Observed source parameters for dynamic rupture with non-uniform initial stress and relatively high fracture energy |
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Institution: | 1. Dpto Geodynamics, Faculty of Geologicas Science, IGEO (UCM-CSIC) Universidad Complutense, Madrid, Spain;2. Área de Geología, Universidad Rey Juan Carlos, C/Tulipán s/n, 28933 Móstoles, Madrid, Spain;3. Geofísica Consultores, c/Ricardo León 33, Torrelodones 28250, Spain;1. Department of Psychiatric Nursing and Human Sciences, University of São Paulo at Ribeirão Preto College of Nursing;2. Bachelor and Teaching Diploma Program in Nursing, University of São Paulo at Ribeirão Preto College of Nursing;3. General and Specialized Nursing Department, University of São Paulo at Ribeirão Preto College of Nursing |
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Abstract: | We have conducted dynamic rupture propagation experiments to establish the relations between in-source stress drop, fracture energy and the resulting particle velocity during slip of an unconfined 2 m long laboratory fault at normal stresses between 4 and 8 MPa. To produce high fracture energy in the source we use a rough fault that has a large slip weakening distance. An artifact of the high fracture energy is that the nucleation zone is large such that precursory slip reduces fault strength over a large fraction of the total fault length prior to dynamic rupture, making the initial stress non-uniform. Shear stress, particle velocity, fault slip and acceleration were recorded coseismically at multiple locations along strike and at small fault-normal distances. Stress drop increases weakly with normal stress. Average slip rate depends linearly on the fault strength loss and on static stress drop, both with a nonzero intercept. A minimum fracture energy of 1.8 J/m2 and a linear slip weakening distance of 33 μm are inferred from the intercept. The large slip weakening distance also affects the average slip rate which is reduced by in-source energy dissipation from on-fault fracture energy.Because of the low normal stress and small per event slip (~86 μm), no thermal weakening such as melting or pore fluid pressurization occurs in these experiments. Despite the relatively high fracture energy, and the very low heat production, energy partitioning during these laboratory earthquakes is very similar to typical earthquake source properties. The product of fracture energy and fault area is larger than the radiated energy. Seismic efficiency is low at ~2%. The ratio of apparent stress to static stress drop is ~27%, consistent with measured overshoot. The fracture efficiency is ~33%. The static and dynamic stress drops when extrapolated to crustal stresses are 2–7.3 MPa and in the range of typical earthquake stress drops. As the relatively high fracture energy reduces the slip velocities in these experiments, the extrapolated average particle velocities for crustal stresses are 0.18–0.6 m/s. That these experiments are consistent with typical earthquake source properties suggests, albeit indirectly, that thermal weakening mechanisms such as thermal pressurization and melting which lead to near complete stress drops, dominate earthquake source properties only for exceptional events unless crustal stresses are low. |
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