**1. Introduction**

20 Will-be-set-by-IN-TECH

104 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

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Earthquake (Mw9.0) as Imaged with Back-Projection of Teleseismic P-waves. *Earth Planets Space*, Special Issue: First Results of the 2011 Off the Pacific Coast of Tohoku By virtue of dense networks of GPS and highly sensitive seismic stations, occurrences of deep low-frequency tremors/earthquakes have been recognized worldwide in the deep portions of subduction plate boundaries (e.g., Schwartz & Rokosky, 2007). Since these tremors and earthquakes have moment release rate smaller than that of regular earthquakes, the low-frequency tremors (LFTs) /earthquakes are classified into "slow-earthquake" group (Ide et al., 2007). The slow-earthquakes occur sometimes independently and sometimes break into chain-reaction propagating at a speed of about 10 km/day. However, a systematic explanation of the origin of such slow-earthquake migrations in some subduction zones is still lacking.

First of this study, we investigate the mechanisms of slow-earthquake migration by comparing observational results and numerical simulation results. Since some LFTs are found to be modulated by Earth tides (e.g., Nakata et al., 2008) and the moment release rate of the slow earthquake group is much smaller than that of regular earthquakes (Ide et al., 2007), they likely have low-stress drop and are sensitive to shear stress perturbations possibly induced by the preseismic slip of nearby megathrust earthquakes.

Second of this study, we propose a new method to detect precursory change around the large asperity prior to a megathrust earthquake, focusing on the spatio-temporal change of migration speed and moment release rate for nearby slow earthquakes. Recently, lowfrequency tremors/earthquakes have been also detected in the shallower portion of frictional transition zone on the subduction plate boundaries (Obana & Kodaira, 2009).

Third of this study, we also perform another numerical simulation of shallow low-frequency tremors/earthquakes, comparing with the characteristics of the deep low-frequency tremors/earthquakes and discussing the strategy of Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) toward an anticipated Tonankai Earthquake.
