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D'Agostino N.,Italian National Institute of Geophysics and Volcanology
Geophysical Research Letters | Year: 2014

Here I compare estimates of tectonic strain rates from dense Global Positioning System measurements with the seismicity released in the last ~500 years in the Apennines (Italy). The rates of seismic moment accumulation from geodesy and of historical seismic release by earthquakes agree within the uncertainties, ruling out significant aseismic deformation. Within the considered 400 km long section of the Apennines, this balance yields an average recurrence interval of 30-75 years for MW≥6.5 events without requiring a future earthquake larger than those observed historically (M W~7). A minimum estimate of unreleased strain allows M W≥6.5 and MW≥6.9 events to be released in ~35% and ~10% of the central-southern Apennines, respectively. The definition of the seismic potential for smaller events is more uncertain, and their occurrence remains a significant threat throughout the Apennines. Key Points Active crustal extension is completely released by earthquakes in the Apennines Last 500 years provide a representative sample of Apennines long-term seismicity Minimum estimate of unreleased strain spatial distribution since 1550 ©2014. American Geophysical Union. All Rights Reserved.

Bizzarri A.,Italian National Institute of Geophysics and Volcanology
Earth and Planetary Science Letters | Year: 2011

A rate- and state-dependent governing law with temperature-dependent constitutive parameters is considered on the basis of laboratory inferences. We model the whole seismic cycle of a homogeneous fault obeying to such a law by adopting a spring-slider dashpot fault analog model. We show that the variations of the parameter a (accounting for the so-called direct effect) with the temperature cause the system to enter, at high speeds, in a conditionally stable regime and also in a velocity strengthening regime. Although we do not observe the complete cessation of slip we can see a severe reduction of the degree of the instability of the fault. In particular, the peaks of the sliding velocity are reduced, as well as the developed temperature due to frictional sliding and the released stress during each instability event. Moreover, the recurrence times are reduced of a factor of two with respect to a reference configuration, where the canonical formulation of rate and state friction (with temporally constant parameters) is assumed. The obtained results can help the interpretation of high velocities laboratory experiments and further illuminate the importance of the temperature in the context of seismic hazard assessment. © 2011 Elsevier B.V.

Bizzarri A.,Italian National Institute of Geophysics and Volcanology
Reviews of Geophysics | Year: 2011

The quantitative estimate of earthquake damage due to ground shaking is of pivotal importance in geosciences, and its knowledge should hopefully lead to the formulation of improved strategies for seismic hazard assessment. Numerical models of the processes occurring during seismogenic faulting represent a powerful tool to explore realistic scenarios that are often far from being fully reproduced in laboratory experiments because of intrinsic, technical limitations. In this paper we discuss the prominent role of the fault governing model, which describes the behavior of the fault traction during a dynamic slip failure and accounts for the different, and potentially competing, chemical and physical dissipative mechanisms. We show in a comprehensive sketch the large number of constitutive models adopted in dynamic modeling of seismic events, and we emphasize their prominent features, limitations, and specific advantages. In a quantitative comparison, we show through numerical simulations that spontaneous dynamic ruptures obeying the idealized, linear slip-weakening (SW) equation and a more elaborated rate-and state-dependent friction law produce very similar results (in terms of rupture times, peaks slip velocity, developed slip, and stress drops), provided that the frictional parameters are adequately comparable and, more importantly, that the fracture energy density is the same. Our numerical experiments also illustrate that the different models predict fault slip velocity time histories characterized by a similar frequency content; a feeble predominance of high frequencies in the SW case emerges in the frequency ranges [0.3, 1] and [11, 50] Hz. These simulations clearly indicate that, even forgiving the frequency band limitation, it would be very difficult (virtually impossible) to discriminate between two different, but energetically identical, constitutive models, on the basis of the seismograms recorded after a natural earthquake. Copyright 2011 by the American Geophysical Union.

Bizzarri A.,Italian National Institute of Geophysics and Volcanology
Journal of Geophysical Research: Solid Earth | Year: 2011

We present a physical model that describes the behavior of spontaneous earthquake ruptures dynamically propagating on a fault zone and that accounts for the presence of frictional melt produced by the sliding surfaces. First, we analytically derive the solution for the temperature evolution inside the melt layer, which generalizes previous approximations. Then we incorporate such a solution into a numerical code for the solution of the elastodynamic problem. When a melt layer is formed, the linear slip-weakening law (initially governing the fault and relying on the Coulomb friction) is no longer valid. Therefore we introduce on the fault a linearly viscous rheology, with a temperature-dependent dynamic viscosity. We explore through numerical simulations the resulting behavior of the traction evolution in the cohesive zone before and after the transition from Coulomb friction and viscous rheology. The predictions of our model are in general agreement with the data from exhumed faults. We also find that the fault, after undergoing the breakdown stress drop controlled by the slip-weakening constitutive equation, experiences a second traction drop controlled by the exponential weakening of fault resistance due to the viscous rheology. This further drop enhances the instability of the fault, increasing the rupture speeds, the peaks in fault slip velocity, and the fracture energy density. Copyright © 2011 by the American Geophysical Union.

Bizzarri A.,Italian National Institute of Geophysics and Volcanology
Earth and Planetary Science Letters | Year: 2012

In this paper we consider a wide catalog of synthetic earthquakes, numerically modeled as spontaneous, fully dynamic, 3-D ruptures on extended faults, governed by different friction laws, including slip-dependent and rate- and state-dependent equations. We analyze the spatial correlations between the peak of fault slip velocity (v peak) and the rupture speed (v r) at which the earthquake spreads over the fault. We found that v peak positively correlates with v r and that the increase of v peak is roughly quadratic. We found that near the transition between sub- and supershear regimes v peak significantly diminishes and then starts to increase again with the square of v r This holds for all the governing models we consider and for both homogeneous and heterogeneous configurations. Moreover, we found that, on average, v peak increases with the magnitude of the event (v peak~M 0 0.18). Our results can be incorporated as constraints in the inverse modeling of faults. © 2011 Elsevier B.V.

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