Abruzzi region (central Italy) producing vast damage in the L'Aquila town and surroundings. In this paper we present the location and geometry of the fault system as obtained by the analysis of main shock and aftershocks recorded by permanent and temporary networks. The distribution of aftershocks, 712 selected events with M L ! 2.3 and 20 with M L ! 4.0, defines a complex, 40 km long, NW trending extensional structure. The main shock fault segment extends for 15-18 km and dips at 45°to the SW, between 10 and 2 km depth. The extent of aftershocks coincides with the surface trace of the Paganica fault, a poorly known normal fault that, after the event, has been quoted to accommodate the extension of the area. We observe a migration of seismicity to the north on an echelon fault that can rupture in future large earthquakes.

S U M M A R YIn Italy, the Mercalli-Cancani-Sieberg (MCS) is the intensity scale in use to describe the level of earthquake ground shaking, and its subsequent effects on communities and on the built environment. This scale differs to some extent from the Mercalli Modified scale in use in other countries and adopted as standard within the USGS-ShakeMap procedure to predict intensities from observed instrumental data. We have assembled a new PGM/MCS-intensity data set from the Italian database of macroseismic information, DBMI04, and the Italian accelerometric database, ITACA. We have determined new regression relations between intensities and PGM parameters (acceleration and velocity). Since both PGM parameters and intensities suffer of consistent uncertainties we have used the orthogonal distance regression technique. The new relations are I MCS = 1.68 ± 0.22 + 2.58 ± 0.14 log PG A, σ = 0.35 and I MCS = 5.11 ± 0.07 + 2.35 ± 0.09 log PGV, σ = 0.26.Tests designed to assess the robustness of the estimated coefficients have shown that singleline parametrizations for the regression are sufficient to model the data within the model uncertainties. The relations have been inserted in the Italian implementation of the USGSShakeMap to determine intensity maps from instrumental data and to determine PGM maps from the sole intensity values. Comparisons carried out for earthquakes where both kinds of data are available have shown the general effectiveness of the relations.

Since 2005, the Italian Civil Protection (Dipartimento della Protezione Cilvile, DPC) has funded several projects driven toward fast assessment of ground motion shaking in Italy-the final goal being that of organizing the emergency and direct the search and rescue (SAR) teams. To this end, the Istituto Nazionale di Geofisica e Vulcanologia (INGV) has started to determine shakemaps using the USGS-ShakeMap package within 30 minutes from event occurrence and adopting a manually revised location. In this paper we present the INGV implementation of USGS-ShakeMap for earthquakes occurring in Italy and immediately neighboring areas. Emphasis is put on data acquisition, the adopted ground motion predictive relations and the site corrections for the local amplifications of the ground motion. Finally, two examples of shakemaps are shown-the first determined for a recent medium size earthquake, the other for the large Irpinia, 1980, M6.9 event. For both events, the maps are compared to the available macroseismic data.

SUMMARY A new non‐parametric multivariate model is provided to characterize the spatio‐temporal distribution of large earthquakes. The method presents several advantages compared to other more traditional approaches. In particular, it allows straightforward testing of a variety of hypotheses, such as any kind of time dependence (i.e. seismic gap, cluster, and Poisson hypotheses). Moreover, it may account for tectonics/physics parameters that can potentially influence the spatio‐temporal variability, and tests their relative importance. The method has been applied to the Italian seismicity of the last four centuries. The results show that large earthquakes in Italy tend to cluster; the instantaneous probability of occurrence in each area is higher immediately after an event and decreases until it reaches, in few years, a constant value representing the average rate of occurrence for that zone. The results also indicate that the clustering is independent of the magnitude of the earthquakes. Finally, a map of the probability of occurrence for the next large earthquakes in Italy is provided.

[1] The main goal of this work is to provide a probability map for the next moderate to large earthquakes (M ! 5.5) in Italy. For this purpose we apply a new nonparametric multivariate model to characterize the spatiotemporal distribution of earthquakes. The method is able to account for tectonics/physics parameters that can potentially influence the spatiotemporal variability and tests their relative importance; moreover, it allows straightforward testing of a variety of hypothesis, such as Seismic Gap, Cluster, and Poisson hypothesis. The method has been applied to Italian seismicity of the last four centuries for earthquakes with M ! 5.5. Italy has been divided into 61 irregular zones representing areas with homogeneous tectonic regime resulting from active stress data. Besides the magnitude and the time of the earthquakes, the model includes information on the tectonic stress regime, the homogeneity of its orientation, the number of active faults, the dimension of the area and the homogeneity of the topography. The time distribution of the M ! 5.5 earthquakes appears clustered in time for a few years after an event, and then the distribution becomes similar to a memoryless Poisson process, leading to a time-dependent probability map. This map shows that the most likely regions where the next moderate to large earthquakes may occur are Friuli, UmbriaMarche, and part of Southern Apennines and the Calabrian arc.