Time-Variation of Later-Arrivals in the Transfer Functions Obtained by a Permanent Seismic Source and Receiver Array (ACROSS)

ACROSS stands for Accurately Controlled, Routinely Operated Signal System. Repeated transmission of a controlled seismic signal and long-term continuous observation of the seismic waves enable precise monitoring of time-evolving physical properties of the Earth’s interior. An observation using the ACROSS source was conducted in the Tokai distinct, Japan to establish a method to monitor the state of coupling of the plate boundary and to understand the time-evolving phenomena such as slow-slip and deep low-frequency tremors (DLFTs). A seismic signal emitted from the ACROSS source was observed by a temporary short-span surface seismometer array and a permanent borehole station (N.HOUH) located at the distance of 57 km from the source for 10 months. Coherent later phases were detected using the semblance analysis of the array of transfer functions and were interpreted as deeper reflections. Here the stability of these phases are discussed using borehole data. The result implies the possibility of seismic monitoring of seismogenic plate-subduction zone using ACROSS.