Chapter 5.2 - Characteristics of ACROSS signals from transmitting stations in the Tokai area 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. Accurately controlled routinely operated signal system (ACROSS) is a powerful tool for monitoring the temporal variation in seismic waves with high accuracy. In this study, using signals transmitted from the Toki and Morimachi seismic ACROSS stations both in the Tokai area, we evaluate seismograms and check the validity of ACROSS signals observed at Hi-net seismic stations. Using ACROSS signals, clear P and S waves can be observed in stations up to about 100km apart in all six components transfer functions with a stacking length of 1–30 days, which varies depending on the transmitter–station distance. The temporal variation in transfer function is also investigated. Clear temporal change above the noise level can be observed at some stations, which indicates the possibility for detecting temporal variations in waveforms using ACROSS signals. These variations correlate well with meteorological parameters, which suggest that the source of temporal variation exists near the Earth’s surface.