[1] We evaluate the effect of coseismic stress changes on the fault slip at midcrustal depth, assuming a velocity-strengthening brittle creep rheology. We show that this model can help reconcile the time evolution of afterslip, as measured from geodesy, with aftershocks decay. We propose an analytical expression for slip of the brittle creeping fault zone (BCFZ) that applies to any dynamic or static stress perturbation, including shear stress and normal stress changes. The model predicts an initial logarithmic increase of slip with time. Postseismic slip rate decays over a characteristic time t r = as _ t that does not depend on the amplitude of the stress perturbation, and it asymptotically joins the longterm creep imposed by interseismic stress buildup _ t. Given that the seismicity rate might be considered proportional to the sliding velocity of the BCFZ, the model predicts a decay rate of aftershocks that follows Omori's law, with a mathematical formalism identical to that of Dieterich [1994] although based on a different mechanical rationale. Our model also differs from Dieterich's model in that it requires that aftershock sequences and deep afterslip, as constrained from geodetic measurements, should follow the same temporal evolution. We test this for the 1999 Chi-Chi earthquake, M w = 7.6 and find that both sets of data are consistent with a model of afterslip due to the response of the BCFZ. The inferred relaxation time t r = 8.5 years requires a value for a = @m/@log(V) (m being the coefficient of friction) in the range between 1.3 10 À3 and 10 À2 .INDEX TERMS: 3210 Mathematical Geophysics: Modeling; 3902 Mineral Physics: Creep and deformation; 5120 Physical Properties of Rocks: Plasticity, diffusion, and creep; 8123 Tectonophysics: Dynamics, seismotectonics; 8159 Tectonophysics: Rheology-crust and lithosphere; KEYWORDS: postseismic relaxation, Omori law, Chi-Chi earthquake Citation: Perfettini, H., and J.-P. Avouac (2004), Postseismic relaxation driven by brittle creep: A possible mechanism to reconcile geodetic measurements and the decay rate of aftershocks, application to the Chi-Chi earthquake, Taiwan,

Slip on a subduction megathrust can be seismic or aseismic, with the two modes of slip complementing each other in time and space to accommodate the long-term plate motions. Although slip is almost purely aseismic at depths greater than about 40 km, heterogeneous surface strain suggests that both modes of slip occur at shallower depths, with aseismic slip resulting from steady or transient creep in the interseismic and postseismic periods. Thus, active faults seem to comprise areas that slip mostly during earthquakes, and areas that mostly slip aseismically. The size, location and frequency of earthquakes that a megathrust can generate thus depend on where and when aseismic creep is taking place, and what fraction of the long-term slip rate it accounts for. Here we address this issue by focusing on the central Peru megathrust. We show that the Pisco earthquake, with moment magnitude M(w) = 8.0, ruptured two asperities within a patch that had remained locked in the interseismic period, and triggered aseismic frictional afterslip on two adjacent patches. The most prominent patch of afterslip coincides with the subducting Nazca ridge, an area also characterized by low interseismic coupling, which seems to have repeatedly acted as a barrier to seismic rupture propagation in the past. The seismogenic portion of the megathrust thus appears to be composed of interfingering rate-weakening and rate-strengthening patches. The rate-strengthening patches contribute to a high proportion of aseismic slip, and determine the extent and frequency of large interplate earthquakes. Aseismic slip accounts for as much as 50-70% of the slip budget on the seismogenic portion of the megathrust in central Peru, and the return period of earthquakes with M(w) = 8.0 in the Pisco area is estimated to be 250 years.

[1] One way to probe the rheology of the lithosphere and fault zones is to analyze the temporal evolution of deformation following a large earthquake. In such a case, the lithosphere responds to a known stress change that can be assessed from earthquake slip models constrained from seismology and geodesy. Here, we model the postseismic response of a fault zone that is assumed to obey a rate-strengthening rheology, where the frictional stress varies as as ln( _ ), _ being the deformation rate and as > 0 a rheological parameter. The model is simple enough that these parameters can be estimated by inversion of postseismic geodetic data. We apply this approach to the analysis of geodetic displacements following the M w 7.3, 1992, Landers earthquake. The model adjusts well the measured displacements and implies as % 0.47-0.53 MPa. In addition, we show that aftershocks and afterslip follow the same temporal evolution and that the spatiotemporal distribution of aftershocks is consistent with the idea that they are driven by reloading of the seismogenic zone resulting from frictional afterslip.

We use about two decades of geodetic measurements to characterize interseismic strain build up along the Central Andes subduction zone from Lima, Peru, to Antofagasta, Chile. These measurements are modeled assuming a 3‐plate model (Nazca, Andean sliver and South America Craton) and spatially varying interseismic coupling (ISC) on the Nazca megathrust interface. We also determine slip models of the 1996 Mw = 7.7 Nazca, the 2001 Mw = 8.4 Arequipa, the 2007 Mw = 8.0 Pisco and the Mw = 7.7 Tocopilla earthquakes. We find that the data require a highly heterogeneous ISC pattern and that, overall, areas with large seismic slip coincide with areas which remain locked in the interseismic period (with high ISC). Offshore Lima where the ISC is high, a Mw∼8.6–8.8 earthquake occurred in 1746. This area ruptured again in a sequence of four Mw∼8.0 earthquakes in 1940, 1966, 1974 and 2007 but these events released only a small fraction of the elastic strain which has built up since 1746 so that enough elastic strain might be available there to generate a Mw > 8.5 earthquake. The region where the Nazca ridge subducts appears to be mostly creeping aseismically in the interseismic period (low ISC) and seems to act as a permanent barrier as no large earthquake ruptured through it in the last 500 years. In southern Peru, ISC is relatively high and the deficit of moment accumulated since the Mw∼8.8 earthquake of 1868 is equivalent to a magnitude Mw∼8.4 earthquake. Two asperities separated by a subtle aseismic creeping patch are revealed there. This aseismic patch may arrest some rupture as happened during the 2001 Arequipa earthquake, but the larger earthquakes of 1604 and 1868 were able to rupture through it. In northern Chile, ISC is very high and the rupture of the 2007 Tocopilla earthquake has released only 4% of the elastic strain that has accumulated since 1877. The deficit of moment which has accumulated there is equivalent to a magnitude Mw∼8.7 earthquake. This study thus provides elements to assess the location, size and magnitude of future large megathurst earthquakes in the Central Andes subduction zone. Caveats of this study are that interseismic strain of the forearc is assumed time invariant and entirely elastic. Also a major source of uncertainty is due to fact that the available data place very little constraints on interseismic coupling at shallow depth near the trench, except offshore Lima where sea bottom geodetic measurements have been collected suggesting strong coupling.

[1] We consider the effect of permanent stress changes on a velocity strengthening rate-and-state fault, through numerical simulations and analytical results on 1-D, 2-D, and 3-D models. We show that slip transients can be triggered by perturbations of size roughly larger than L b = G d c /bs, where G is the shear modulus, d c and b are the characteristic slip distance and the coefficient of the evolution effect of rate-and-state friction, respectively, and s is the effective normal stress. Perturbations that increase the Coulomb stress lead to the strongest transients, but creep bursts can also be triggered by perturbations that decrease the Coulomb stress. In the latter case, peak slip velocity is attained long after the perturbation, so that it may be difficult in practice to identify their origin. The evolution of slip in a creep transient shares many features with the nucleation process of a rate-and-state weakening fault: slip initially localizes over a region of size not smaller than L b and then accelerates transiently and finally expands as a quasi-static propagating crack. The characteristic size L b implies a constraint on the grid resolution of numerical models, even on strengthening faults, that is more stringent than classical criteria. In the transition zone between the velocity weakening and strengthening regions, the peak slip velocity may be arbitrarily large and may approach seismic slip velocities. Postseismic slip may represent the response of the creeping parts of the fault to a stress perturbation of large scale (comparable to the extent of the main shock rupture) and high amplitude, while slow earthquakes may represent the response of the creeping zones to a more localized stress perturbation. Our results indicate that superficial afterslip, observed at usually seismogenic depths, is governed by a rate-strengthening rheology and is not likely to correspond to stable weakening zones. The predictions of the full rate-and-state framework reduce to a pure velocity strengthening law on a timescale longer than the duration of the acceleration transient, only when the triggering perturbation extends over length scales much larger than L b .Citation: Perfettini, H., and J.-P. Ampuero (2008), Dynamics of a velocity strengthening fault region: Implications for slow earthquakes and postseismic slip,

No abstract

Abstract. We study the effect of time-variable normal stress perturbations on a creeping fault which satisfies a velocity-weakening rate-and state-dependent friction law and is slipping at constant speed. We use the spring-block model and include the effect of inertia. To account for the variable normal stress, we use the description introduced by Linker and Dieterich [1992], which links normal stress fluctuations to changes of the state variable. We consider periodic perturbations of the normal stress in time (as caused, for instance, by tides) and compare the behavior for two commonly used friction laws (the "slip" and the "ageing" laws). Their mechanical response is shown to be significantly different for normal stress fluctuations. It could be used to probe these two laws during laboratory friction experiments. We show that there is a resonance phenomenon, involving strong amplification of the shear and velocity response of the interface, when the spring stiffness is modestly above its critical value (or when, at a given stiffness, the normal stress is modestly below its critical value). We show that such an amplification is also observed when periodic fluctuations of the shear loading are considered, making the resonance phenomenon a general feature of the response of a near-critical creeping surface to periodic fluctuations of the external loading. Analytical solutions are based on a linear expansion for low amplitude of normal or shear stress variations and are in very good agreement with numerical solutions. A method to find the evolution of friction in the case of an arbitrary perturbation of the normal stress is also presented. The results show that a creeping fault may be destabilized and enter a stick-slip regime owing to small normal stress oscillations. This may also account for a mechanism for the generation of "creep bursts." However, these phenomena require very specific parameter ranges to excite the resonance, which may not be met very generally in nature. This study illustrates the importance of the normal stress fluctuations on stable sliding and suggests further friction laboratory experiments.

We refer to periods of subduction strain accumulation beneath the Mentawai Islands, Sumatra, as "supercycles," because each culminates in a series of partial ruptures of the megathrust in its final decades. The finale of the previous supercycle comprised two giant earthquakes in 1797 and 1833 and whatever happened in between. This behavior between two great ruptures has implications for how the megathrust will behave between its more recent partial failure, during the M w 8.4 earthquake of 2007, and subsequent large ruptures. We synthesize previously published coral microatoll records and a large new coral data set to constrain not only these two giant ruptures but also the intervening interseismic megathrust behavior. We present detailed maps of coseismic uplift during the two earthquakes and of interseismic deformation during the periods 1755-1833 and 1950-2000, as well as models of the corresponding slip and coupling on the underlying megathrust. The large magnitudes we derive (M w 8.6-8.8 for 1797 and M w 8.8-8.9 for 1833) confirm that the 2007 earthquakes released only a fraction of the moment released during the previous rupture sequence. Whereas megathrust behavior leading up to the 1797 and 2007 earthquakes was similar and comparatively simple, behavior between 1797 and 1833 was markedly different and more complex: several patches of the megathrust became weakly coupled following the 1797 earthquake. We conclude that while major earthquakes generally do not involve rupture of the entire Mentawai segment, they may significantly change the state of coupling on the megathrust for decades to follow, influencing the progression of subsequent ruptures.