LABORATORY SIMULATIONS OF INDUCED SEISMICITY
Wei Wu1, 2
1
2
Department of Geophysics, Stanford University, Stanford, CA 94305, United States
State Key Laboratory of Geomechanics and Geotechnical Engineering, Institute of Rock and Soil
Mechanics, Chinese Academy of Sciences, Wuhan 430071, China
ABSTRACT
Induced seismicity occurs when human activities break frictional equilibrium of pre-existing fractures in
fractured rock masses. Pre-existing fractures are commonly found in brittle rocks and may be close to
failure. Stress perturbation triggers frictional slip when shear stress on a pre-existing fracture reaches or
exceeds its frictional strength. The perturbation induced by human activities modifies stress state in the
pre-existing fracture and results in slip failure and energy radiation.
A direct-shear model, consists of two thin rock plates and a layer of simulated granular gouges, is used to
study frictional slip triggering of a pre-existing fracture under quasi-static and dynamic shear stresses. A
servo-controlled quasi-static loading system induces the quasi-static triggering of frictional slip on the preexisting fracture. A dynamic loading system instantaneously launches a striker plate to impact the incident
plate and to generate an incident P-wave. The P-wave propagates in the incident plate as a shear stress and
induces the dynamic triggering of frictional slip. The dynamic triggering of frictional slip is solely induced
by the P-wave before wave reflection at the plate end.
Both quasi-static and dynamic shear stresses induce non-uniform shear stress distributed along the preexisting fracture. There is a shear stress at the trailing edge, which controls the frictional slip, and a
rebound stress at the leading edge, which is caused by a small moment. The frictional slip is triggered
when the maximum shear stress reaches a critical value at the trailing edge and is accompanied by shear
stress drop. The quasi-statically triggered frictional slip is unrecoverable and includes a main slip and a few
short slips before and after the main slip. The dynamically triggered frictional slip can be partially
recovered after the P-wave and consists of a few unrecovered slips. The duration of the dynamically
triggered frictional slip is a few microseconds, while the duration of the quasi-statically triggered frictional
slip is from a few seconds to many years.
To the end, the experimental results are compared with previous experimental and field observations. It is
found that fault strengthening takes a long time between slips, while seismic waves may successively
induce fault slip and restrict fault self-healing. Laboratory simulations of induced seismicity have
advantages to verify numerical and analytical predictions and to explore unknown field observations under
controllable indoor environments.
KEYWORDS
Laboratory testing, Induced seismicity, Direct shear, Frictional slip