Monitoring and Forecasting Injection Induced Fault Reactivation and
Seismicity in the Laboratory using Active UltrasonicMethods
Abstract
Induced earthquakes are still highly unpredictable, and often caused by
variations in pore fluid pressure. Monitoring and understanding the
mechanisms of fluid-induced fault slip is essential for seismic risk
mitigation and seismicity forecasting. Fluid-induced slip experiments
were performed on critically stressed faulted sandstone samples, and the
evolution of the actively sent ultrasonic waves throughout the
experiment was measured. Two different fault types were used: smooth
saw-cut fault samples at a 35º angle, and a rough fault created by
in-situ faulting of the samples. Variations in the seismic slip velocity
and friction along the fault plane were identified by the coda of the
ultrasonic waves. Additionally, ultrasonic amplitudes show precursory
signals to laboratory fault reactivation. Our results show that small
and local variations in stress before fault failure can be inferred
using coda wave interferometry for time-lapse monitoring, as coda waves
are more sensitive to small perturbations in a medium than direct waves.
Hence, these signals can be used as precursors to laboratory fault slip
and to give insight into reactivation mechanisms. Our results show that
time-lapse monitoring of coda waves can be used to monitor local stress
changes associated with fault reactivation in this laboratory setting of
fluid-induced fault reactivation. This is a critical first step towards
a method for continuous monitoring of natural fault zones, contributing
to seismic risk mitigation of induced and natural earthquakes.