Influence of initial stress and rupture initiation parameters on forbidden zone rupture propagation
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The Authors 2015. Published by Oxford University Press on behalf of The Royal Astronomical Society. Well established theoretical and numerical results of 2-D ruptures have been accepted for years to limit the speed of mode II cracks to be below the Rayleigh velocity or above the shear wave speed. However, recent work has numerically produced rupture speeds in this so-called 'forbidden zone', that is the region of rupture velocities between the Rayleigh wave speed and the shear wave speed, for 3-D simulations. We verify that finding here and further examine the dependence of that behaviour on initial stress and rupture initiation parameters. Using a 3-D finite element model for dynamic rupture propagation, numerical experiments were performed for different initial stress conditions as well as different size initiation patches and forced rupture velocities. It is shown that the initial stress on the fault has a strong influence on the resulting rupture, specifically with regards to the distance at which the rupture transitions to supershear speeds, the maximum rupture velocity attained on the fault, and how rapidly the rupture passes through the forbidden zone. It is also demonstrated that for the same initial stress, increasing the size of the nucleation patch or the speed of forced rupture can artificially increase the gradient of the rupture velocity within the forbidden zone. This suggests that the rupture is uniquely predetermined by the stress state and material properties of the fault and surrounding medium in these models.
