Migration Episode of Shallow Low-frequency Tremor at the Nankai Trough Subduction Zone: Seismological Evidence for Episodic Slow Slip Event Occurring at the Shallow Transition Zone

Friday, 19 December 2014
Yusuke Yamashita1, Hiroshi Yakiwara2, Hiroshi Shimizu3, Kazunari Uchida3, Shuichiro Hirano2, Hiroki Miyamachi2, Kodo Umakoshi4, Manami Nakamoto3, Miyo Fukui3, Megumi Kamizono3, Hisao Kanehara5, Tomoaki Yamada1, Masanao Shinohara1 and Kazushige Obara1, (1)Earthquake Research Institute, University of Tokyo, Tokyo, Japan, (2)Nansei-Toko Observatory for Earthquakes and Volcanoes, Kagoshima University, Kagoshima, Japan, (3)Institute of Seismology and Volcanology, Kyushu University, Shimabara, Japan, (4)Graduate School of Fisheries Science and Environmental Studies, Nagasaki University, Nagasaki, Japan, (5)Faculty of Fisheries, Nagasaki University, Nagasaki, Japan
To understand the shallow part of plate interface between megathrust seismogenic zone and trench axis is very important for development of huge earthquake rupture and generation of tsunami. Monitoring of offshore seismicity near the Nankai trough by temporal ocean bottom seismographic observation in 2013 revealed that low-frequency tremor occurred associated with shallow very-low-frequency earthquakes (VLFEs) in the shallow part of plate interface. The shallow tremor episode lasted for approximately 1 month, which is almost consistent with the shallow VLFE activity observed from land broad-band seismic stations [Asano, 2014]. The horizontal location of shallow tremor estimate by envelope correlation method [Obara, 2002] shows a belt-like distribution along trench strike with narrow width than the deep tremor. The most remarkable feature of the shallow tremor activity is migration. There are two migration modes including diffusive slower migration and rapid tremor reversal (RTR), which are very similar to the deep tremor as a part of the ETS. This strongly indicates a possibility of the occurrence of episodic slow slip event in the shallow transition zone. That is to say, the migration of shallow tremor is supposed to be caused by migrating rupture front of SSE. In addition, the migration was detouring around the subducted Kyushu-Palau ridge. This suggests that the occurrence of tremor is sensitive to change in the shape of plate interface and seeks to propagate along almost the same depth range, in other word, a specified temperature and pressure condition. The narrow width distribution of shallow tremor also indicates that the shallow tremor is strongly related to dehydration process of a specified mineral under a narrow limited range of temperature and pressure condition compared to the deep tremor.