Seasonal crustal seismic velocity changes：We benefit from the dense High-Sensitivity Seismograph Network (Hi-net) that covers the entire Japan to measure continuous changes in seismic velocities over a few years, using noise-based seismic monitoring. The accurate daily seismic δv/v in 1 – 7 s in the subsurface show coseismic and postseismic responses to the different earthquakes such as the Mw 9.0, 2011 Tohoku-oki earthquake. In addition to the relative seismic velocity decreases (103) that are coincident with different earthquakes, some seasonal seismic velocity changes with amplitudes of 104 are observed in large region in Japan. Similar to GPS and GRACE observations, crustal seismic velocities are affected by external environmental perturbations. We introduce a linear model to remove the seasonal components of seismic velocity changes to improve the detection capability of the crustal responses to small earthquakes that are previously hidden in these strong seasonal perturbations.
Depth variability of the crustal response to the Mw 9.0, 2011 Tohoku-oki earthquake：To understand how stress changes in the deep crust, we use the Hi-net tilt- meter recordings to measure seismic velocity changes in two period ranges 8 – 30 s and 15 – 50 s in northeastern Honshu. The results first prove the feasibility of using tiltmeter data to track seismic velocity changes at long period. Temporal evolution of seismic velocity changes in various periods show evidence of depth-dependent crustal rheology.