Accurate relative location estimates for the North Korean nuclear tests using empirical slowness corrections

Abstract

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

Yield Estimates for the Six North Korean Nuclear Tests From TeleseismicPWave Modeling and Intercorrelation ofPandPnRecordings

“…Based on the published estimates of burial depths in Table , we specify depths of 380 m for NK1, 600 m for NK2, 420 m for NK3, and 750 m for NK4, NK5, and NK6. These depths are generally consistent with absolute estimates from precise relative location estimates (e.g., Gibbons et al, ; Pasyanos & Myers, ), but significant uncertainty remains, and we will evaluate the effect of these choices. Filtered synthetics for a distance of 5,500 km are then constructed for the chosen source depth for a range of yield spanning the estimates in Table along with Futterman t* values from 0.4 to 1.3 s. While some paths likely have frequency‐dependent attenuation, Chaves et al () found that overall their waveform predictions of the NK6 event favored a constant‐ Q model.…”

“…The events were readily detected and identified (e.g., Walter et al, ), in addition to being announced. Precise relative locations for the six explosions, with uncertainties on the order of a few hundred meters, have been determined by numerous seismic differential arrival time studies (e.g., Carluccio et al, ; Gibbons et al, , ; He et al, ; Myers et al, ; Murphy et al, ; Selby, ; Wang et al, ; Wen & Long, ; Yao et al, ; Zhang & Wen, ; Zhao et al, ; Zhao, Xie, He, et al, ). Absolute locations are much more difficult to resolve, and various strategies have been used to place the suite of precise relative locations in absolute position with respect to the source region topography, locations of tunnel portals, and/or deformation features on the surface inferred from InSAR or satellite photography (e.g., Murphy et al, ; Myers et al, ; Pabian & Coblentz, ; Pabian & Hecker, ; Patton & Pabian, ; Wei, ), with recent results having less than ~0.5‐km variations.…”

Relative surface wave amplitude and phase anomalies from the Democratic People's Republic of Korea announced nuclear tests

“…The direction of the steepest grade for the 200905 and 201302 events ranges between 130°N and 140°N which is also the direction of the largest amplitude ratio and phase shift Geophysical Research Letters 10.1002/2017GL074577 anomalies. Gibbons et al [2016] also attribute topographic effects to surprising differences with Pn waveform similarity across the dense network of Japan. There is no correspondence between the amplitude ratio or phase shift anomalies with the size of the isotropic component from the RMT which only differ between events by an average of ±8% (ranging between 66 and 81%).…”

“…If we used 0.9 threshold, only 2% would be removed. Table S1 in the supporting information lists the Bayesloc body wave, surface wave grid search, and Norwegian Seismic Array (NORSAR) body wave relative locations from Gibbons et al [2016]. Figure 2 shows the measured surface wave and the predicted δt computed from the best fit relative location obtained through the grid search.…”

Seismic Spectral Ratios Between North Korean Nuclear Tests: Implications for Their Seismic Sources

“…At most regional and teleseismic stations with data accessible, the frequency band with enough signal‐to‐noise ratios (SNR) for NKT2006 is quite narrow. Additionally, according to accurate relative location results (e.g., Gibbons et al, ), the distances between NKT2006 and other Korean nuclear tests are significantly larger than distances among the others, which would increases the scatter in the observed spectral ratios at different stations especially at high frequencies.…”