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We review the previously compiled seismic magnitudes for underground nuclear explosions at the Union of the Soviet Socialist Republics Novaya Zemlya Test Site, and analyze these data in the context of published details about individual tests and official Russian reports of annual cumulative explosive energy release at the test site. We derive relationships between teleseismic body wave magnitudem b and explosive yield Y by predicting the yield of each test usingm b and minimizing the difference between predicted and reported annual cumulative yield. A linear relationship between mb and logY with a slope of 0.82 provides a good fit to the annual total yield, with a root mean square (rms) misfit of 0.095 logY units. A spectral magnitude relationship based on the explosion source model of Mueller and Murphy (1971) with a single-free-parameter provides a slightly better fit. For the 10 tests for which individual yields can be directly deduced from the official reports, the rms misfit of reported yields is less than 0.1 logY units or about 25%. More complex relationships that differentiate between the northern and southern test areas, or take account of variations in the scaled depth of burial, do not provide substantially better fits to the reported yields. [ABSTRACT FROM AUTHOR]

We present a regional short-period Sn magnitude scale mb(Sn) for small earthquakes along the northern Mid-Atlantic Ridge. Surface-wave magnitudes, teleseismic body-wave magnitudes, and seismic moments cannot be reliably determined for small earthquakes along this and other midocean ridges. Local magnitudes that rely on Lg waves are likewise not generally useful due to the substantial oceanic paths for earthquakes along midocean ridges. In contrast, Pn and Sn arrivals for earthquakes along the northern Mid-Atlantic Ridge are generally well recorded by the existing seismographic networks, and, in fact, Sn arrivals are larger than Pn arrivals for about one-third of the ridge events. For this reason, we have developed a new regional Sn magnitude scale that is tied to Mw, so that seismic moments can be readily approximated. In our least-squares fit of peak amplitudes from 120 earthquakes having a published moment magnitude, we solved for the attenuation curve for paths in the oceanic mantle lid, for event magnitude adjustments (EMAs) to account for differences between long-period moment magnitude Mw and short-period Sn magnitude, and for station corrections. We find regional EMAs that are well correlated with the style of faulting: they are positive for normal-faulting earthquakes along spreading ridges and negative for strike-slip earthquakes along transform faults. These source-specific EMAs are approximately +0.11 magnitude units for normal-fault earthquakes and -0.26 magnitude units for strike-slip earthquakes on transform faults, and are consistent with previously reported apparent stresses from these regions. The amplitude distance curve determined for Sn for the northern Atlantic Ocean is similar to that determined for Pn in the northern Atlantic out to a distance of about 500 km, but at larger distances is more similar to the western U.S. Pn curve, likely reflective of the warmer temperatures at greater upper-mantle depths. [ABSTRACT FROM AUTHOR]

We assess seismological evidence bearing on claims that North Korea conducted a small nuclear test on 12 May 2010 in the vicinity of known underground nuclear tests (UNTs) in 2006, 2009, 2013, and 2016. First, we use Lg-wave cross correlation and more traditional methods to locate the 2010 event between about 4 and 10 km southwest of the 2009 test. Second, we compare the relative sizes of regional P and S waves, using stations within 400 km of the known North Korean nuclear tests, to assess the nature of the event. We measured P/S ratios at different frequencies, at first using data from the open station MDJ in northeast China, for training sets of earthquakes and of explosions. We developed a linear discriminant function (LDF) that, in application to P/S measured at MDJ, is most effective in separating the earthquake and explosion populations. MDJ lacks usable data for the event of interest, but we obtained regional data from stations of the nearby Dongbei Broadband Seismographic Network (DBSN) for the 12 May 2010 event and for nearby UNTs conducted in 2006 and 2009. When our LDF is applied to DBSN data, and to data from stations SMT and NE3C in China, the LDF values measured from P/S ratios from known explosions are explosion-like; but for the 12 May 2010 event, the LDF values are earthquake-like for frequencies between 6 and 12 Hz. Our method for characterizing earthquakes and explosions on the basis of their regional signals can be widely applied. Measurements of P/S based on the three-component waveform data provide better discrimination power than do those based on vertical-component data alone. [ABSTRACT FROM AUTHOR]

Waveform correlation is garnering attention as a method for detecting, locating, and characterizing similar seismic events. To explore the opportunities for using waveform con-elation in broad regional monitoring, we applied the technique to a large region of central Asia over a three-year period, monitoring for events at regional distances using three high-quality stations. We discuss methods for choosing quality templates and introduce a method for choosing correlation detection thresholds, tailored for each template, for a desired false alarm rate. Our SeisCorr software found more than 10,000 detections during the three-year period using almost 2000 templates. We discuss and evaluate three methods of confirming detections: bulletin confirmation, high correlation with a template, and multistation validation. At each station, 65%-75% of our detections could be confirmed, most by multistation validation. We confirmed over 6500 unique detections. For monitoring applications, it is of interest that a significant portion of the Comprehensive Nuclear-Test-Ban Treaty Organization's Late Event Bulletin (LEB) catalog events was detected and that adding our confirmed detections for the LEB catalog would more than double the catalog size. Waveform correlation also allows for relative magnitude calculation, and we explore the magnitudes of detected events. The results of our study suggest that doing broad regional monitoring using historical and real-time-generated templates is feasible and will increase detection capabilities. [ABSTRACT FROM AUTHOR]

Seismological monitoring of the Comprehensive Test Ban Treaty (CTBT) requires detailed knowledge of travel-time characteristics of seismic phases across large areas. We use first-arrival travel times from several of Russian Deep Seismic Sounding (DSS) profiles, primarily those from Peaceful Nuclear Explosions (PNEs), to construct a 3D regional travel-time model of northern Eurasia. The method used, which can be viewed as a generalization of the existing regionalization techniques, is based on apparent-velocity-based spatial interpolation of the travel times picked from DSS records. The resulting travel-time field is described in terms of an apparent velocity model in the 3D space of geographic coordinates and ray parameter. In the travel-time calibration, this model could be used to construct approximate source-specific station correction surfaces for any location within the region and also as a region-specific reference model to be used as a background for further calibration effort. This approach allows incremental refinement of the model as additional travel-time data become available. By converting the travel-time model into depth, a 3D regional velocity model is obtained, providing a description of the general features of the upper mantle in northern Eurasia. [ABSTRACT FROM AUTHOR]

For simple theoretical models of an underground explosion, two apparently different relationships between isotropic moment and volume change have been published. We show that there is no inconsistency, because these relationships are based on two different definitions of volume change at the explosion source. We report observations of three 25-ton chemical explosions conducted in 1997 at the former nuclear test site near Semipalatinsk, Kazakhstan. These show the theoretically predicted decrease of signal strength as the shot depth increases. We argue that the relationship between isotropic moment and volume change proposed by Müller (1973) is preferred. [ABSTRACT FROM AUTHOR]

Seismic-event location within the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty entails a priori knowledge of the travel time of seismic phases for a given source to stations of the International Monitoring System (IMS). Such travel-time information (or ground truth, GT) is provided empirically by seismic reference events, events that have well-determined hypocenter locations (epicenters typically known to ± 5 km with high confidence) and origin times. In this study we present new reference events for the calibration of six seismic stations of the IMS in China, a region with high seismic activity. We use the Annual Bulletin of Chinese Earthquakes, which lists about 1000 earthquakes in and near China each year with consistent phase picks at regional stations, to determine precise relative earthquake locations from double-difference cluster analysis. The resulting high-resolution image of active faulting at seismogenic depths in areas of dense seismicity is correlated with the tectonic structure derived from mapped fault information at the surface to validate the absolute locations. We generated 59 reference events with M ≥3.5, distributed in six clusters in central and eastern China, and recorded by at least one of the six IMS stations. The scatter in relative travel-time residuals is reduced from 1.28 sec before to 0.61 sec after relocation, consistent with the relocated positions of the events. The degree of correlation between seismicity structure and well-characterized fault data indicates that, in four clusters, the locations of the new reference events are accurate to within 5 km (GT5), and in two clusters within 10 km (GT10). [ABSTRACT FROM AUTHOR]

We have used seismic signals recorded at regional and teleseismic distances to determine precise locations of 19 underground nuclear explosions (13 in vertical shafts, and 6 in horizontal tunnels) carried out between October 1976 and July 1996 at the Lop Nor test site in the southern Xinjiang province of China. In addition to first- and later-arriving phase-pick data from the International Seismological Centre and Chinese bulletins, we use waveform cross-correlation methods to measure relative arrival times between the explosions with an accuracy of about 10 msec. We adapted the double-difference algorithm to work with regional and global network data, and applied it to the combined Lop Nor data set to remove the effects of uncertainty in the Earth model. Specifically, we determined locations of more recent tests to the accuracy of the cross-correlation data while simultaneously determining the relative locations of the older tests, for which digital waveforms are not available, to the accuracy of the phase-pick data. In general, our locations are consistent, at the 90% confidence level, with previously published locations (when compared in a common reference frame), with anthropogenic features from satellite imagery, and with high-resolution elevation data. For four explosions, however, our results indicate that a particular explosion may have been carried out in a different shaft than previously noted, or previously associated features must be ruled out because of their location outside a particular error ellipse. Mislocations of explosions from associated satellite features are less than 1 km for all 13 shaft events. The pattern of tunnel-explosion locations falls within a region of suitable overburden required for containment. Eighteen Lop Nor locations have solution qualities at the GT2 level or better, and are well suited to calibrate IMS stations for the purpose of monitoring compliance with the Comprehensive Nuclear-Test-Ban Treaty. [ABSTRACT FROM AUTHOR]

A surprising discovery has been made that in some cases the complex, highly scattered Lg wave is found to be similar for clusters of events. We analyze in detail a subset of 28 out of 90 events from the 1999 Xiuyan sequence. Cross correlations provide highly accurate differential travel-time measurements. Their error estimated from the internal consistency is about 7 msec. These travel-time differences are then inverted by the double-difference technique to obtain epicenter estimates that have location precision on the order of 150 m. The locations are computed with waveform data from four to five regional stations 500 to 1000 km away. The epicenter estimates are not substantially affected by the sparseness of stations or large azimuthal gaps. Comparison with a surface trace a few kilometers away and location estimates based on much more dense networks led us to conclude that the absolute positions are accurate to the 5-km level. Regional event locations must often be based on a small number of phases and stations due to weak signal-to-noise ratios and sparse station coverage. This is especially true for monitoring work that seeks to locate smaller magnitude seismic events with a handful of regional stations. Two primary advantages of using Lg for detection and location are that it is commonly the largest amplitude regional wave (enabling detection of smaller events) and it propagates more slowly than P waves or Sn (resulting in smaller uncertainty in distance, for a given uncertainty in travel time). [ABSTRACT FROM AUTHOR]