Laboratory Investigations of the Origin of Pulverized Rocks

Zones of pulverized rock have been observed in surface outcrops adjacent to the fault cores of the San Andreas and other major faults in Southern California. These pulverized rocks consist of highly fractured fragments that still fit together and essentially preserve the original rock texture. The origin of these pulverized rocks is not clear, but their structural context indicates that they are clearly associated with faulting; an understanding of their origin might allow inferences to be drawn about the nature of dynamic slip on faults, energy balance of earthquakes, and implications for ground motions and radiation patterns near faults. Our overall experimental study will include both quasi-static and dynamic loading to determine whether pulverized rocks can be produced in the laboratory and whether they indicate anything about the rate of deformation. In the present preliminary study, the first of a series of experiments to be undertaken, laboratory experiments are conducted on Westerly granite samples to investigate whether pulverized rocks can be produced under stress-wave loading conditions and whether they are diagnostic of any particular process of formation. In the first group of experiments a Split Hopkinson pressure bar (SHPB) is utilized to subject cylindrical rock specimens to well-defined uniaxial compressive stress-wave loading. In these experiments the amplitude as well as the duration of the compressive loading pulse is systematically varied to study the initiation and progression of fragmentation in both confined and unconfined granite samples. In the second group of experiments, plate-impact experiments are conducted to obtain the stress threshold for inelasticity in Westerly granite by estimating its Hugoniot Elastic Limit (HEL) under shock-induced compression. These experiments are also designed to provide spall (tensile) strength following shock-induced compression loading in the granite samples. It is expected that the measured spall strength will provide an indication of the extent of the shock-induced damage during compression in the rock samples. The results of the SHPB experiments indicate that the peak stress for Westerly granite under uniaxial compression is ~210 MPa (with a strain to failure of about 0.7%) in the unconfined state; the peak stress increases to 1 GPa under a confinement pressure of 60 MPa. In the unconfined state, the post-impact samples show no apparent fragmentation when the applied compression stress is lower than 180 MPa. However, at stress levels above 210 MPa extensive fragmentation is observed. Under confinement, the post- impact samples appear to be intact macroscopically when the applied compression stress is lower than 1 GPa; they show a few cracks on the impact surface but no extensive fragmentation when the applied compression stress is between 1.1 and 2.2 GPa, and they show extensive fragmentation when the applied compression stress is higher than 2.6 GPa. The HEL for the granite is estimated to be between 4.2 to 5.0 GPa. The spall strength following the shock-compression is measured to be small (~50 MPa), and nearly independent of the applied compression level in the range of 1.2 to 5.0 GPa. Detailed microstructural analysis of the post-test recovered specimens from both the SHPB and plate-impact experiments is currently underway to understand texture of the impacted rocks? The answers to these questions are expected to shed light concerning the origin and significance of the pulverized rocks adjacent to fault cores of major faults.