These approaches can be applied to regional-scale seismic arrays in a specific target region.Īt 21:18 (JST) on June 23, 2015, an M 6.8 ( Mw 6.5) deep-focus earthquake (focal sphere shown in Fig. These techniques enable detection of smaller, local earthquakes hidden in noisy waveforms or the overlapping coda waves of larger earthquakes, and the catalogs updated with the resulting newly detected events have revealed new insights into earthquake occurrences, such as triggering mechanisms, migration processes, and stress changes after large earthquakes. ![]() 2012) and high-frequency seismogram analysis (e.g., Miyazawa 2011 Sawazaki and Enescu 2014), have been developed. ![]() To overcome these difficulties, several methods, such as the matched filter technique (e.g., Kato et al. Nevertheless, misdetection of earthquakes occurs frequently because of emergent arrivals, overlapping larger events, earthquake swarms, and low signal-to-noise ratios (for smaller or distant earthquakes). Through the development of instruments and observation networks, detection capabilities for small earthquakes have improved significantly (e.g., Obara et al. Therefore, to reveal the detailed characteristics of the seismicity and subsurface structure beneath the Japanese Islands, various seismic networks have been developed (e.g., Okada et al. 1), because two oceanic plates are subducting, the seismic velocity structure of the region is complex and the seismicity is high. Three magnitude-6-class earthquakes occurred sequentially within 4 min in the Pacific slab at 480 km depth, where complex heterogeneities exist within the slab. Furthermore, based on the ratios of the S-wave amplitudes to that of the first event, the magnitudes of the latter events are estimated as M 6.5 ± 0.02 and M 5.8 ± 0.02, respectively. The maximum-amplitude patterns of the latter two earthquakes were similar to that of the first Bonin earthquake, which indicates similar locations and mechanisms. The United States Geological Survey catalog includes three Bonin deep-focus earthquakes with similar hypocenter locations, but their estimated magnitudes are inconsistent with seismograms from across Japan. Snapshots of the seismic energy propagation illustrate that after one deep-focus earthquake occurred beneath the Sea of Japan, two deep-focus earthquakes occurred sequentially after the first ( Mw 6.5) event beneath the Bonin Islands in the next 4 min. ![]() Observed high-frequency (>1 Hz) seismograms across Japan, which contain several sets of P- and S-wave arrivals for the 10 min after the origin time, indicate that moderate-to-large earthquakes occurred sequentially around Japan. An M 6.8 ( Mw 6.5) deep-focus earthquake occurred beneath the Bonin Islands at 21:18 (JST) on June 23, 2015.
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