Insights into pulverized rock formation from dynamic rupture models of earthquakes

Abstract:

Pulverized rocks (PR) are extremely incohesive and highly fractured rocks found within the damage zones of several large strike-slip faults around the world. They maintain their crystal structure, show little evidence of shearing or chemical alteration, and are believed to be produced by strong tensile forces. Several mechanisms for pulverization have been proposed such as a normal stress reduction at the rupture tip that results in unloading failure similar to rock bursts, or a rapid straining from the passing rupture that forces multiple fractures to propagate simultaneously as in the case of an impact. Each mechanism has flaws and for now lab and field studies are at an impasse. Numerical modeling, however, offers new insights into what is needed to produce PR and likely conditions of formation. We perform dynamic rupture simulations of different earthquakes, varying the magnitude, the slip distribution, and the rupture speed (supershear and subshear), while measuring the stresses produced away from the fault. A basic threshold of 10 MPa is set as the tensile strength of the rock mass and recordings are made of where, when, and by how much this threshold is exceeded for each earthquake type. Guided by field observations, we discern that a large (> Mw 7.3) supershear earthquake along a bimaterial fault produces a pulverized rock distribution most consistent with observations. The damage is asymmetric with the majority on the stiffer side of the fault extending out for several hundred meters. These results indicate that PR are not necessarily a cumulative damage product, but instead can be formed by a single large event. The results also suggest that the presence of PR may be a marker for past supershear rupture propagation, something with significant repercussions for seismic hazard assessments.