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Gentili S.,Centro Ricerche Sismologiche
Physics of the Earth and Planetary Interiors

In this paper, I propose a new equation describing the evolution of seismic sequences, based on radiated energy. The evolution of radiated energy in time is described as function of the p and c parameters of the Modified Omori Law and of the energy radiated within a short time following the mainshock. By using the energy rather than the number of events in describing seismic sequences, I circumvent the problem of missing weak aftershocks close in time to the mainshock, because most of the total energy is contained in largest events. In addition, I propose an equation describing the difference in magnitude between the mainshock and the strongest aftershock as function of the energy radiated in a short time after the mainshock, of p, c, and of the parameter b of the Gutenberg Richter equation. An application to California sequences shows values of p in the range [0.65,. 1.5] and c in the range [0,. 0.25] and no dependence of c on the difference between mainshock and cutoff magnitude. © 2012 Elsevier B.V. Source

Gentili S.,Centro Ricerche Sismologiche | Franceschina G.,Italian National Institute of Geophysics and Volcanology
Geophysical Journal International

We investigated the high frequency attenuation of S waves in the southeastern Alps and northern External Dinarides using waveforms from 331 earthquakes (3.0 < Mw < 6.5). The spectral decay parameter, k, was computed using 1345 three component high quality records, collected by the Italian Strong Motion Network (RAN) and by the Short-Period Seismometric Network of northeastern Italy (NEI) in the period 1976-2007. Weak motion data from 11 stations of the NEI network and strong motion data collected by five accelerometers of the RAN were analysed. The k parameter was estimated in the 0-250 km distance range, in a frequency band extending from the corner frequency of the event up to 25 or 45 Hz, using the amplitude acceleration Fourier spectra of S waves. The observed record-to-record variability of k was modelled by applying a generalized inversion procedure, using both parametric and non-parametric approaches. Our results evidence that k is independent on earthquake size, while it shows both site and distance dependence. Stations of the NEI network present the same increase of k with epicentral distance, RE, and show values of the zero-distance k parameter, k0(S), between 0.017 and 0.053 s. For the whole region, the k increase with distance can be described through a linear model with slope dk/dRE= (1.4 ± 0.1) × 10-4 s km-1. Assuming an average S-wave velocity, km s-1 between 5 and 15 km depth, we estimate an average frequency independent quality factor, for the corresponding crustal layer. The non-parametric approach evidences a weak positive concavity of the curve that describes the k increase with RE at about 90 km distance. This result can be approximated through a piecewise linear function with slopes of 1.0 × 10-4 and 1.7 × 10-4 s km-1, in accordance with a three layers model where moving from the intermediate to the bottom layer both and decrease. Two regional dependences were found: data from earthquakes located westward to the NEI network evidence weaker attenuation properties, probably because of S-wave reflections from different parts of the Moho discontinuity under the eastern Po Plain, at about 25-30 km depth, while earthquakes located eastward (in western Slovenia), where the Moho deepens up to 45-50 km, evidence a higher attenuation. Moreover, the k estimates obtained with data from earthquakes located in the area of the 1998 (Mw= 5.7) and 2004 (Mw= 5.2) Kobarid events are 0.017 s higher than the values predicted for the whole region, probably because of the high level of fracturing that characterizes fault zones. The comparison between measured and theoretical values of k, computed at a few stations with available S-wave velocity profiles, reveals that the major contribution to the total k0(S) is due to the sedimentary column (from surface to 800 m depth). The hard rock section contribution is limited to 0.005 s, in accordance with a maximum contribution of 0.010 s predicted by the non-parametric inversion. © 2011 The Authors Geophysical Journal International © 2011 RAS. Source

Caporali A.,University of Padua | Neubauer F.,University of Salzburg | Ostini L.,University of Padua | Stangl G.,Austrian Academy of Sciences | Zuliani D.,Centro Ricerche Sismologiche

The indentation of the Adria plate into the Southern and Eastern Alps is an ongoing collisional process accompanied by seismicity, surface and rock uplift and lateral escape. We present a 3D quantitative description of the process by combining GPS and structural data with an elastic dislocation model. Horizontal velocities of 70 Austrian and Italian permanent GPS stations in the Eastern and Southern Alps serve as boundary condition on the free surface of an elastic half space containing six rectangular faults, each with a uniform slip rate. The geometry of the rectangular faults and the slip rate vector are constrained by least squares, taking into account the structural setting of the area and the geographic distribution of the velocity data. We find that the surface velocities of the order of some mm/yr require reverse (North side of the Tauern Window), transpressional (Giudicarie, North Alpine Wrench Corridor, Pustertal, Dinarides) and normal (Brenner fault) slips ranging from 10 to 30. mm/yr at crustal depths. The regional stress pattern computed from fault plane solutions agrees with the principal directions of our rectangular fault planes. The model, although constrained by horizontal velocities only, predicts a pattern of vertical motion, which qualitatively agrees with known phenomena such as the surface uplift in the Tauern Window area, of the order of up to few mm/yr. If the heat on the shearing fault planes is removed mostly by upwards diffusion, the absence of large heat anomalies on the Earth's surface suggests, for nominal geotherms, shear stresses and concentration of subcrustal radiogenic elements, that the time of initiation of the slip dates to Pliocene, hence more recent than Late Oligocene-Miocene time of collision of the Adria indenter. © 2013 Elsevier B.V. Source

Devoti R.,Italian National Institute of Geophysics and Volcanology | Zuliani D.,Centro Ricerche Sismologiche | Braitenberg C.,University of Trieste | Fabris P.,Centro Ricerche Sismologiche | Grillo B.,University of Trieste
Earth and Planetary Science Letters

Changes in groundwater or surface water level may cause observable deformation of the drainage basins in different ways. We describe an active slope deformation monitored with GPS and tiltmeter stations in a karstic limestone plateau in southeastern Alps (Cansiglio Plateau). The observed transient GPS deformation clearly correlates with the rainfall. Both GPS and tiltmeter equipments react instantly to heavy rains displaying abrupt offsets, but with different time constants, demonstrating the response to different catchment volumes. The GPS movement is mostly confined in the horizontal plane (SSW direction) showing a systematic tendency to rebound in the weeks following the rain. Four GPS stations concur to define a coherent deformation pattern of a wide area (12×5 km2), concerning the whole southeastern slope of the plateau. The plateau expands and rebounds radially after rain by an amount up to a few centimeters and causing only small vertical deformation. The effect is largest where karstic features are mostly developed, at the margin of the plateau where a thick succession of Cretaceous peritidal carbonates faces the Venetian lowland. A couple of tiltmeters installed in a cave at the top of the plateau, detect a much faster deformation, that has the tendency to rebound in less than 6 h. The correlation to rainfall is less straightforward, and shows a more complex behavior during rainy weather. The different responses demonstrate a fast hydrologic flow in the more permeable epikarst for the tiltmeters, drained by open fractures and fissures in the neighborhood of the cave, and a rapid tensile dislocation of the bedrock measured at the GPS stations that affect the whole slope of the mountain. In the days following the rain, both tiltmeter and GPS data show a tendency to retrieve the displacement which is consistent with the phreatic discharge curve. We propose that hydrologically active fractures recharged by rainfall are the most likely features capable to induce the observed strain variations. © 2015 Elsevier B.V. Source

Sugan M.,Centro Ricerche Sismologiche | Peruzza L.,Centro Ricerche Sismologiche
Bollettino di Geofisica Teorica ed Applicata

In this paper we gather and review seismological database and seismotectonic literature for one of the north-eastern Italian regions to which OGS (Istituto Nazionale di Oceanografia e di Geofisica Sperimentale) provides seismic monitoring and alarm system, the Veneto region. We subdivide the region into 9 seismic districts (Fig. 1), according to some common criteria about seismogenic processes, or data availability. This zonation has a basic operational purpose: it helps the personnel of the Centro Ricerche Sismologiche (CRS) department involved in the rapid availability service, in having an updated and technical overview of the earthquake space-time distribution, and a more detailed description about the main events of the past and about the available knowledge of potential seismogenic sources. This state-of-the-art is a quick manual set up for the local authorities too, as the Regional Civil Defence system is charged to intervene in case of potentially damaging earthquakes. In chapter 2 we describe the dataset available and the ones we used: the data are described separately by historical database (par. 2.1), instrumental archives (par. 2.2), seismogenic sources (par. 2.3), and seismic hazard and regulation (par. 2.4), taking into account the most updated and reliable sources available at the time of the analysis (mid 2011); a synthetic geological and structural framework of eastern Southern Alps and northern Apennines is given as well in par. 2.5, based on literature data only. In chapter 3 some methodological considerations about data quality are given, in order to compile a reference dataset and to represent/analyze the seismicity. Heterogeneous data completeness and magnitude estimates are the main problems pertaining the national and regional instrumental dataset; as no authoritative region is set up for different data providers in north-eastern Italy (from 1977 OGS, but then Istituto Nazionale di Geofisica e Vulcanologia - INGV and Provincia Autonoma di Trento - PAT, with their independent stations and/or data processing), a composition of different datasets is suggested in Table 3.1 for two districts (Giudicarie and Pianura Veneto Ovest) that fall in marginal position with respect to the bulk of OGS monitoring system. Nevertheless, some mislocations, omissions or questionable differences in location parameters have been detected; they have been tracked in the description of individual districts. Chapter 4 describes separately each district, following a common scheme (structural and geological context, historical and instrumental seismicity, neotectonic and active sources, damaging effects and seismic regulation). The Veneto region confirms to be a mid-to-high seismic region, like documented by the historical records. Instrumental data are available since 1977, but their completeness is strongly heterogeneous in time and space; the seismicity pictures of the last decades should take this fact into account. Similarly, many Veneto municipalities exhibit a seismic protection deficit, as they entered in seismic regulation in 2003 only. Earthquakes above the perception threshold occur along a NE-SW wide stripe of mountain-to-plain transition in the Southern Alps, and along the last trait of the Po river. In the Venetian Prealps there are several geomorphological signatures of active deformation, but rates and geometries of potential sources are in many cases still doubtful, or controversial; similarly, the comprehension of mechanisms and activity rates of seismogenic sources buried above thick alluvial deposits, in the lower plain, is tricky. Even if the seismometric monitoring of the Veneto region performed by OGS since 1977 has not been constant in time and coverage, the regional OGS database of instrumental seismicity is by far the most homogeneous and reliable dataset available, for all but the two districts on the westward border of the Lombardia region, and Emilia Romagna region southwards; in these areas, a manual revision of earthquake location is needed by integrating original phases of different seismometric networks, and by common data processing in earthquake location and magnitude assignment. The new seismometric stations installed in the Veneto region during the last 5 years, in the frame of the agreements between Protezione Civile Regionale and OGS, and the uniform processing of data belonging to other networks (the PAT network, and some INGV stations) have increased the detection capability and location quality of small earthquakes. Lastly, some districts (namely Lessini-Schio, Pedemontana Sud and Pianura Veneto Ovest) have been affected in the second half of 2011, by some of the most important earthquakes (widely felt, till to sporadic damages) ever experienced during the modern instrumental period: preliminary comments on this activity have been added to the initial manuscript during its revision, but additional analyses are still ongoing and they will probably increase the level of knowledge about seismogenesis in these areas. The seismological information here gathered and reviewed has been set up into electronic archives in the institutional OGS-CRS web site devoted to real time seismology (http://rts.crs.inogs.it), for dynamic queries under development. © 2011 - OGS. Source

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