MJAR is not diminished by the signal incoherence that makes conventional array processing problematic at this array. We demonstrate that false alarm elimination by f-k analysis of single channel detection statistic traces is crucial for maintaining a low detection threshold. Correlation detectors are to be advocated as a routine complement to the existing pipeline detectors, both for reducing the detection threshold for sites of interest and providing automatic classification of signals from repeating sources.

Seismic Location Calibration of the European Arctic

Abstract — A crustal velocity model has been developed for Fennoscandia, the Baltic shield and adjacent areas. This model represents a simplified average of various models developed for parts of this region. We show that P-wave travel times calculated with this model provide an excellent fit to observations at the Fennoscandian, KRSC and IRIS station networks for a set of seismic events with known or very well-constrained locations. The station-event paths cover large parts of Western Russia and the Barents Sea, thus indicating that this model, which we denote the Barents model, is appropriate for this entire region. We show by examples that significant improvements in event location precision can be achieved compared to using the IASPEI model. We finally use the Barents model to calculate locations of recent small seismic events in the Novaya Zemlya region of interest in a CTBT monitoring context.

Joint seismic-infrasonic processing of recordings from a repeating source of atmospheric explosions

A database has been established of seismic and infrasonic recordings from more than 100 well-constrained surface explosions, conducted by the Finnish military to destroy old ammunition. The recorded seismic signals are essentially identical and indicate that the variation in source location and magnitude is negligible. In contrast, the infrasonic arrivals on both seismic and infrasound sensors exhibit significant variation both with regard to the number of detected phases, phase travel times, and phase amplitudes, which would be attributable to atmospheric factors. This data set provides an excellent database for studies in sound propagation, infrasound array detection, and direction estimation.

Detection Capability of the Seismic Network of the International Monitoring System for the Comprehensive Nuclear‐Test‐Ban Treaty

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.

Earthquake detectability estimates for 478 globally distributed seismograph stations

Abstract Using the P-wave reportings to the ISC for the four years 1971–1974, the earthquake detectability for 478 globally distributed seismograph stations in individual distance interval 30–90° has been estimated in terms of the 50% incremental detection threshold expressed in ISC m b units.

Seismicity of the North Sea Area

The North Sea area is well removed from major tectonic plate boundaries, and in consequence the seismicity of the region is modest in comparison to many other areas of the world. Nevertheless, a few significant earthquakes (M ~ 6.0) are known to have occurred in the North Sea and adjacent areas within the past 100 years, the largest of these being the 1904 Oslofjord earthquake, the 1927 earthquake off the west coast of Norway and the 1931 Doggerbank earthquake. A major problem in assessing North Sea seismicity is the lack of adequate instrumental coverage, and only recently can earthquakes in this region be detected at magnitude 4.0 or lower. Based on the sparse available data, there is still evidence that the North Sea seismicity level is higher than that of adjacent land areas, with most of the known earthquakes occurring off the west coast of Norway. There is at present no evidence to rule out the possibility of future occurrence of larger earthquakes (M ~ 7.0) in the North Sea area, although the probability of occurrence of such events must be rated very low.

Phase Identification and Event Location at Regional Distance Using Small-Aperture Array Data

As part of ongoing research aimed at a better understanding of seismic wave propagation at regional distances, one of the NORSAR subarrays has been modified to a small aperture array with station distances from 125 to 2051 meters. Regional phases including Pn, P, Sn, Lg and Rg from reported events within 10° have been analyzed with respect to phase velocity and azimuth. This is done by computing frequency-wave number spectra for time series of 100 samples each (20 Hz data), which for most phases include both onset and amplitude maximum. Derived values for phase velocity and azimuth are in general accordance with expected values, even at frequencies of 6–7 Hz. Discrimination between fast (P) and slow types of phases (Sn, Lg) is possible using data from this array. A location alogrithm for the small aray data is described that enables us to locate most regional events with a difference from the location by the whole Fennoscandian network that is within the uncertainty limits of the latter.

Observed Characteristics of Regional Seismic Phases and Implications for P/S Discrimination in the European Arctic

Abstract — In this paper, we use data from seismic stations operated by NORSAR, the Kola Regional Seismological Centre (KRSC) and IRIS to study the characteristics of regional phases in the European Arctic, with emphasis on the P/S ratio discriminant. While the detection and location capability of the regional station network is outstanding, source classification of small seismic events has proved very difficult. For example, the mb = 3.5 seismic event near Novaya Zemlya on 16 August, 1997 has been the subject of extensive analysis in order to locate it reliably and to classify the source type. We consider the application of the P/S discriminant in the context of this event and other events observed at regional distances in the European Arctic. We show that the P/S ratios of Novaya Zemlya nuclear explosions measured in the 1–3 Hz filter band scale with magnitude, indicating a need for caution and further research when applying P/S discriminants. Using mainly data from the large NORSAR array, we note that observed P/S amplitude ratios in the European Arctic show large variability for the same source type and similar propagation paths, even when considering closely spaced observation points. This effect is most pronounced at far regional distances and relatively low frequencies (typically 1–3 Hz), but it is also significant on closer recordings (around 10 degrees) and at higher frequencies (up to about 8 Hz). Our conclusion from this study is that the P/S ratio at high frequencies (e.g., 6–8 Hz) shows promise as a discriminant between low-magnitude earthquakes and explosions in the European Arctic, but its application will require further research, including extensive regional calibration and detailed station-source corrections. Such research should also focus on combining the P/S ratio with other short-period discriminants, such as complexity and spectral ratios.

Spatial characteristics of the NORESS noise field and implications for array detection processing

Abstract The spatial characteristics of the NORESS noise field have been studied by computing cross-correlations as functions of intersensor separation for various frequency bands. The correlation curves exhibit distinct negative minima. The sensor separations for which these minima are attained are found to vary systematically with frequency. These observations suggest that waves propagating at Rayleigh wave velocities are a major contributor to the NORESS noise field. In order to assess the capabilities of NORESS to suppress noise and hence provide useful beam-forming gains, noise suppression spectra were computed hourly for various NORESS sub-configurations over a one-year period. The negative minima of the noise correlation functions find a counterpart in these computations. For several NORESS sub-geometries there is a frequency range for which the noise suppression spectrum exceeds the N 1 2 level. This feature is shown to be stable over time, and furthermore shows no strong dependency on the actual noise level.

Event detectability of seismograph stations in Fennoscandia

Abstract The event detection probability as a function of ISC magnitude shows significant variations between seismic stations in Fennoscandia. In the teleseismic range, the 50% incremental detection thresholds extend from m b = 4.5 to 5.5 among those stations that reported to the ISC during 1964–1969; the best stations being located in Finland. Most of this variation, though not all, can be explained from noise level fluctuation and regional signal attenuation effects. A common feature for all stations studied is a significantly better detectatbility for signals from eastern Asia than from western North America at a similar distance range. This is interpreted as resulting from higher attenuation in the mantle of high-frequency signals from the latter region.