Measuring time-lapse changes from noise and earthquakes

Time-lapse changes in the subsurface or laboratory studies reveals how earth materials are behaving. Seismic or ultrasonic waves that are extremely complicated can successfully be used to extract time-lapse changes. Coda waves that are are strongly scattered can be used to measure velocity changes as small as 0.1%. I will show how such measurements can be used to unravel reversible and irreversible changes in rock samples. Earthquakes recorded over more than 10 years at KiK-net stations all over Japan are a rich source of data to monitor changes in seismic velocity in the near-surface. I will show that the shear velocity changes with the seasons, and that it is reduced by earthquakes. After the Tohoku-Oki earthquake the shear wave velocity is reduced all over the eastern half of Japan.