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  • 1
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    Along-strike rupture directivity of earthquakes of the 2009 L'Aquila, central Italy, seismic sequence (2015)
    Oxford University Press
    Details
    Publication Date: 2015-08-29
    Description: We perform a systematic investigation of along-strike rupture directivity of 70 earthquakes (3.0 ≤ M w ≤ 6.1) of the 2009 L'Aquila seismic sequence by analysing azimuthal variations of broad-band seismograms recorded in the distance range 60 km 〈 R 〈 230 km. We use reference spectra of events with little directivity (similar to the empirical Green's function method) to deconvolve propagation-site effects and focus on source properties. A directivity index (0 ≤ I DIR ≤ 1) calculated for each earthquake quantifies the spectral separation above the corner frequency of the target event at opposite along-strike directions. A large number (73 per cent) of events including the M w 6.1 main shock show high (〉0.7) I DIR values indicating predominantly unilateral rupture propagation. The preferred rupture propagation direction is generally to the southeast with no dependence on the earthquake magnitude or occurrence time. Events on two main faults (L'Aquila and Campotosto) show somewhat different behaviour. Almost all earthquakes on the L'Aquila fault have strong unilateral directivity to the southeast, whereas earthquakes on the Campotosto fault show more diverse behaviour. However, there is a predominance of unilateral ruptures (14 out of 22) also on the Campotosto fault, and the few (five) earthquakes with ruptures to the northwest are limited to the most northwestern segment of the fault. The spectral results are consistent with time-domain analysis when the latter samples adequately the frequency band above corner frequency. The preferred rupture direction may be produced at least in part by a velocity contrast across the fault. The results provide important input for estimates of seismic motion and physics of earthquake ruptures.
    Keywords: Seismology
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 2
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    Stress drop and source scaling of the 2009 April L'Aquila earthquakes (2012)
    Oxford University Press
    Details
    Publication Date: 2012-12-19
    Description: The empirical Green's function (EGF) technique is applied in the frequency domain to 962 broad-band seismograms (3.3 ≤ M W ≤ 6.1) to determine stress drop and source scaling of the 2009 April L’Aquila earthquakes. The station distance varies in the range 100–250 km from the source. Ground motions of several L’Aquila earthquakes are characterized by large azimuthal variations due to source directivity, even at low magnitudes. Thus, the individual-station stress-drop estimates are significantly biased when source directivity is not taken into account properly. To reduce the bias, we use single-station spectral ratios with pairs of earthquakes showing a similar degree of source directivity. The superiority of constant versus varying stress-drop models is assessed through minimization of misfit in a least-mean-square sense. For this analysis, seismograms of 26 earthquakes occurring within 10 km from the hypocentres of the three strongest shocks are used. We find that a source model where stress drop increases with the earthquake size has the minimum misfit: as compared to the best constant stress-drop model the improvement in the fit is of the order of 40 per cent. We also estimate the stress-drop scaling on a larger data set of 64 earthquakes, all of them having an independent estimate of seismic moment and consistent focal mechanism. An earthquake which shows no directivity is chosen as EGF event. This analysis confirms the former trend and yields individual-event stress drops very close to 10 MPa at magnitudes M W  〉 4.5 that decrease to 1 MPa, on the average, at the smallest magnitudes. A varying stress-drop scaling of L’Aquila earthquakes is consistent with results from other studies using EGF techniques but contrasts with results of authors that used inversion techniques to separate source from site and propagation effects. We find that there is a systematic difference for small events between the results of the two methods, with lower and less scattered values of stress drop resulting from the EGF approach.
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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  • 3
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    Isotopic carbon discrimination and leaf nitrogen content of Erica arborea L. along a CO2 concentration gradient in a CO2 spring in Italy (1995)
    Oxford University Press
    Details
    Publication Date: 1995-05-01
    Print ISSN: 0829-318X
    Electronic ISSN: 1758-4469
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Published by Oxford University Press
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  • 4
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    Along-strike rupture directivity of earthquakes of the 2009 L'Aquila, central Italy, seismic sequence (2015)
    Oxford University Press
    Details
    Publication Date: 2015-08-27
    Print ISSN: 0956-540X
    Electronic ISSN: 1365-246X
    Topics: Geosciences
    Published by Oxford University Press on behalf of The Royal Astronomical Society.
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  • 5
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    Large amplitude variations recorded by an on‐fault seismological station during the L’Aquila earthquakes: Evidence for a complex fault‐induced site effect (2011)
    AGU
    Details
    Publication Date: 2012-02-03
    Description: A station (FAGN) installed on a segment of the fault system that generated the April 2009 L’Aquila earthquakes shows larger ground motions compared to nearby stations. Spectral ratios using 304 earthquakes result in a station amplification significantly varying event by event in the frequency band 1–8 Hz. The resulting pattern of amplitude dependence on causative earthquake location reveals that the strongest (up to a factor of 10) amplifications occur for tightly clustered aftershocks aligned with the fault dip beneath FAGN thus indicating a fault‐guided effect. Fault models are investigated in a grid‐search approach by varying velocity, Q, width and depth of the fault zone. Although the problem solution is not unique and there are strong trade‐offs among the model parameters, constraints from observations yield a deep trapping structure model where the most likely values of velocity reduction, Q and damage zone width are 25%, 20, and 280 m, respectively.
    Description: Published
    Description: L24305
    Description: 3.1. Fisica dei terremoti
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: JCR Journal
    Description: reserved
    Keywords: L'Aquila earthquake ; fault zone ; trapped waves ; site amplification ; 04. Solid Earth::04.06. Seismology::04.06.99. General or miscellaneous ; 04. Solid Earth::04.06. Seismology::04.06.01. Earthquake faults: properties and evolution ; 04. Solid Earth::04.06. Seismology::04.06.04. Ground motion ; 04. Solid Earth::04.06. Seismology::04.06.09. Waves and wave analysis
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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