Large-scale aseismic motion identified through 4-D P-wave tomography

In 2000, a large water injection (over 23 000 m3) has been conducted in granite through a 5-km-deep borehole at Soultz-sous–Forêts, in the Upper Rhine Graben (northeastern France). The microseismicity induced by this hydraulic stimulation was monitored with a network of 14 seismic stations deployed at ground surface. Some 7215 well-located events have been used to conduct a 4-D tomography of P-wave velocities. The method combines a double-difference tomography method with an averaging post-processing that corrects for parameter dependence effects. The total set of 7 215 events has been divided into 14 subsets that explore periods defined with respect to the injection scheme. Particular attention is given to changes in injected flow rates, periods of stationary injection conditions and post-injection periods. Fast changes in VP velocities are identified in large rock mass volumes precisely when the injection flow rate varies while little velocity variation is detected during stationary injection periods. The VP anomalies observed during stationary injection conditions are interpreted as being caused by effective stress variations linked to fluid diffusion, while the fast changes observed concomitantly to changes in flow rate are considered to be caused by non-seismic motions.