金曜日セミナー(2019年11月1日) Dr. Jean-Pierre Vilotte (IPGP)

Low-frequency earthquakes in western Shikoku (Japan): from detection to space-time activity, through source characteristics


Jean-Pierre Vilotte (IPGP)


Abstract
Low-frequency earthquakes (LFEs) are slow earthquakes observed in the transition zones of subduction zones and active faults. LFEs are associated with impulsive transient signal events mixed within complex tremor signal. Slow earthquake signals provide sparse observations probing different scales of the energy-release process – that still remains to be fully understood – along plate boundaries slowly driven out of equilibrium. Detailed analysis of LFEs sources and space-time activity in relation to SSEs and VLFEs can provide important information on the state and the dynamics of the subduction plate boundary.

We derived a catalog of LFEs in western Shikoku (Japan) by applying array-based detection and location method to 4-year (2013-2016) continuous waveforms  period of recorded at Hi-net seismic stations of NIED. Advanced time-frequency statistical signal processing scheme, combined with network-based coherency analysis provides an efficient tool for extracting the LFEs from noisy signals and estimating source location without a prior assumption about the event source (i.e., template event). The derived catalog of over 150,000 events.

LFEs’ sources parameters were inferred from S-wave displacement amplitude spectrum using a Bayesian method. Estimated moment magnitudes range between Mw 0.9 and 2.2, while corner frequencies vary between 0.8 and 12 Hz. The LFEs’ sources exhibit a power-law scaling between corner frequency and seismic moment similar to the scaling observed for classical earthquakes.

The derived LFE catalogue also provides detailed information on the LFE space-time activity during tectonic tremor sequences and inter-sequence periods. We will discuss on-going developments to extract scale-dependent space-time characteristics and dynamic interactions of the LFE activity, in relation with short-term SSEs.