Spatiotemporal distribution of seismic energy radiation from low‐frequency tremor in western Shikoku, Japan

Abstract

The spatiotemporal patterns of the radiated seismic energy from low‐frequency tremor associated with slow slip events in western Shikoku, Japan, have been estimated. A spatially decaying mean square amplitude and differential traveltime measurement from envelope correlations observed at Hi‐net stations were used to locate tremors and to estimate their seismic energies. Tremor amplitude was corrected for the site amplification factors estimated using the coda normalization method. The location of the tremor is estimated as the position where both the decay of the observed energy and the differential travel times are well explained. The temporal pattern of the total energy released agrees very well with the observed tilt records. Although the spatial patterns of the energy radiation vary strongly among the six slow slip events (SSEs) in western Shikoku, the region where they overlap always radiates relatively high levels of energy. The estimated band‐limited energy rate of the tremor for a 2 to 10 Hz bandwidth was typically between 105 and 106 J/min, though there is temporal variation of the radiation energy. The released total energy associated with the SSEs whose magnitude is 5.9–6.1 is on the order of 108 J within the duration of SSEs of 8–13 days. This is quite small energy radiation compared to the moment release of accompanying SSE. The scaled energy, which is the ratio of the seismic moment and the seismic energy, for the slow slip and a very low frequency earthquake was estimated to be 5 orders of magnitude smaller than that of a regular earthquake.