![]() ![]() Our analyses also confirm that the recent southern events of June 2019 and May 2020 are both tectonic earthquakes that occurred naturally. We used the two ratio types jointly in a quadratic discriminant function and successfully classified the six declared nuclear tests and the triggered earthquakes that followed the September 2017 explosion. The performed discriminant analyses suggest that combining Pg/Lg and Pn/Lg ratios results in improved discriminant power compared with any of the ratio types alone. However, based on P/Lg ratios, the distinction between the earthquakes and the cavity collapse is ambiguous. We were able to separate the cavity collapse from the population of nuclear explosions. In this work, we used P/Lg ratios from regional stations to categorize seismic events that occurred in NK from 2006 to May 2020, including these two recent events, the six declared NK nuclear tests, and the cavity collapse more » and triggered earthquakes that followed the 3 September 2017 nuclear explosion. Although these events were located ~330–400 km from the known nuclear test site, the fact that they occurred within the territory of NK, a country with a recent history of underground nuclear tests, made them events of interest for the monitoring community. Two events of magnitude (m b) 3.6–3.8 occurred in southern North Korea (NK) on 27 June 2019 and. The lack of improvement may be attributed to a greater variance in Lg travel times. Typically, we added 6 Lg arrival times to a total of 15 Pn and Pg arrival times. The addition of Lg arrivals to locate events did not result in better locations. ![]() The average location errors from the true epicenters using regional data are approximately 6 km. When average location errors are compared against errors with the standard method, the correlation matrix is about 3 km better, e.g., about 17 kilometers as compared to 20 kilometers the regional models method is 3 to 6 km better, e.g., about 5 kilometers as compared to 9 kilometers, depending on whether near-regional stations are used. They can be used to locate seismic events in any source region, with any set of stations. Furthermore, these methods do not use station corrections therefore the locations more » obtained are not station- or region-dependent. All the methods evaluated in this report appear to give smaller absolute epicenter and depth errors than the standard method of locating events. The new methods include: Using P travel times in laterally heterogeneous media: A station travel time residual correlation matrix in the normal equations Location with simultaneous determination of Pn, Pg, and Lg velocities Combination of the correlation matrix method and the simultaneous determination method. Several location methods developed at SDAC are evaluated by comparing location errors of these methods against errors computed with the standard method. The results of this analysis show that backazimuth can be determined to within +- 5/sup 0/ of the true backazimuth, about 90% of the time, after systematic variations are taken = , ![]() All calculations and observations are made after the events have been filtered between 0.3 and 6 Hz. The local magnitude (M/sub L/) of the events range from 3.7 to 5.6 with the small events barely visible over the microseisms. The stations range from 230 km to 400 km away from the events. The data set used consists of the seismic signals from 11 nuclear events at the Nevada Test Site (NTS) and recorded at the four Lawrence Livermore National Laboratory (LLNL) seismic stations: Elko, Kanab, Landers, and Mina. In this study locations are obtained using a backazimuth determined by the particle motion of Pn and a distance determined by differential travel times between Pn, Pg, and Lg relative to a master event of known location. This report describes the progress being made in event location using a single 3-component station. ![]()
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