327 ANNALS OF GEOPHYSICS, VOL. 48, N. 2, April 2005 Key words dynamic rupture – fault constitutive law – fault friction – stress heterogeneities 1. Introduction Modeling the dynamic propagation of earth- quake ruptures requires the adoption of a con- stitutive relation, which governs the traction evolution within the cohesive zone (Ida, 1972; Andrews, 1976a; Okubo and Dieterich, 1984; Cocco and Bizzarri, 2002). In particular, the Rate- and State-dependent friction laws (RS) derived from laboratory experiments (Di- eterich, 1979a,b; Ruina, 1983) have been wide- ly used in numerical simulations of earthquake ruptures, both at the laboratory scale (Bizzarri and Cocco, 2003 and reference therein) and for real-world faults (Guatteri et al., 2001, 2003). The frictional stress in RS laws depends on the slip rate and on the state variable. This class of constitutive laws includes an evolution law for the state variable and involves a friction de- pendence on time. Since RS laws account for fault restrengthening after dynamic failure, they can be used to simulate repeated seismic events Modeling the dynamic rupture propagation on heterogeneous faults with rate- and state-dependent friction Elisa Tinti, Andrea Bizzarri and Massimo Cocco Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy Abstract We investigate the effects of non-uniform distribution of constitutive parameters on the dynamic propagation of an earthquake rupture. We use a 2D finite difference numerical method and we assume that the dynamic rupture propagation is governed by a rate- and state-dependent constitutive law. We first discuss the results of several numerical experiments performed with different values of the constitutive parameters a (to account for the di- rect effect of friction), b (controlling the friction evolution) and L (the characteristic length-scale parameter) to simulate the dynamic rupture propagation on homogeneous faults. Spontaneous dynamic ruptures can be simu- lated on velocity weakening (a < b) fault patches: our results point out the dependence of the traction and slip ve- locity evolution on the adopted constitutive parameters. We therefore model the dynamic rupture propagation on heterogeneous faults. We use in this study the characterization of different frictional regimes proposed by Boatwright and Cocco (1996) based on different values of the constitutive parameters a, b and L. Our numeri- cal simulations show that the heterogeneities of the L parameter affect the dynamic rupture propagation, control the peak slip velocity and weakly modify the dynamic stress drop and the rupture velocity. Moreover, a barrier can be simulated through a large contrast of L parameter. The heterogeneity of a and b parameters affects the dynamic rupture propagation in a more complex way. A velocity strengthening area (a > b) can arrest a dynam- ic rupture, but can be driven to an instability if suddenly loaded by the dynamic rupture front. Our simulations provide a picture of the complex interactions between fault patches having different frictional properties and il- lustrate how the traction and slip velocity evolutions are modified during the propagation on heterogeneous faults. These results involve interesting implications for slip duration and fracture energy. Mailing address: Dr. Elisa Tinti, Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Roma, Italy; e-mail: tinti@ingv.it