The role of cohesion in fault slip style

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The shear strength of rocks and sediments in and around fault zones controls whether the fault moves stably, or unstably as potentially damaging earthquakes. Shear strength has two parts: friction, which is the resistance to sliding for surfaces under pressure; and cohesion, which is an intrinsic strength that exists when the surfaces are completely unloaded. Up to now, previous fault studies have focused almost exclusively on how friction controls fault movement, but ignore the cohesion. However, cohesion is likely an important quantity for faults, for two main reasons: (1) earthquakes tend to occur in hard, lithified rocks rather than sediments, and (2) previous experiments show that stick-slip sliding, which is a laboratory earthquake, tends to occur in hard, competent rocks. Direct cohesion measurements are rare, but we have developed a proven method for successful measurements in the Marum laboratory. Here, we propose to conduct a systematic dataset of cohesion measurements, and to explore the role of cohesion in seismogenic fault slip. We will use drilling samples from ICDP projects, which may include the Alpine Fault in New Zealand, the San Andreas Fault, the Oman Ophiolite, the North Anatolian Fault and the Pärvie Fault System in Sweden. Other test materials may include basement rock from Oklahoma, USA, exhumed fault rock from the Shimanto Belt, Japan, and a variety of common rocks and pure minerals, used as testing standards. The main goal is to assemble cohesion measurements for a wide range of rock types under consistent conditions, and compare these measurements with the results of more conventional friction experiments on the same rock types under the same conditions. Based on this overall project, we will conduct more focused sub-projects aimed at how specific conditions affect cohesion and evaluate what this means for fault slip. These specific conditions include: temperature, pressure, the presence of fluid, sliding speed, and rock fabric. The results of these tests will allow us to compare the relative importance of cohesion compared to friction, with cohesion being a possible new powerful tool for understanding fault motion and earthquakes.