Noise‐based crustal seismic velocity changes are known to be affected by environmental perturbations, such as rainfall, atmospheric pressure loading, and temperature changes. Similar to geodetic observations, these external perturbations can mask the effects of tectonic and volcanic processes. In this study, we benefit from the dense Hi‐net short‐period seismic network that covers the entire Japan to measure continuous changes in seismic velocities over a few years, using noise‐based seismic monitoring. Some strong seasonal seismic velocity changes are observed in both southern Japan (Kyushu Island) and northern Japan (Hokkaido Island). Decreasing of seismic velocities in summer in southern Japan can be clearly explained by a model of increased crustal fluid pore pressure associated with high rainfall. In northern Japan, it is necessary to adopt a more complex model to explain the observed seismic velocity variations, which takes into account precipitation, snow depth, and sea level changes. Moreover, western and eastern Hokkaido Island show very different responses to these different external perturbations. The models developed are used to remove the seasonal components of the seismic velocity changes. The minimum remaining detectable seismic velocity change reduces to 10−5, which allows detection of crustal responses to small earthquakes that are previously hidden in the strong seasonal perturbations.

We performed time lapse measurements of velocity variations using empirical Green's functions reconstructed by autocorrelation of seismic noise recorded during a period of 17 years in the region of l'Aquila, Italy. The time lapse approach permitted us to evaluate the spatial (depth) dependence of velocity variation (dv/v). By quantitatively comparing the 17 years of dv/v time series with independent data (e.g. strain induced by earthquakes, hydrological loading) we unravel a group of physical processes inducing velocity variations in the crust over multiple time and spatial scales. We find that rapid shaking due to three magnitude 6+ earthquakes mainly induced near surface velocity variations. On the other hand, Slow strain perturbation (period 5 years) associated with hydrological cycles, induced velocity changes primarily in the middle-crust. The observed behavior suggests the existence of a large volume of fluid filled cracks exist deep in the crust. Our study, beyond shedding new light into the depth dependent rheology of crustal rocks in the region or l'Aquila, highlights the possibility of using seasonal and multiyear perturbations to probe the physical properties of seismogenic fault volumes. 1. Introduction Detailed laboratory protocols exist to estimate how rocks respond to strain perturbations, and show that a variety of non-linear responses exists for variable rock types with different physical properties (e.g. cracks density, microstructure, presence of fluid, temperature and pressure effects