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Alessandrini, Bruno
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Alessandrini, Bruno
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bruno.alessandrini@ingv.it
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- PublicationRestrictedForeshock statistics and their possible relationship to earthquake prediction in the italian regionThis study focuses on the foreshocks of earthquake sequences recorded by the Italian national seismological network between 1975 and 1991. We consider as foreshocks events exceeding a given threshold magnitude that follow a period of quiescence and come before a mainshock occurring within a given time-distance range. The total number of potential foreshocks identified by the algorithm depends on the size and the duration of the predefined quiescence area, whereas the number of mainshocks preceded by foreshocks depends also on the size of the area in which the event is expected and the duration of the alarm period. Our aim was to optimize the choice of the parameters defining potential foreshocks as precursors of stronger earthquakes, in order to improve their validity and reliability. A preliminary investigation carried over the whole Italian territory allowed the definition of zones where foreshocks-mainshocks series are likely to occur. One of these zones is an area of about 32,000 km^2 in Central Italy including a total of 2671 events of M >= 2.5, where there is a significant tendency of moderate shocks to be followed at short distance and in short time by larger shocks. Our method would lead to issuing 63 alarms with the optimal definition of foreshocks as events of magnitude exceeding 3.0 after at least 80 days of quiescence in an area of 140 km radius. The rate of occurrence for mainshocks of magnitude exceeding 4.0 during the following 48 hours, in the specified area of 30 km radius, is 6 of 23 events. The corresponding probability gain is about 150. For comparison, another larger zone (Po Valley) with 453 events of M >= 2.5 exhibits only a rate of I of 82 precursors. In this area most events are single shocks. Our analysis shows that the foreshock activity can be regarded as a significant earthquake precursor, so suggesting that real time observation of seismicity can be used to improve the quality of an earthquake prediction system.
83 4 - PublicationOpen AccessA simple P-wave polarization analysis: its application to earthquake location(1994-09)
; ; ; ; ; ;Alessandrini, B.; Istituto Nazionale di Geofisica, Roma, Italy ;Cattaneo, M.; Dister, Sezione Geofisica, Università di Genova, Italy ;Demartin, M.; C.N.R., Istituto di Ricerca sul Rischio Sismico, Milano, Italy ;Gasperini, M.; C.N.R., Istituto di Geologia Marina, Bologna, Italy ;Lanza, V.; Dister, Sezione Geofisica, Università di Genova, Italy; ; ; ; We present a method for hypocentral location which takes into account all three components of ground motion and not only the vertical one, as it is usually done by standard least-square techniques applied to arrival times. Assuming that P-wave particle motion direction corresponds to the propagation direction of the seismic wave, we carried out a simple statistical analysis of ground motion amplitudes, carefully using three-component records. We obtained the azimuth and the emersion angle of the seismic ray, which, added to Pg and Sg arrival times, allowed us to find reliable hypocentral coordinates of some local events by means of a ray-tracing technique. We compared our locations to those obtained using a least-square technique: our polarization method's dependence on the accuracy of the model used (on the contrary, the least-square technique proved to be quite stable with respect to changes in the model's velocity parameters) led us to conclude that polarization data provide coherent information on the true ray-path and can be successfully used for both location procedures and seismic wave propagation studizs in strongly heterogeneous media.272 1759 - PublicationOpen Access3-D crustal P-wave velocity tomography of the Italian region using local and regional seismicity data(1995-05)
; ; ; ;Alessandrini, B.; Istituto Nazionale di Geofisica, Roma, ltaly ;Beranzoli, L.; Istituto Nazionale di Geofisica, Roma, ltaly ;Mele, F. M.; Istituto Nazionale di Geofisica, Roma, ltaly; ; A tomographic experiment was performed in the Italian region using local and regional arrivaI times of p and S seismological phases selected from the Italian National Bulletin in the time interval 1984-1991. We deter- mined a 3-D crustal P-wave velocity model using a simultaneous inversion method that iteratively re1ocates the hypocenters and computes the unknown model parameters. A fast two-point ray tracing algorithm was adopted to compute the ray paths and travel times of P", S", P g' Sg phases with good accuracy. Synthetic tests were performed using the "true" hypocenter and station distribution to rough1y evaluate the extension of the areas most densely spanned by the ray paths; the agreement between synthetic and computed models is more satisfactory at Moho depths than in the upper crust. The qua1ity of the model resulting from inversion of real data is examined by the ca1culation of the Spread Function (Toomey and Foulger, 1989). The 3-D crustal P-wave velocity mode1 of the Italian region shows remarkab1e trends at Moho depths: the areas east of the Apennines call for positive adjustments of the initial velocity va1ue, while the west region shows negative ad- justments. The correspondence among the main features of the velocity field, the map of Moho isobaths and the map of the gravity anoma1ies is also outlined.175 505 - PublicationOpen AccessActive and remnant subducted slabs beneath Italy: evidence from seismic tomography and seismicity(1993)
; ; ; ; ; ;Amato, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Alessandrini, B.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Cimini, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Frepoli, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Selvaggi, G.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; 201 727 - PublicationOpen AccessTomographic image of the crust and uppermost mantle of the Ionian and Aegean regions(1997-01)
; ; ; ; ; ; ; ;Alessandrini, B.; Istituto Nazionale di Geofisica, Roma, Italy ;Beranzoli, L.; Istituto Nazionale di Geofisica, Roma, Italy ;Drakatos, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Falcone, C.; Istituto Nazionale di Geofisica, Roma, Italy ;Karantonis, G.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece ;Mele, F. M.; Istituto Nazionale di Geofisica, Roma, Italy ;Stavrakakis, G. N.; National Observatory of Athens, Institute of Geodynamics, Athens, Greece; ; ; ; ; ; We present a tomographic view of the crust and uppermost mantle beneath the Central Mediterranean area obtained from P-wave arrival times of regional earthquakes selected from the ISC bulletin. The P-wave velocity anomalies are obtained using Thurber's algorithm that jointly relocates earthquakes and computes velocity adjustments with respect to a starting model. A specific algorithm has been applied to achieve a distribution of epicentres as even as possible. A data set of 1009 events and 49072 Pg and Pn phases was selected. We find a low velocity belt in the crust, evident in the map view at 25 km of depth, beneath the Hellenic arc. A low velocity anomaly extends at 40 km of depth under the Aegean back arc basin. High velocities are present at Moho depth beneath the Ionian sea close to the Calabrian and Aegean arcs. The tomographic images suggest a close relationship between P-wave velocity pattern and the subduction systems of the studied area.240 164