ALBERT

Description: Using template waveforms from aftershocks of the Wenchuan earthquake (12 May 2008, M s  7.9) listed in a global bulletin and continuous data from eight regional stations, we detected more than 6000 additional events in the mainshock source region from 1 May to 12 August 2008. These new detections obey Omori’s law, extend the magnitude of completeness downward by 1.1 magnitude units, and lead to a more than fivefold increase in number of known aftershocks compared with the global bulletins published by the International Data Centre and the International Seismological Centre. Moreover, we detected more M 〉2 events than were listed by the Sichuan Seismograph Network. Several clusters of these detections were then relocated using the double-difference method, yielding locations that reduced travel-time residuals by a factor of 32 compared with the initial bulletin locations. Our results suggest that using waveform correlation on a few regional stations can find aftershock events very effectively and locate them with precision.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Eleven small seismic events that occurred after the North Korean underground nuclear test (UNT) on 3 September 2017 have been reported. We detected three additional events not found using traditional methods by using a waveform‐matching technique. The magnitude of these fourteen shocks ranges from Mw 2.1 to 4.0. The first and largest of them, occurring about 8½ minutes after the nuclear explosion itself, has been identified by several authors as a cavity collapse. We analyzed the remaining 13 events, first by using regional 〈span〉P〈/span〉‐ and 〈span〉S〈/span〉‐wave spectral amplitude ratios observed at station MDJ (Mudanjiang, China), applying Mahalanobis methods to identify them. Applying a previously established linear discriminant function for the region to observed 〈span〉P/S〈/span〉 spectral ratios, 12 of the events are classified as earthquakes, and 1 other, the first of a doublet pair on 9 December 2017, appeared to be an explosion. However, waveform similarity of this event with its doublet pair, and application of the MDJ discriminant function to data from two other regional stations suggests that it, too, is likely to be an earthquake. It appears that damage caused by strong ground motion from the large UNT of 3 September 2017 has led to slow adjustments in the surrounding region entailing stress relief via aftershocks. Additional small seismic events (earthquakes) may be expected in the region.〈/span〉

Description: We assess seismological evidence bearing on claims that North Korea conducted a small nuclear test on 12 May 2010 in the vicinity of known underground nuclear tests (UNTs) in 2006, 2009, 2013, and 2016. First, we use Lg -wave cross correlation and more traditional methods to locate the 2010 event between about 4 and 10 km southwest of the 2009 test. Second, we compare the relative sizes of regional P and S waves, using stations within 400 km of the known North Korean nuclear tests, to assess the nature of the event. We measured P / S ratios at different frequencies, at first using data from the open station MDJ in northeast China, for training sets of earthquakes and of explosions. We developed a linear discriminant function (LDF) that, in application to P / S measured at MDJ, is most effective in separating the earthquake and explosion populations. MDJ lacks usable data for the event of interest, but we obtained regional data from stations of the nearby Dongbei Broadband Seismographic Network (DBSN) for the 12 May 2010 event and for nearby UNTs conducted in 2006 and 2009. When our LDF is applied to DBSN data, and to data from stations SMT and NE3C in China, the LDF values measured from P / S ratios from known explosions are explosion-like; but for the 12 May 2010 event, the LDF values are earthquake-like for frequencies between 6 and 12 Hz. Our method for characterizing earthquakes and explosions on the basis of their regional signals can be widely applied. Measurements of P / S based on the three-component waveform data provide better discrimination power than do those based on vertical-component data alone. Electronic Supplement: Tutorial material on the Mahalanobis distance-squared measure, three-component linear discriminant function (LDF) analysis, tables of measurements of the log 10 P / S spectral ratios obtained from waveforms recorded at station MDJ for the two training sets and three-component discrimination analysis, and figures of log ( P / S ) values measured at 8 Hz from vertical-component waveforms at station MDJ for two training sets and probability distributions for D .