Relationship between triggered deep low-frequency events in western Japan and strain changes due to the Rayleigh waves from the 2004 Sumatra-Andaman earthquake

The dilatational strain changes due to the Rayleigh waves from the 2004 Sumatra-Andaman earthquake, triggered periodic deep low-frequency seismic events in western Japan. The triggered events pose constraints on the possible source mechanisms of slab tremor. The observations are that the triggering is coincident with the large dilatation at the source region at depth of about 30 km, the dilatation is related with the triggered signal amplitude, shear waves do not contributed to the triggering, and short-period (15--30 sec) Rayleigh waves much strongly triggered the events than the first arrival long-period (> 40 sec) ones. The two normal strain changes, in radial and vertical directions, rather than volumetric changes, seem to be much related with the triggering signals. The similar triggering from the other large earthquake, the 2003 Tokachi-oki earthquake (M8.1), also shows that the triggered signal amplitudes are well related with the Rayleigh wave amplitudes. The triggering from dilatational components rather than shear ones suggests that the fluid supplied by the dehydration from the subducting slab plays an important role for the excitation. We compared the signal amplitudes with the dilatations due to the Rayleigh waves, at source regions. We found a clear exponential relationship between them. The possible triggering mechanism is that the fluid moves to fault zone due to expansion and effectively reduces the friction on the fault, resulting in the deep low-frequency event, where the strain energies are released relatively slowly. Our result is one of the clear seismic evidences that the deep low-frequency events have fluid-flow processes.