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

Description: Artifacts often affect seismic catalogs. Among them, the presence of man-made contaminations such as quarry blasts and explosions is a well-known problem. Using a contaminated dataset reduces the statistical significance of results and can lead to erroneous conclusions, thus the removal of such nonnatural events should be the first step for a data analyst. Blasts misclassified as natural earthquakes, indeed, may artificially alter the seismicity rates and then the b-value of the Gutenberg and Richter relationship, an essential ingredient of several forecasting models. At present, datasets collect useful information beyond the parameters to locate the earthquakes in space and time, allowing the users to discriminate between natural and nonnatural events. However, selecting them from webservices queries is neither easy nor clear, and part of such supplementary but fundamental information can be lost during downloading. As a consequence, most of statistical seismologists ignore the presence in seismic catalog of explosions and quarry blasts and assume that they were not located by seismic networks or in case they were eliminated. We here show the example of the Italian Seismological Instrumental and Parametric Database. What happens when artificial seismicity is mixed with natural one?

Description: Real‐time earthquake rIsk reduction for a reSilient Europe (RISE) project, funded by the European Union’s Horizon 2020 research and innovation program under Grant Agreement Number 821115 and partially funded by the Pianeta Dinamico‐Working Earth INGV‐MUR project

Description: Published

Description: 3538-3551

Description: 4T. Sismicità dell'Italia

Description: 6T. Studi di pericolosità sismica e da maremoto

Description: JCR Journal

Description: Taroni et al. (2021) published a statistical framework to reliably estimate the b-value and its uncertainties, with the goal being the interpretation in a seismotectonic context and improving earthquake forecasting capabilities. In this comment, we show that the results presented for the Italian region and the conclusions drawn by the authors, are heavily biased due to quarry-blast events in the Italian earthquake catalog used in the analysis. Without removing this anthropogenic component in the data, a meaningful analysis of the earthquake- size distribution for natural seismicity is, in our opinion, not possible. This comment highlights the need for basic data quality analysis before sophisticated statistical tools are applied to a dataset.

Description: European Union’s Horizon 2020 research and innovation program under Grant Agreement Number 821115 Pianeta Dinamico-Working Earth INGV-MUR project.

Description: Published

Description: 1089-1094

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: The Mw 7.1 Ridgecrest earthquake sequence in California in July 2019 offered an opportunity to evaluate in near‐real time the temporal and spatial variations in the average earthquake size distribution (the b‐value) and the performance of the newly introduced foreshock traffic‐light system. In normally decaying aftershock sequences, in the past studies, the b‐value of the aftershocks was found, on average, to be 10%–30% higher than the background b‐value. A drop of 10% or more in “aftershock” b‐values was postulated to indicate that the region is still highly stressed and that a subsequent larger event is likely. In this Ridgecrest case study, after analyzing the magnitude of completeness of the sequences, we find that the quality of the monitoring network is excellent, which allows us to determine reliable b‐values over a large range of magnitudes within hours of the two mainshocks. We then find that in the hours after the first Mw 6.4 Ridgecrest event, the b‐value drops by 23% on average, compared to the background value, triggering a red foreshock traffic light. Spatially mapping the changes in b-values, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. After the second, magnitude 7.1 mainshock, which did occur in that location as anticipated, the b‐value increased by 26% over the background value, triggering a green traffic light. Finally, comparing the 2019 sequence with the Mw 5.8 sequence in 1995, in which no mainshock followed, we find a b‐value increase of 29% after the mainshock. Our results suggest that the real‐time monitoring of b‐values is feasible in California and may add important information for aftershock hazard assessment.

Description: Published

Description: 2828-2842

Description: 6T. Studi di pericolosità sismica e da maremoto

Description: 7T. Variazioni delle caratteristiche crostali e precursori sismici

Description: 8T. Sismologia in tempo reale

Description: JCR Journal