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

Abstract

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.

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

“…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 . []. Figure shows the measured surface wave and the predicted δt computed from the best fit relative location obtained through the grid search.…”

“…Figure shows the measured surface wave and the predicted δt computed from the best fit relative location obtained through the grid search. In addition, the predicted differential times given for both the Bayesloc and NORSAR [ Gibbons et al ., ] body wave relative locations were computed using equation .…”

“…Gibbons et al . [] 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%).…”

“…The four recent explosions were clustered under the 2205 m granite peak of Mount Mant'ap. Recent studies of relative locations for various DPRK explosion event pairs using regional distance body wave phases [e.g., Wen and Long , ; Zhang and Wen , ; Gibbons et al ., ] and teleseismic P [e.g., Selby , ; Gibbons et al ., ] were precise and consistent in relative separation patterns. However, the surface wave‐based relative locations for the DPRK explosions are inconsistent with body wave‐based relative locations.…”

High‐Precision Relocation With the Burial Depths of the North Korean Underground Nuclear Explosions by Combining Pn and Pg Differential Traveltimes

“…Relative location methods have frequently been used to provide accurate epicenters relative to a reference event for a group of closely distributed sources such as aftershock sequences or nuclear explosions. Accordingly, this approach has been successfully applied to obtain the relative locations of the NKTS nuclear explosions (e.g., Gibbons et al., 2017; He et al., 2018; Murphy et al., 2013; Schaff et al., 2018; Selby, 2010; Tian et al., 2018; Tibuleac et al., 2008; Wen & Long, 2010; Zhang & Wen, 2013; Zhao et al., 2014, 2016, 2017). However, most of these relative location methods do not provide satisfactory information on the burial depth due to the strong trade‐off between the burial depth and origin time (e.g., Waldhauser & Ellsworth, 2000; Zhao et al., 2014).…”