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Enescu, B.
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Enescu, B.
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- PublicationOpen AccessAftershock modeling based on uncertain stress calculations(2009)
; ; ; ; ; ; ; ;Hainzl, S.; GFZ German Research Centre for Geosciences, Potsdam, Germany ;Enescu, B.; GFZ German Research Centre for Geosciences, Potsdam, Germany ;Cocco, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Woessner, J.; Institute of Geophysics, ETH Zurich, Zurich, Switzerland ;Catalli, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Wang, R.; Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences,Potsdam, Germany ;Roth, F.; Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences,Potsdam, Germany; ; ; ; ; ; We discuss the impact of uncertainties in computed coseismic stress perturbations on the seismicity rate changes forecasted through a rate- and state-dependent frictional model. We aim to understand how the variability of Coulomb stress changes affects the correlation between predicted and observed changes in the rate of earthquake production. We use the aftershock activity following the 1992 M7.3 Landers (California) earthquake as a case study. To accomplish these tasks, we first analyze the variability of stress changes resulting from the use of different published slip distributions. We find that the standard deviation of the uncertainty is of the same size as the absolute stress change and that their ratio, the coefficient of variation (CV), is approximately constant in space. This uncertainty has a strong impact on the forecasted aftershock activity if a rate-and-state frictional model is considered. We use the early aftershocks to invert for friction parameters and the coefficient of variation by means of the maximum likelihood method. We show that, when the uncertainties are properly taken into account, the inversion yields stable results, which fit the spatiotemporal aftershock sequence. The analysis of the 1992 Landers sequence demonstrates that accounting for realistic uncertainties in stress changes strongly improves the correlation between modeled and observed seismicity rate changes. For this sequence, we measure a friction parameter Aσn ≈ 0.017 MPa and a coefficient of stress variation CV = 0.95.183 1072 - PublicationOpen AccessSensitivity study of forecasted aftershock seismicity based on Coulomb stress calculation and rate- and state-dependent frictional response(2010)
; ; ; ; ; ; ;Cocco, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Hainzl, S.; Helmholtz Centre Potsdam: GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany ;Catalli, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Enescu, B.; National Research Institute for Earth Science and Disaster Prevention (NIED), 3-1 Tennodai, Tsukuba, Ibaraki, 305-0006, Japan ;Woessner, J.; ETH Zurich, Swiss Seismological Service, Zurich, Switzerland ;Lombardi, A. M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; ; We use the Dieterich (1994) physics-based approach to simulate the spatio- temporal evolution of seismicity caused by stress changes applied to an infinite population of nucleating patches modeled through a rate- and state- dependent friction law. According to this model, seismicity rate changes depend on the amplitude of stress perturbation, the physical constitutive properties of faults (represented by the parameter Aσ), the stressing rate and the background seismicity rate of the study area. In order to apply this model in a predictive manner, we need to understand the impact of physical model parameters and the correlations between them. Firstly we discuss different definitions of the reference seismicity rate and show their impact on the computed rate of earthquake production for the 1992 Landers earthquake sequence as a case study. Furthermore, we demonstrate that all model parameters are strongly correlated for physical and statistical reasons. We discuss this correlation emphasizing that the estimations of the background seismicity rate, stressing rate and Aσ are strongly correlated to reproduce the observed aftershock productivity. Our analytically derived relation demonstrates the impact of these model parameters on the Omori-like aftershock decay: the c- value and the productivity of the Omori law, implying a p-value smaller or equal to 1. Finally, we discuss an optimal strategy to constrain model parameters for near-real time forecasts.206 179 - PublicationOpen AccessThe Effect of a Mainshock on the Size Distribution of the AftershocksA systematic decay of the aftershock rate over time is one of the most fundamental empirical laws in Earth science. However, the equally fundamental effect of a mainshock on the size distribution of subsequent earthquakes has still not been quantified today and is therefore not used in earthquake hazard assessment. We apply a stacking approach to well-recorded earthquake sequences to extract this effect. Immediately after a mainshock, the mean size distribution of events, or b-value, increases by 20-30%, considerably decreasing the chance of subsequent larger events. This increase is strongest in the immediate vicinity of the mainshock, decreasing rapidly with distance but only gradually over time. We present a model that explains these observations as a consequence of the stress changes in the surrounding area caused by the mainshocks slip. Our results have substantial implications for how seismic risk during earthquake sequences is assessed.
179 147 - PublicationRestrictedA retrospective comparative forecast test on the 1992 Landers sequence(2011)
; ; ; ; ; ; ; ; ; ; ;Woessner, J.; ETH, Zurich ;Hainzl, S.; GFZ, Potsdam ;Marzocchi, W.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Werner, M. J.; ETH, Zurich ;Lombardi, A. M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Catalli, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Enescu, B.; GFZ, Potsdam ;Cocco, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Gerstenberger, M.; GNS, New Zealand ;Wiemer, S.; ETH, Zurich; ; ; ; ; ; ; ; ; We perform a retrospective forecast experiment on the 1992 Landers sequence comparing the predictive power of commonly used model frameworks for short‐term earthquake forecasting. We compare a modified short‐term earthquake probability (STEP) model, six realizations of the epidemic‐type aftershock sequence (ETAS) model, and four models that combine Coulomb stress changes calculations and rate‐and‐state theory to generate seismicity rates (CRS models). We perform the experiment under the premise of a controlled environment with predefined conditions for the testing region and data for all modelers. We evaluate the forecasts with likelihood tests to analyze spatial consistency and the total amount of forecasted events versus observed data. We find that (1) 9 of the 11 models perform superior compared to a simple reference model, (2) ETAS models forecast the spatial evolution of seismicity best and perform best in the entire test suite, (3) the modified STEP model matches best the total number of events, (4) CRS models can only compete with empirical statistical models by introducing stochasticity in these models considering uncertainties in the finite‐fault source model, and (5) resolving Coulomb stress changes on 3‐D optimally oriented planes is more adequate for forecasting purposes than using the specified receiver fault concept. We conclude that statistical models perform generally better than the tested physics‐based models and parameter value updates using the occurrence of aftershocks generally improve the predictive power in particular for the purely statistical models in space and time.199 16