We analyse the instrumental seismicity of the Abruzzo region in the period 1981-2003 in order to obtain a catalogue as homogeneous as possible in terms of location procedure and quality of the results. We analyse four temporal datasets: 1981-1991; 1992-1996; 1997-1999 and 2000-2003. The 1981-1991 dataset is taken from the CSTI catalogue, opportunely selected by using quality criteria. The datasets from 1992 to 2003 are relocated by integrating the recordings of the national seismic network with the recordings of the local Abruzzo seismic network (operating from 1992 to 1999). Particular attention is paid to the velocity models, in order to account for the different stratigraphic/tectonic domains which characterize the Abruzzo region. In particular, we used 8 velocity models, applied to stations or groups of stations lying within relatively homogeneous areas. We obtained a database, selected for RMS ≤0.5s and hypocentral errors ≤5 km, of 985 events with 0.5≤M≤4.4 plus two events of moderate magnitude (Mw=5.9, Mw=5.5) corresponding to the largest shocks of the May 1984 Sangro Valley seismic sequence. We also computed 17 new focal mechanisms. The seismotectonic implications mainly concern the thickness of the seismogenic layer. A robust statistical estimate of the base of the seismogenic layer is given by the depth above which the 90% of seismicity occurs (D90). The maximum thickness (15-17 km) is found in the eastern Abruzzo Apennines (surface heat flow ≤40 mW/m2). A thickness of 12-14 km is found in the western Abruzzo Apennines (40< surface heat flow ≤60 mW/m2). The observed depths are consistent with independent rheological data (B-D transition). The connection between the background seismicity and the geometry at depth of the active faults is feasible only rarely (e.g. M. Gorzano normal fault in northern Abruzzo). More often the seismicity is spread within the seismogenic volume. Locally, it concentrates close to structural complexities or defines small seismic sequences activating inherited structures. The active faults south of L'Aquila are almost free from microseismic activity. The new focal mechanisms computed from the 1992-1999 database confirm and reinforce the existence of a dominating extensional regime across the Abruzzo Apennines.
Abstract. Tectonic styles and distributions of nodal planes are an essential input for probabilistic seismic hazard assessment. As a part of a recent elaboration of a new seismic hazard model for Italy, we adopted a cascade criteria approach to parametrize the tectonic style of expected earthquake ruptures and their uncertainty in an area-based seismicity model. Using available or recomputed seismic moment tensors for relevant seismic events (Mw starting from 4.5), first arrival focal mechanisms for less recent earthquakes, and also geological data on past activated faults, we collected a database for the last ~ 100 yrs gathering a thousand of data all over the Italian peninsula and regions around it. The adopted procedure consists, in each seismic zone, of separating the available seismic moment tensors in the three main tectonic styles, making summation within each group, identifying possible nodal plane(s) taking into account the different percentages of tectonic styles and including, where necessary, total or partial random source contributions. Referring to the used area source model, for several seismic zones we obtained robust results, e.g. along the southern Apennines we expect future earthquakes to be mostly extensional, although in the outer part of the chain strike-slip events are possible. In the Northern part of the Apennines we also expect different tectonic styles for different hypocentral depths. In zones characterized by a low seismic moment release, the possible tectonic style of future earthquakes is less clear and it has been represented using different combination (total or partial) of random sources.
Field observations and modelling indicate that elastic interaction between active faults can lead to variations in earthquake recurrence intervals measured on timescales of 102-104 yr. Fault geometry strongly influences the nature of the interaction between adjacent structures as it controls the spatial redistribution of stress when rupture occurs. In this paper, we use a previously published numerical model for elastic interaction between spontaneously growing faults to investigate the relationships between fault geometry, fault slip rate variations and the statistics of earthquake recurrence. These relationships develop and become systematic as a long-term consequence of stress redistribution in individual rupture events even though on short timescales earthquake activity appears to be stochastic. We characterize fault behaviour using the coefficient of variation (CV) of earthquake recurrence intervals and introduce a new measure, slip-rate variability (SRV) that takes into account the size and time ordering of slip events. CV generally increases when the strain is partitioned on more than one fault but the relationship between long-term fault slip rate (SRmean) and CV is poorly defined. In contrast, SRV increases systematically where faulting is more distributed and SRmean is lower. To first order, SRV is inversely proportional to SRmean. We also extract earthquake recurrence statistics and compare these to previously published probability density functions used in earthquake forecasting. The histograms of earthquake recurrence vary systematically as a function of fault geometry and are best characterized by a Weibull distribution with fitting parameters that vary from site to site along the fault array. We explain these phenomena in terms of a time-varying, geometrical control on stress loading of individual faults arising from the history of elastic interactions and compare our results with published data on SRV and earthquake recurrence along normal faults in New Zealand and in the Italian Apennines. Our results suggest that palaeoseismic data should be collected and analysed with structural geometry in mind and that information on SRV, CV and SRmean should be integrated with data from earthquake catalogues when evaluating seismic hazard.
Abstract. Italy is one of the most seismically active countries in Europe. Moderate to strong earthquakes, with magnitudes of up to ∼ 7, have been historically recorded for many active faults. Currently, probabilistic seismic hazard assessments in Italy are mainly based on area source models, in which seismicity is modelled using a number of seismotectonic zones and the occurrence of earthquakes is assumed uniform. However, in the past decade, efforts have increasingly been directed towards using fault sources in seismic hazard models to obtain more detailed and potentially more realistic patterns of ground motion. In our model, we used two categories of earthquake sources. The first involves active faults, and using geological slip rates to quantify the seismic activity rate. We produced an inventory of all fault sources with details of their geometric, kinematic, and energetic properties. The associated parameters were used to compute the total seismic moment rate of each fault. We evaluated the magnitude–frequency distribution (MFD) of each fault source using two models: a characteristic Gaussian model centred at the maximum magnitude and a truncated Gutenberg–Richter model. The second earthquake source category involves grid-point seismicity, with a fixed-radius smoothed approach and a historical catalogue were used to evaluate seismic activity. Under the assumption that deformation is concentrated along faults, we combined the MFD derived from the geometry and slip rates of active faults with the MFD from the spatially smoothed earthquake sources and assumed that the smoothed seismic activity in the vicinity of an active fault gradually decreases by a fault-size-driven factor. Additionally, we computed horizontal peak ground acceleration (PGA) maps for return periods of 475 and 2475 years. Although the ranges and gross spatial distributions of the expected accelerations obtained here are comparable to those obtained through methods involving seismic catalogues and classical zonation models, the spatial pattern of the hazard maps obtained with our model is far more detailed. Our model is characterized by areas that are more hazardous and that correspond to mapped active faults, while previous models yield expected accelerations that are almost uniformly distributed across large regions. In addition, we conducted sensitivity tests to determine the impact on the hazard results of the earthquake rates derived from two MFD models for faults and to determine the relative contributions of faults versus distributed seismic activity. We believe that our model represents advancements in terms of the input data (quantity and quality) and methodology used in the field of fault-based regional seismic hazard modelling in Italy.
Faults have been increasingly integrated into seismic‐hazard assessments. We have developed a package of MATLAB ( , last accessed January 2016) tools (called FiSH ), designed to help seismic‐hazard modelers analyze fault data. These tools enable the derivation of expected earthquake rates, given common fault data, and allow researchers to test the consistency between the magnitude–frequency distributions (MFDs) assigned to a fault and some available observations. The basic assumption of FiSH is that the geometric and kinematic features of a fault are the expression of its seismogenic potential. Three tools have been designed to integrate the variable levels of information available: (1) the first tool allows users to convert fault geometry and slip rates into a global budget of the seismic moment released in a given time frame, taking uncertainties into account; (2) the second tool computes the recurrence parameters and associated uncertainties from historical and/or paleoseismological data; and (3) the third tool outputs time‐independent or time‐dependent earthquake rates for different MFD models. We present a test case to illustrate the capabilities of FiSH , on the Paganica normal fault in central Italy that ruptured during the 2009 L’Aquila earthquake sequence (mainshock M w 6.3). The source codes are open, and we encourage users to handle the scripts, communicate with us regarding bugs, and/or suggest further improvements. Our intent is to distribute these tools in order to help researchers to pinpoint potential inconsistencies and obtain reliable fault‐based seismic‐hazard evaluations.
Abstract Central Italy is characterized by a network of active faults that interact in a complex manner. Coseismic Coulomb stress changes have been invoked by several authors to explain the concentration of moderate‐to‐strong earthquakes in this region, but none has considered the time‐dependent viscoelastic relaxation of the lower crust and upper mantle as a possible additional source of stress changes at a regional scale. Here starting from the 1915 M w 6.9 ± 0.2 Fucino earthquake, we calculated the coseismic plus postseismic Coulomb failure stress changes (ΔCFS) due to eight moderate‐to‐strong earthquakes that have struck Central Italy in the last century and culminated with the 2016–2017 sequence. Results from this modeling coupled with some synthetic tests simulating normal fault earthquakes with different magnitudes allowed us to highlight the importance of postseismic processes. In particular, the viscoelastic stress transfer due to events of M w ≥ 6.5 can modify the spatial distribution of ΔCFS on a centennial timescale and therefore trigger events at larger distances. In addition, using these results, we identified other earthquake clusters in the historical catalogue (last 618 years), which, like the 1915–2017 series, were potentially modulated by both coseismic and postseismic processes. Finally, considering our calculations combined with historical and paleoseismological data, we suggest that several faults in Central Italy may be at present close to failure.
Central Italy is characterized by a network of active faults that interact in a complex manner. Coseismic Coulomb stress changes have been invoked by several authors to explain the concentration of moderate-to-strong earthquakes in this region, but none has considered the time-dependent viscoelastic relaxation of the lower crust and upper mantle as a possible additional source of stress changes at a regional scale. Here starting from the 1915 M w 6.9 ± 0.2 Fucino earthquake, we calculated the coseismic plus postseismic Coulomb failure stress changes (ΔCFS) due to eight moderate-to-strong earthquakes that have struck Central Italy in the last century and culminated with the 2016-2017 sequence. Results from this modeling coupled with some synthetic tests simulating normal fault earthquakes with different magnitudes allowed us to highlight the importance of postseismic processes. In particular, the viscoelastic stress transfer due to events of M w ≥ 6.5 can modify the spatial distribution of ΔCFS on a centennial timescale and therefore trigger events at larger distances. In addition, using these results, we identified other earthquake clusters in the historical catalogue (last 618 years), which, like the 1915-2017 series, were potentially modulated by both coseismic and postseismic processes. Finally, considering our calculations combined with historical and paleoseismological data, we suggest that several faults in Central Italy may be at present close to failure.
<p>Probabilistic fault displacement hazard analysis (PFDHA) estimates the probability of occurrence and the expected exceedance of on-fault (principal fault rupturing; PF) and off-fault (dist ributed rupturing; DR) surface displacement during an earthquake. Here we concent rate on off-fault rupturing on dip-slip earthquakes, and present an original probability model for the occurrence of DR and for the expected exceedance of displacement dist ribution based on an approach named &#8220;slicing&#8221; (an alternative to the &#8220;gridding&#8221; approach commonly used). The method is developed based on the compilation and reappraisal of surface ruptures from 32 historical crustal dip-slip earthquakes, with magnitudes ranging from M<sub>w</sub> 4.9 to 7.9. A ranking scheme is applied to distinguish PF (rank 1) from simple DR (rank 2) and t riggered faulting (rank 3). Thus modellers can use prediction equations based on or excluding ruptures st rongly related to local st ructural setting depending on the site of concern. In the case of a st ructural setting at a site where large-scale bending (rank 21, 22) and pre-existing faults (rank 1.5, 3) is considered irrelevant, modelling can be performed considering only the unpredictable DR (rank 2). To minimize bias due to the incomplete nature of the database, we int roduce the &#8220;slicing&#8221; approach, which considers that the probability of having a surface rupture within slices parallel to the PF is homogeneous along the st rike of each slice. &#8220;Slicing&#8221; probabilities, computed as a function of magnitude of the earthquake and distance from the PF, are then combined with Monte Carlo simulations that model the dependence of the probability of occurrence of rupture and exceedance of displacement with the dimensions and position of the site of interest with respect to the PF. Finally, both probabilities are combined with existing predictive equations of exceedance of displacement on the PF to calculate fault-displacement hazard curves for sites of interest.</p>
