Abstract
We discovered a secular change in the travel time of direct S-waves over a 10-year observation period by means of continuous operation of an artificial and stable seismic source, called Accurately Controlled Routinely Operated Signal System (ACROSS), which is deployed in the central part of Japan along the Nankai Trough. We used 13 High Sensitivity Seismograph Network Japan (Hi-net) stations around the ACROSS source to monitor the temporal variation in travel time. Green’s functions were calculated for each station daily from March 29, 2007, through October 30, 2017. Secular advance in the temporal variation in travel time was seen for the whole operation period, in addition to a steplike delay associated with the 2011 Tohoku earthquake. We estimated the rate of secular change and the amount of coseismic step by modeling the transfer function of S-waves with a linear trend and the coseismic step of the 2011 Tohoku earthquake. Distance dependences of the travel time changes can be explained as a combination of common bias and dispersion for each station, for both the secular and coseismic changes. This can be interpreted as a randomly distributed change in seismic velocity over the range of the observation region. An azimuthal dependence exists for both changes and shows larger changes in the NE–SW direction than in the NW–SE direction from the ACROSS source.
Highlights
Temporal variation in the propagation property of seismic waves has been studied to understand the state of the subsurface medium under various tectonic circumstances
As secular advance and coseismic step are commonly recognizable for the stations with good signal-to-noise ratio (SNR), we modeled this variation with a simple equation to examine whether these characteristics held for the stations with lower SNR
The rates of the secular change ranged from 0.0 to 1.4 ms/year, and the amounts of coseismic step at the Tohoku earthquake ranged from − 4.0 to 0 ms (Additional file 3)
Summary
Temporal variation in the propagation property of seismic waves has been studied to understand the state of the subsurface medium under various tectonic circumstances. Stress/strain, which changes crack density, and pore pressure, which changes fluid saturation, can be detected by measuring variation of seismic velocity. Yamaoka et al (2001) demonstrated variation of travel time with a resolution of 0.1 ms, and Ikuta et al (2002) were the first to detect coseismic change of seismic velocity induced by the strong motion of earthquakes using ACROSS with a 15-month observation period.
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