Chapter 25 – Characteristics of ACROSS Signals Transmitted from the Tono Transmitting Station and Observed by Hi-net

Abstract Monitoring the temporal variation of physical properties around the plate boundary region is important for detecting subtle changes before the occurrence of a large earthquake. However, few studies have been done that involve continuously monitoring seismic waves. ACROSS is a newly developed tool for monitoring the temporal variation in seismic waves with high accuracy. In this study, using signals transmitted from the Tono seismic ACROSS station, we evaluate seismograms and check the validity of ACROSS signals observed at Hi-net seismic stations deployed by NIED. Using ACROSS signals, we tested our capability for detecting temporal variations in travel times for P and S waves. A clear onset can be observed in stations up to about 60 km apart. The wave train of P and S waves vary from station to station, suggesting that the effect of local velocity structure under the station is large within the frequency range used (10-20 Hz). The temporal variation in transfer function is also investigated, with transfer functions calculated every 10 days for about one and half years. A clear temporal change above the noise level, apparently an annual variation, is observed at the N.HOUH station. This variation, the cause of which is not clear at this time, indicates the possibility for detecting temporal variations in waveforms using ACROSS signals.