2014 年 66 巻 4 号 p. 97-112
NIED Hi-net is the nation-wide high sensitivity seismograph network which has been constructed by the National Research Institute for Earth Science and Disaster Prevention. The high quality observation data of the Hi-net brought us outstanding contribution to seismology. Recently, the detection of minute changes of seismic travel times with the use of the Hi-net data is tried by various methods such as seismic ACROSS (Accurately Controlled, Routinely Operated, Signal System), seismic interferometry and so on. Although time accuracy is essential for precise measurement of temporal variations of seismic travel times, reliability evaluation of the sampling timing of the Hi-net data has been not always sufficient so far. This paper reports the temporal variations of the sampling timings at the twenty Hi-net stations for the eight years from 2004 to 2012 and proposes an approach to detect minute changes in seismic travel times in sub-millisecond order. The sensor check signal which is usually used to calibrate the seismometer, is enabled on 1PPS of GPS receiver independently of the sampling clock. Therefore the sampling clock can be calibrated by the GPS clock using the delay time of the sensor check signal. The delay times are usually stable, but stepwise changes were observed at restarts of ADC (Analog-to-Digital Converter). This method was applied to the TOKI seismic ACROSS signals observed by Hi-net Yaotsu (11.3 km distance from TOKI). Consequently, sub-millisecond to several milliseconds stepwise travel time changes were removed, and travel time changes owing to large earthquakes, and annual and secular travel time changes became clear. This correction method is useful to not only the seismic source-receiver system such as ACROSS but also other methods to detect seismic travel time changes of utilizing arrival time difference between observation stations.