Seismological Characterization of Micro- and Macrofracturing Processes in a Fault Zone: Experiences from Laboratory Stick-Slip Friction Experiments and Close-By Monitoring of MW 1.9 Fault in a Deep South African Gold Mine.

Abstract:

Nearly 50 years have passed since the seminal work of Brace and Byerlee who first suggested that the stick-slip events frequently accompanying frictional sliding in laboratory experiments may be regarded as an experimental analog of shallow earthquakes. Since then numerous studies have investigated physical processes and characteristics of frictional sliding. Extrapolating these laboratory observations to the field scale provide important insights into the physics governing source processes of earthquakes. This can be achieved by a direct near-field in-situ fault observations. In the Earth crust deep mines provide a unique opportunity for this purpose by giving access to the focal depth of earthquakes.

In this study we compare the physical and statistical properties of the attoseismicity (M_W -8 to -6) recorded in the laboratory experiments on rock samples with nano- and picoseismicity (M_W -4 to 0) recorded in-situ after a M_W 1.9 earthquake. The microfracturing processes were investigated for the two laboratory stick-slip friction experiments performed on Westerly granite samples with different fault surface roughness (saw-cut fault, fault developed from a fresh fracture). We examined temporal changes of seismic moment tensors, source parameters and b-values of acoustic emission events that occurred during multiple seismic cycles. This study is compared with characteristics of seismicity preceding and following the M_W 1.9 earthquake that occurred in Mponeng mine, South Africa. The mainshock was followed by an aftershock sequence composed of more than 25000 nano- and picoearthquakes recorded using high frequency seismic network composed of accelerometers and acoustic emission sensors. Close source-receiver distances (<30m from the hypocenter) allowed to investigate the physical processes occurring in the fault zone before and after the failure in very fine details allowing to form a link between laboratory and in-situ observations.