ALBERT

Description: Earthquake catalogues, seismotectonic zonations and ground-motion prediction equations (GMPE) are the basic ingredients for probabilistic seismic hazard assessment (PSHA). Seismotectonic zones are commonly defined considering the style-of-faulting; contemporary GMPE’s also differentiate by the style-of-faulting. Here we present a case study for Italy to show that style-of-faulting should also be incorporated into the recurrence rates estimation. In the past 40 years many studies relating b-values of the Gutenberg and Richter law to physical properties have been performed, from laboratory rock specimens to observations in different tectonic regimes. Various authors analyzed the correlation between b-value and tectonic regimes and the results are generally consistent: as power laws indicate scale invariance, the inverse dependence of the b-value on the differential stress is universally valid and the parameter can therefore be interpreted as a ‘stressmeter’ in the Earth’s crust. A consequence of the inverse dependency of the b-value on differential stress is that tectonics regimes with different dominant faulting styles should exhibit significantly different b-values, in particular the highest values for normal events (bNR), followed by strike-slip ( bSS) and reverse (bTH): bTH 〈 bSS 〈 bNR. In this study, we evaluate this hypothesis for the first time, using data from the Italian Peninsula, whose complex geology is reflected in a strongly variable stress field and distinctly different faulting regimes. Extensional, compressional and strike-slip regimes are simultaneously present. T he study region fulfils two other critical requirements: 1)the regional seismic monitoring of the microseismicity of the past two decades was good enough to allow detailed mapping of the b-value and 2) a rich catalogue of focal mechanism exists that allows a detailed seismotectonic zonation. Because the b–value is a critical parameter in PSHA, linking it firmly to regional faulting style has significant implications for future regional PSHA studies. At present the b-values are not used for zonation purposes, but they are either assigned regionally or computed for each zone, where zones are in general defined based on expert judgment. We suggest that future seismotectonic zonation models should take into account the knowledge on faulting style dependence of b-values. T here are a variety of way how this can be achieved, for example using high resolution mapping of b as an input for zonation, or by using the b-values of the large scale tectonic zones as a prior, deviating only if local b-values are found to be significantly different from the regional ones.

Description: Published

Description: Montpellier

Description: 3.1. Fisica dei terremoti

Description: open

Description: T he Asperity Likelihood Model (ALM) was first developed for forecasting earthquakes in California (Wiemer and Schorlemmer, SRL, 2007) and is now being tested for performance in the US testing center of the Collaboratory for the Study of Earthquake Predictability (CSEP). T he model hypothesizes that small-scale spatial variations in b-value of the Gutenberg and Richter relationship play a central role in forecasting future seismicity. T he physical basis of the model is the concept that the local b-value depends inversely on applied shear stress. T hus, low b-values (b〈0.7) characterize locked patches of faults—asperities—from which future main shocks are more likely to nucleate, whereas high b-vaues (b〉1.1), found for example in creeping sections of faults, suggest a lower probability of nucleating large events. Here, we calibrate this model for the Italian testing region, the first region in the CSEP European testing center. Italian seismicity is lower, more distributed, and less fault-centric than seismicity in California. Comparison of forecasts of the same model in different regions is a key element in making progress in the study of earthquake forecast models. We also explore two modified versions of this model: in the ALM.IT model, we in addition decluster the input catalog and smooth the node-wise rates of the declustered catalog with a gaussian filter. Completeness values for each node are determined using the probability-based magnitude of completeness method (Schorlemmer and Woessner,BSSA, 2008). In the HALM (Hybrid Asperity Likelihood Model), a ‘hybrid’ between a grid-based and a zoning model, the Italian territory is divided into 8 distinct regions depending on the main tectonic regime, and the local b-value variabilily is thus mapped using regional b-values for each tectonic zone.

Description: Published

Description: Montpellier

Description: 4.2. TTC - Modelli per la stima della pericolosità sismica a scala nazionale

Description: open