1,721,117 research outputs found
Anomalously deep earthquakes in northwestern Italy
No full textIt is usually assumed that earthquakes in intraplate regions occur in the upper crust, and northwestern Italy is generally assigned to this kind of 'normal' seismicity. In this work, the depth distribution of the events localized in this area by the Istituto Geofisico Geodetico (IGG) seismic network in the period 1991-1997 is analyzed in detail. In particular, the location capability of the network is discussed, adopting as reference quarry blasts (for the epicentral position) and the locations obtained from a dense temporary network (for the depth estimate). Within the so-obtained error limits, the depth distribution of events show a characteristic pattern: while for most of the area covered by the network the well-located seismicity lies within the first 20 km of depth, in a band following the inner arc of the Western Alps, numerous events have anomalously large focal depths, reaching a maximum of 114 km. These depth determinations cannot be attributed to instabilities of the location procedure: different choices of the propagation models used for the hypocentral determination led to very similar depth values, always significantly larger than the standard values for the surrounding areas. A strong correlation has been found between the 3-dimensional distribution of these foci and the P-wave propagation anomalies obtained from tomographic studies, suggesting a direct link between elastic and rheological properties of lower crust and upper mantle in this area
Source parameters of small events using constrained deconvolution with empirical Green's functions
No full textDependence of the seismotectonic regime on the thermal state in the northern italian apennines
No full textIn order to study the relationships between recent seismotectonic processes and thermal conditions of the lithosphere, we analyzed the lateral variations both in surface heat-flow density and in seismic activity of the northern Italian Apennines. The focal depths have been related to the mechanical properties of the lithosphere. The hypocentre distribution indicates that the thickness of the seismogenic crustal layer increases towards the outer part of the range. Rheological models confirm this result, showing that in the western areas, marked by heat-flow density greater than 60 mW/m2 and by positive gravity anomalies, the depth of the crustal brittle-ductile transition is rather shallow, being about 10 km. In the cold external parts of the chain, this depth increases, ranging from 15 to 25 km; brittle seismogenic levels can be also found in the upper mantle, below an essentially aseismic, ductile crustal layer. The estimated limiting temperatures of the brittle strength zones are about 320°-380°C and 550°-600°C for crustal and upper mantle materials, respectively. © 1993
Scaling properties of the spatio-temporal distribution of earthquakes: a multifractal approach applied to a Californian catalogue
No full textSource Parameters Estimated from the Aftershocks of the 1997 Umbria-Marche (Italy) Seismic Sequence
No full textReliability of earthquake location procedure in heterogeneous areas: synthetic tests in the South Western Alps, Italy
No full textThe accuracy of procedures for locating earthquakes and tomographically imaging 3-D velocity structures, is influenced by factors such as network geometry, errors in picking seismic phases, modeling errors and velocity model uncertainties. The main purpose of this study is to investigate the importance of different error sources through synthetic experiments. The synthetic data sets were generated for a realistic 3-D velocity model of the South Western Alps adopting a precise raytracing technique. The actual station configuration and recorded seismicity during the passive seismological experiment Geofrance 3D-Alps, conducted between August 1996 and February 1997, were used. The station configuration corresponded to an enhancement of the regional networks operating in the area, with the addition of temporary stations. In addition, synthetic data sets were corrupted by applying different error schemes. Furthermore, to assess the locating capabilities of the regional networks, synthetic data sets were constructed for synthetic events placed in correspondence to the nodes of a regular three dimensional (3-D) grid. We mimicked the actual data management scheme for synthetic data processing: we re-located the earthquakes, performed an inversion for a minimum 1-D velocity model with station corrections and, then, ran a complete 3-D velocity model inversion. One of the main results, confirming earlier studies was that earthquake locations in strongly heterogeneous areas, obtained using a regional 1-D model, can have significant errors. Even a minimum 1-D model with station corrections does not significantly improve the accuracy of parameter locations in the South Western Alps. For high precision locations, a 3-D model is needed. However, a 3-D model cannot be reconstructed from noise contaminated data recorded by a sparse regional network. © 2001 Elsevier Science B.V. All rights reserved
Reconstruction of seismogenetic structures by multiplet analysis: an example of Western Liguria
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