SUMMARYDeclared North Korean nuclear tests in 2006, 2009, 2013, and 2016 were observed seismically at regional and teleseismic distances. Waveform similarity allows the events to be located relatively with far greater accuracy than the absolute locations can be determined from seismic data alone. There is now significant redundancy in the data given the large number of regional and teleseismic stations that have recorded multiple events, and relative location estimates can be confirmed independently by performing calculations on many mutually exclusive sets of measurements. Using a 1-dimensional global velocity model, the distances between the events estimated using teleseismic P phases are found to be approximately 25% shorter than the distances between events estimated using regional Pn phases. The 2009The , 2013, and 2016 events all take place within 1 km of each other and the discrepancy between the regional and teleseismic relative location estimates is no more than about 150 m. The discrepancy is much more significant when estimating the location of the more distant 2006 event relative to the later explosions with regional and teleseismic estimates varying by many hundreds of meters. The relative location of the 2006 event is challenging given the smaller number of observing stations, the lower signal-to-noise ratio, and significant waveform dissimilarity at some regional 2 S. J. Gibbons et al.stations. The 2006 event is however highly significant in constraining the absolute locations in the terrain at the Punggye-ri test-site in relation to observed surface infrastructure.For each seismic arrival used to estimate the relative locations, we define a slowness scaling factor which multiplies the gradient of seismic traveltime versus distance, evaluated at the source, relative to the applied 1-d velocity model. A procedure for estimating correction terms which reduce the double-difference time residual vector norms is presented together with a discussion of the associated uncertainty. The modified velocity gradients reduce the residuals, the relative location uncertainties, and the sensitivity to the combination of stations used. The traveltime gradients appear to be overestimated for the regional phases, and teleseismic relative location estimates are likely to be more accurate despite an apparent lower precision. Calibrations for regional phases are essential given that smaller magnitude events are likely not to be recorded teleseismically. We discuss

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Declared North Korean nuclear tests in 2006, 2009, 2013 and 2016 were observed seismically at regional and teleseismic distances. Waveform similarity allows the events to be located relatively with far greater accuracy than the absolute locations can be determined from seismic data alone. There is now significant redundancy in the data given the large number of regional and teleseismic stations that have recorded multiple events, and relative location estimates can be confirmed independently by performing calculations on many mutually exclusive sets of measurements. Using a 1-D global velocity model, the distances between the events estimated using teleseismic P phases are found to be approximately 25 per cent shorter than the distances between events estimated using regional Pn phases. The 2009, 2013 and 2016 events all take place within 1 km of each other and the discrepancy between the regional and teleseismic relative location estimates is no more than about 150 m. The discrepancy is much more significant when estimating the location of the more distant 2006 event relative to the later explosions with regional and teleseismic estimates varying by many hundreds of metres. The relative location of the 2006 event is challenging given the smaller number of observing stations, the lower signal-to-noise ratio and significant waveform dissimilarity at some regional stations. The 2006 event is however highly significant in constraining the absolute locations in the terrain at the Punggye-ri test-site in relation to observed surface infrastructure. For each seismic arrival used to estimate the relative locations, we define a slowness scaling factor which multiplies the gradient of seismic traveltime versus distance, evaluated at the source, relative to the applied 1-D velocity model. A procedure for estimating correction terms which reduce the double-difference time residual vector norms is presented together with a discussion of the associated uncertainty. The modified velocity gradients reduce the residuals, the relative location uncertainties and the sensitivity to the combination of stations used. The traveltime gradients appear to be overestimated for the regional phases, and teleseismic relative location estimates are likely to be more accurate despite an apparent lower precision. Calibrations for regional phases are essential given that smaller magnitude events are likely not to be recorded teleseismically. We discuss the implications for the absolute event locations. Placing the 2006 event under a local maximum of overburden at 41.293°N, 129.105°E would imply a location of 41.299°N, 129.075°E for the January 2016 event, providing almost optimal overburden for the later four events.

This paper describes how commercial satellite imagery was used along with news reports and published scientific articles to investigate the December 1995 allegations of Indian nuclear test preparations in the Rajasthan Desert. Taking the allegations against India as an example of a future CTB compliance dispute, the investigation was conducted to test the utility of commercial satellite imagery for CTB verification. The technical inquiry produced a series of findings on India's nuclear testing history and on the recent nuclear test allegations. These findings included the exact location of the subsidence crater created by the May 18, 1974 nuclear test, the discovery of an adjacent military range near Khetolai village, and the observation of recent large-scale, unusual activity at this military range in the immediate vicinity of the 1974 test site. The image-derived information was used to sift fact from fiction in the conflicting media reports. It was then integrated into the collection of credible evidence and analyzed to determine whether the observed activity at the Khetolai military range was conventional, missile testing, nuclear, or innocuous. Gupta and PabianOur analysis led to three main conclusions. First, the Khetolai military range has a history of nuclear test activity. It was used for the May 18, 1974 nuclear test, and there is credible evidence from four different sources that indicates shafts were constructed in the early 1980s for two additional nuclear tests there. Second, there is believable evidence that supports the claim of planned Prithvi field testing at the Khetolai military range. An Indian news report with four verified factual claims described the planned missile activity, and 1:500,000 scale US DMA maps show five nearby military operation areas that could be well-suited as missile impact points. Third, a significant fraction of the image-derived evidence was consistent with nuclear test preparations, planned Prithvi missile testing, or a combination of both. Thus, nuclear test preparations and planned Prithvi field testing are each plausible, non-exclusive explanations for the observed new activity at the Khetolai military range.The paper "ends with an assessment of the implications of the case study to the broader issue of CTB verification and compliance. The conclusions include a discussion on the lessons that were learned from the verification exercise, and commentary on the value of existing commercial satellite imagery for addressing future CTB compliance issues. INTRODUCTIONOn December 15,1995, the iVeu; York Times reported that India was preparing to conduct a nuclear test in the Rajasthan Desert. 1 The following day, the Washington Post ran an article that also reported Indian preparations for a nuclear explosive test in northwest India. 2 These two articles generated a cacophony of expert and official responses, ranging from strongly worded declarations that the allegations of test preparations were definitely false to opposing statements that the reports were definitely true....