Neil D. Selby

Relative Locations of the October 2006 and May 2009 DPRK Announced Nuclear Tests Using International Monitoring System Seismometer Arrays

Abstract Waveform data recorded at seven of the International Monitoring System (IMS) seismometer arrays are used to measure the relative times of teleseismic P signals with three methods: analyst picks, and two methods that make full use of the capabilities of seismometer arrays, the cross correlation of array beams and the average of channel-by-channel cross correlations. These times are used to estimate the relative locations of the 9 October 2006 and 25 May 2009 Democratic People’s Republic of Korea announced underground nuclear tests. The 2009 test is found to have occurred about 1.8±0.8 km to the west and 0.4±0.6 km to the north of the 2006 test. The use of cross correlation reduces the standard deviation of the travel-time residuals from ∼0.05 to ∼0.01 sec, enabling the two epicenters to be statistically distinguished with high confidence. This result demonstrates the power of a small number of IMS seismometer arrays at teleseismic distances to detect and relatively locate small explosions with high precision.

mb:Ms Event Screening Revisited

The ratio of body‐wave to surface‐wave magnitude, m b: M s, has historically been one of the most effective methods for distinguishing earthquakes from underground explosions. In the context of the Comprehensive Nuclear‐Test‐Ban Treaty (CTBT), m b: M s is currently one of the experimental standard event‐screening criteria being provisionally tested at the International Data Centre (IDC). An event in the IDC analyst‐reviewed bulletin is screened out if the hypothesis that it is an underground explosion can be rejected with high confidence. Recently, two announced nuclear tests by the Democratic People’s Republic of Korea have raised interest because the M s values for these explosions are high compared to historical explosions with similar m b. On an m b: M s plot, both explosions lie close to the contemporary IDC experimental screening line, M s=1.25 m b −2.2. Although neither explosion was screened out by the IDC, the two explosions indicate that a revision of the line is advisable to ensure with high confidence that any future underground nuclear explosion is not screened out. Here, m b and M s magnitudes for 409 past underground nuclear explosions are collated and presented. The magnitudes include new measurements, an archive of historical measurements made over the years at AWE Blacknest, and a reworking of bulletin data. The revised m b: M s screening line based on these magnitudes is M s= m b −0.64. The effect of the revised line on event screening at the IDC is assessed. It is found that the new criterion screens out 42% of a set of events from 2008, whereas the old criterion screened out 87%, which is a large reduction. The revised provisional m b: M s screening line was agreed upon by the Waveform Expert Group at Working Group B of the CTBT Preparatory Commission in February 2010 and has been tested in operations at the IDC since 3 June 2010. Online Material: Tables of supplementary source parameters.

Improved Teleseismic Signal Detection at Small-Aperture Arrays

Abstract Here, the implementation of the generalized F detector (Selby, 2008) as an automatic teleseismic signal detector is described. The method is applied to 10 days of waveform data from 13 small-aperture arrays of the primary seismic network of the International Monitoring System (IMS), which is being set up to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The results demonstrate that the generalized F method can be used as a signal detector at small-aperture arrays, despite the correlated noise that prevents the original F detector being useful at such arrays. By comparing lists of detections with predicted arrival times of first-arriving P -type phases from earthquakes in the Reviewed Event Bulletin (REB, produced by the International Data Centre, IDC, being set up to monitor the CTBT), it is shown here that the generalized F detector compares favorably with traditional signal detection methods used at the IDC, with more candidate associations being made despite the total number of F detections being about half of those made by the IDC. Increasing the proportion of associated detections should improve the efficiency with which automatic bulletins can be constructed, and reduce analyst workload. Further advantages over traditional methods are that the F detector approach is simple to explain, based on probability theory and physical models of noise and signals, treats each array equally and objectively, and does not require subjective tuning. Analysis of the vector slowness distribution of associated and unassociated detections shows that associated detections have similar distributions for both F and the IDC, whereas the distribution of unassociated detections can be different, with unassociated IDC detections at some arrays apparently being associated with correlated noise sources.

Seismic Discrimination in Southern Xinjiang: The 13 March 2003 Lop Nor Earthquake

Earthquakes that occur near known nuclear test sites are invaluable for evaluating the capabilities of the International Monitoring System (ims), currently being established to monitor compliance with the Comprehensive Nuclear-Test-Ban Treaty (ctbt). On 13 March 2003 a seismic disturbance with magnitude m b (National Earthquake Information Center [neic]) 4.8 occurred near the Chinese nuclear test site at Lop Nor, southern Xinjiang. Here we attempt to identify this disturbance as an earthquake by using three types of seismic data: (1) teleseismic P waves recorded at ims stations; (2) long-period surface waves recorded at a network of stations in Eurasia that simulates the proposed ims seismic network in this region, and (3) long-period full waveform ( P, S , and surface wave) modeling at station wmq, which is approximately 250 km from the reported epicenter of the 13 March 2003 disturbance. We find that all impulsive teleseismic P -wave onsets show compressive first motion and that discrimination using the m  b : M s criterion requires assumptions that cannot be justified on theoretical grounds. However, by combining the three data types, we conclude that the observations are consistent with a double-couple source with strike φ = 125 ± 10°, dip δ = 40 ± 10°, rake λ = 90 ± 10°, and moment M 5.5 ± 1 × 1015 N m. That said, consistency of the seismic wave field with a double couple does not immediately rule out an explosion source, because tectonic release accompanying an explosion can make it impossible to resolve the isotropic moment using long-period data. The strongest argument for an earthquake is that both the regional surface waves, and waveform modeling at wmq, give an estimated focal depth of 6 ± 1 km. This result reinforces the importance of focal depth determination to ctbt monitoring, in particular, for shallow earthquakes with mechanisms that are close to perfect 45° reverse-dip-slip, which are difficult to discriminate by using other methods. Fortunately, depth determination using surface waves is especially favorable for this type of earthquake, if a sufficient number of stations can be used. We therefore recommend that long-period data recorded at auxiliary seismic stations of the ims be utilized by the International Data Centre to monitor compliance with the ctbt.

Comment on “Unexplained Sets of Seismographic Station Reports and a Set Consistent with a Quark Nugget Passage” by David P. Anderson, Eugene T. Herrin, Vigdor L. Teplitz, and Ileana M. Tibuleac

Anderson et al. (2003) interpret a set of unassociated seismic arrivals observed on 24 November 1993 as a line source, which they claim is consistent with the passage of a “strange quark nugget” through the Earth. In fact, these arrivals can be convincingly interpreted as an earthquake source on the Pacific-Antarctic ridge. We attempt to form an event using the arrivals listed in table 6 of Anderson et al. (2003) and find, as the authors do, that this is not possible without a large residual at station stk. We then use the bulletin of the ISC (International Seismological Centre, 2001) to investigate residuals at stk during November 1993. The results are shown in Figure 1. We find that the residuals show a clear trend with values around −9 sec on 24 November. Consequently we believe …

Seismic Detections of Small-Scale Heterogeneities in the Deep Earth

We report the detection of coherent scattered energy related to the phase PKPPKP (P′P′) in the data of medium aperture arrays. The scattered energy (P′•P′) is weak and requires array processing techniques to extract the signal from the noise. The arrival time window of P′•P′ is mostly free from other interfering body wave energy and can be detected over a large distance range. P′•P′ has been detected in the data of large aperture arrays previously, but the detection in the data of smaller arrays shows its potential for the study of the small-scale structure of the Earth. Here, we show that P′•P′ can detect scattering off small-scale heterogeneities throughout the Earth’s mantle from crust to core making this one of the most versatile scattering probes available. We compare the results of P′•P′ to a related scattering probe (PK•KP). The detected energy is in agreement with stronger scattering, i.e., more heterogeneous structure, in the upper mantle and in an approximately 800-km-thick layer above the core–mantle boundary. Lateral variations in heterogeneity structure can also be detected through differences in scattered energy amplitude. We use an application of the F-statistic in the array processing allowing us a precise measurement of the incidence angles (slowness and backazimuth ) of the scattered energy. The directivity information of the array data allows an accurate location of the scattering origin. The combination of high-resolution array processing and the scattering of P′•P′ as probe for small-scale heterogeneities throughout the Earth’s mantle will provide constraints on mantle convection , mantle structure , and mixing related to the subduction process.