[1] On the basis of elastodynamic stress fields for singular crack and nonsingular slipweakening models of propagating rupture, we develop preliminary answers to such questions as follows: If a rupturing fault is intersected by another, providing a possible bend in the failure path, when will stressing be consistent with rupture along the bend? What secondary fault locations and orientations, in a damaged region bordering a major fault, will be stressed to failure by the main rupture? Stresses that could initiate rupture on a bend are shown to increase dramatically with crack speed, especially near the limiting speed (Rayleigh for mode II, shear for mode III). Whether a bend path, once begun, can be continued to larger scales depends on principal stress directions and ratios in the prestress field. Conditions should often be met in mode II for which bend paths encouraged by stressing very near the rupture tip are discouraged by the larger-scale stressing, a basis for intermittent rupture propagation and spontaneous arrest. Secondary failure in the damage zone likewise increases markedly as the limiting speed is approached. Such may make the fracture energy much greater than for slip on a single surface. The extent of secondary faulting is strongly affected by prestress directions and the ratio of residual to peak strength. For mode II, prestress controls whether activation occurs primarily on the extensional side, which we show to be the typical case, or on the compressional side too. Natural examples are consistent with the concepts developed.INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7260 Seismology: Theory and modeling; 8020 Structural Geology: Mechanics; 8168 Tectonophysics: Evolution of the Earth: Stresses-general; 7230 Seismology: Seismicity and seismotectonics; KEYWORDS: fault mechanics, fault branching, rupture dynamics, earthquakes Citation: Poliakov, A. N. B., R. Dmowska, and J. R. Rice, Dynamic shear rupture interactions with fault bends and off-axis secondary faulting,

[1] We consider a mode II rupture which propagates along a planar main fault and encounters an intersection with a branching fault. Using an elastodynamic boundary integral equation formulation, allowing the failure path to be dynamically self-chosen, we study the following questions: Does the rupture initiate along the branch? Does it continue? Is the extensional or compressional side most favored for branching? Does rupture continue on the main fault too? Failure is described by a slip-weakening law for which the strength at any amount of slip is proportional to normal stress. Our results show that dynamic stresses around the rupture tip, which increase with rupture velocity at locations off the main fault plane relative to those on it, could initiate rupture on a branching fault. As suggested by prior work, whether branched rupture can be continued to a larger scale depends on principal stress directions in the prestress state and on rupture velocity. The most favored side for branching rupture switches from the extensional to the compressional side as we consider progressively shallower angles of the direction of maximum compressive prestress with the main fault. Simultaneous rupturing on both faults can be activated when the branching angle is wide but is usually difficult for a narrow branching angle due to strong stress interactions between faults. However, it can be also be activated by enhanced dynamic stressing when the rupture velocity is very near the Rayleigh velocity. Natural examples seem consistent with the simulations that we present.INDEX TERMS: 7209 Seismology: Earthquake dynamics and mechanics; 7223 Seismology: Seismic hazard assessment and prediction; 7260 Seismology: Theory and modeling; 8010 Structural Geology: Fractures and faults; KEYWORDS: branching, fault, rupture propagation, boundary integral equation method (BIEM), fracture Citation: Kame, N., J. R. Rice, and R. Dmowska, Effects of prestress state and rupture velocity on dynamic fault branching,

SUMMARY The different phases of the earthquake cycle can produce measurable deformation of the Earth's surface. This work is aimed at describing the evolution of that deformation in space and time, as well as the distribution of causal slip on the fault at depth. We have applied GPS and synthetic aperture radar (SAR) interferometry (InSAR) techniques to northern Chile, where fast plate convergence rates are associated with large subduction earthquakes and extensive crustal deformation. The region of northern Chile between 18°S and 23°S is one of the most important seismic gaps in the world, with no rupture having occurred since 1877. In 1995, the Mw= 8.1 Antofagasta earthquake ruptured the subduction interface over a length of 180 km in the region immediately to the south of this 450 km long gap. The coseismic deformation associated with this event has been documented previously. Here we use GPS position time‐series for 40 benchmarks (measured between 1996 and 2000) and ERS SAR interferograms (for the interval between 1995 and 1999) to map both the post‐seismic deformation following the 1995 event and the ongoing interseismic deformation in the adjacent gap region. In the seismic gap, the interseismic velocities of 20–30 mm yr−1 to the east with respect to South America are mapped. Both the GPS and the InSAR measurements can be modelled with 100 per cent coupling of the thrust interface of the subduction to a depth of 35 km, with a transition zone extending down to 55 km depth. The slip rate in that zone increases linearly from zero to the plate convergence rate. South of the gap, the interferometric map shows interseismic deformation superimposed with deformation following the 1995 earthquake and covering the same area as the coseismic deformation. Some 40 per cent of this deformation is related to seismic activity in the 3.3 yr following the 1995 event, in particular slip during a Mw= 7.1 earthquake in 1998. However, most of the signal (60 per cent) corresponds to post‐seismic deformation resulting from widespread aseismic slip in the subduction interface. The afterslip appears to have occurred down‐dip in the transition zone at 35–55 km depth and to have propagated laterally northwards at 25–45 km depth under the Mejillones Peninsula, which is a prominent geomorphological feature at the boundary between the 1877 and 1995 rupture zones. We propose a simple slip model for the seismic cycle associated with the Antofagasta earthquake, where the transition zone alternates between aseismic shear and seismic slip.