Advantages of high-resolution seismic velocity monitoring using coda wave interferometry with an accurately controlled seismic source

SUMMARY The Accurately Controlled Routinely Operated Signal System (ACROSS) is an artificial seismic source that generates highly repeatable and stable seismic waves for high-resolution temporal monitoring. While direct P and S waves from ACROSS have been widely used to monitor earthquakes, volcanic activity and environmental changes, the scattered waves that arrive later, known as coda waves, have received limited attention. In this study, we used coda waves generated by ACROSS to monitor subsurface seismic velocity changes (dv/v) over 10 months. The ACROSS signals were recorded by a seismic array of 14 seismometres deployed approximately 3 km from the source in Morimachi, central Japan. By deconvolving the records with a known source function, we obtained transfer functions corresponding to band-limited Green’s functions. Coda wave interferometry applied to the coda portions of these transfer functions detected temporal variations in dv/v, exhibiting both seasonal long-term variations and rainfall-induced short-term variations. Comparative analyses using direct P- and S-wave traveltime changes from the same ACROSS data and ambient-noise interferometry show that, under the present observational conditions, these conventional methods primarily detect long-term variations; however, they do not resolve the rapid, transient short-term variations. In contrast, the stable ACROSS source combined with the broader sampling of coda waves provides superior sensitivity to environmental changes across both timescales over kilometre-scale distances, highlighting its effectiveness for high-temporal-resolution monitoring. To investigate the mechanisms responsible for the observed dual-timescale variations, we used a poroelastic model with precipitation input and considered thermoelastic effects and groundwater-induced changes as possible contributors to the long-term variations.