Accurate Tremor Locations in Japan from Coherent S-Waves

Abstract:

The tremor detectors developed for accurately locating tectonic tremor in Cascadia [Armbruster et al., JGR 2014] have been applied to data from the HINET seismic network in Japan. The best results were obtained in the Tokai region with stations ASU, ASH and TYE having relatively close spacing (11-18 km). 330 days with active tremor, 2004-2014, near these stations were found on the daily epicentral distributions of tremor on the HINET web site. The detector sees numbers of detections per day comparable to minor tremor episodes in Cascadia. Major tremor episodes in Cascadia are associated with geodetic signals stronger than those seen in Japan. If the tremor is located by constraining it to the plate interface, a pattern of persistent sources is seen, with some intense sources. This is similar to what was seen in Cascadia. In southwest Shikoku 139 days with tremor were identified. Stations UWA, OOZ and IKT see tremor with persistent patterns and strong sources but with approximately one fifth as many detections per day on active days, compared to ASU-ASH-TYE. The web site tremor distributions show activity here as strong as in Tokai. We believe the lesser number of detections in Shikoku is primarily the result of wider station spacing, 19-30 km, than in Tokai, although there may be other factors. Yabe and Ide [EPS 2013] detect and locate tremor in Kyushu on July 17-18 2005 and December 4-6 2008. A detector with stations NRA, SUK and KTM, station spacing 21-22 km, sees tremor which resembles minor episodes in Cascadia. The relative arrival times are consistent with their locations. We conclude that the methods developed in Cascadia will work in Japan but the typical spacing of HINET stations, ~20 km, is greater than the optimum distance found in analysis of data from Cascadia, 8 to 15 km.