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  • 1
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    Mws of Seismic Sources under Thick Sediments (2017)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2017-05-31
    Description: Moment magnitudes differing by up to 0.5 units have been published for the same events of the 2012 Ferrara seismic sequence. With respect to the mainshock that occurred on 20 May 2012, results by Malagnini et al. (2012) and Pondrelli et al. (2012) represent opposite extremes: although the former used model Padania, a region-specific velocity structure based on all the available geological and geophysical information from local studies, the latter used a global crustal model with a set of phase corrections calibrated over the central Apennines by Ekström et al. (1998) . Model Padania well reproduces the observed dispersion of surface-wave group velocities in a band of shorter periods, between 33 and 100 s, whereas Pondrelli et al. (2012) performed their inversions in the 50–150 s period band. Here, we show that because surface waves generated within the thick sediments of the Po river floodplain dominated the seismograms, the source excitation terms that came out of a regression scheme performed on the ground motions recorded during the sequence were systematically affected by a broadband increase of the spectral amplitudes at frequencies below 0.4 Hz (frequency range of the regressions: from 0.1 to 22.5 Hz). As a consequence, the two largest events of the sequence share a common true moment magnitude M w ~5.6, even though their enhanced spectral level from 0.1 to 0.4 Hz is consistent with M w ~6.0. Electronic Supplement: Figures of hypocentral distances, moment tensor (MT) solutions, and site terms from the ground-motion regression; and tables of velocity models and MT solutions.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 2
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    On the Relationship between Mw and ML for Small Earthquakes (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-10-08
    Description: Estimating the moment magnitudes ( M w ) of a small earthquake is a challenging task. One viable option to measure its size is to calculate its local magnitude ( M L ) and convert it to the physically based M w . Unfortunately, to correctly perform such a conversion is not easy; moreover, even though many studies demonstrate that the equivalence between M L and M w is incorrect for small events, these two parameters are sometimes thought to be strictly equivalent, regardless of the earthquake’s size. Using random vibration theory, we show that, below M w ~4, the M L of a small earthquake is proportional to the logarithm of its seismic moment, and the following relationship holds: We test our findings on a high-quality data set in the Upper Tiber Valley (northern Apennines, Italy), composed of events in the range of 0≤ M L ≤3.8, for which we compute accurate estimates of M L and M w . Online Material: Details of the processing procedure, figures of the empirical regional attenuation functional, and source terms of 1191 events from the Alto Tiberina fault (ATF) data set and earthquake catalog.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 3
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    Modulation of Seismic Attenuation at Parkfield, Before and After the 2004 M 6 Earthquake (2019)
    American Geophysical Union
    Details
    Publication Date: 2019-06-01
    Print ISSN: 2169-9313
    Electronic ISSN: 2169-9356
    Topics: Geosciences , Physics
    Published by American Geophysical Union
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  • 4
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    Regional Moment Tensors of the 2009 L'Aquila Earthquake Sequence (2011)
    Seismological Society of America
    Details
    Publication Date: 2011-05-29
    Description: Broadband waveform inversion of ground velocities in the 0.02-0.10 Hz frequency band is successfully applied to 181 earthquakes with M (sub L) 〉 or =3 of the April 2009 L'Aquila, Italy, earthquake sequence. This was made possible by the development of a new regional crustal velocity model constrained by deep crustal profiles, surface-wave dispersion and teleseismic P-wave receiver functions, and tested through waveform fit. Although all earthquakes exhibit normal faulting, with the fault plane dipping southwest at about 55 degrees for the majority of events, a subset of events had much shallower dips. The issue of confidence in the derived parameters was investigated by applying the same inversion procedure by two groups who subjectively selected different traces for inversion. The unexpected difficulty in modeling the regional broadband waveforms of the mainshock as a point source was investigated through an extensive finite-fault modeling of broadband velocity and accelerometer data, which placed the location of major moment release up-dip and about 4-7 s after the initial first-arrival hypocentral parameters.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America
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  • 5
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    Regional Moment Tensors of the 2009 L'Aquila Earthquake Sequence (2011)
    Seismological Society of America
    Details
    Publication Date: 2017-04-04
    Description: Broadband waveform inversion of ground velocities in the 0.02 0.10 Hz frequency band is successfully applied to 181 earthquakes with ML ≥ 3 of the April, 2009, L'Aquila, Italy, earthquake sequence. This was made possible by the development of a new regional crustal velocity model constrained by deep crustal profiles, surfacewave dispersion and teleseismic Pwave receiver functions and tested through waveform fit. Although all earthquakes exhibit normal faulting, with the fault plane dipping southwest at about 55º for the majority of events, a subset of events had much shallower dips. The issue of confidence in the derived parameters was investigated by applying the same inversion procedure by two groups who subjectively selected different traces for inversion. The unexpected difficulty in modeling the regional broadband waveforms of the mainshock as a point source was investigated through an extensive finitefault modeling of broadband velocity and accelerometer data, which placed the location of major moment release updip and about 47 seconds after the initial firstarrival hypocentral parameters.
    Description: Published
    Description: 975-993
    Description: 3.1. Fisica dei terremoti
    Description: JCR Journal
    Description: open
    Keywords: L'Aquila sequence ; focal mechanisms ; 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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  • 6
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    Characteristics of high frequency ground motions in the Maule region (Chile), obtained from aftershocks of the 2010 Mw 8.8 earthquake (2014)
    Details
    Publication Date: 2017-04-04
    Description: The Mw 8.8 Maule earthquake occurred off the coast of central Chile on 2010 February 27, and was followed by thousands of aftershocks. In this study, we modeled 172 aftershocks recorded by more than 100 temporary broadband stations deployed between March 2010 and January 2011. Each of these earthquakes is characterized by a well-determined hypocentral location and well-constrained focal mechanism and moment magnitudes in the range M 3.7 to 6.2. Most of these earthquakes are characterized by shallow, eastward-dipping, thrust-type focal mechanisms consistent with faulting at or near the plate interface, where the Nazca plate is subducting beneath the South America plate at approximately 74 mm/yr. This study provides a unique opportunity to quantify high-frequency earthquake ground motion in a subduction zone due to the quality and quantity of observations in the frequency and distance range of 0.2-30 Hz and 40-500 km, respectively. The analysis was done using a two-step modeling procedure. A regression is performed to characterize source duration and excitation, source-receiver distance dependence, and station site effects. A point source forward model is then constructed in terms of geometrical spreading, duration, site effects and source scaling to match the regression results. This procedure provides the necessary point source parameters for stochastic finite-fault modeling of peak ground motions for future earthquakes in this subduction zone.
    Description: Published
    Description: Trieste
    Description: 4T. Fisica dei terremoti e scenari cosismici
    Description: open
    Keywords: seismology
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: Poster session
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  • 7
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    Control of pore fluid pressure diffusion on fault failure mode:Insights from the 2009 L’Aquila seismic sequence (2012)
    Details
    Publication Date: 2017-04-04
    Description: The MW 6.13 L’Aquila earthquake ruptured the Paganica fault on 2009/04/06 at 01:32 UTC, and started a strong sequence of aftershocks. For the first four days, the region north of the hypocenter of the main quake was shaken by three large events (MW 5.0) that ruptured different patches of the Monti della Laga fault (hereafter “Campotosto”). In our hypothesis, these aftershocks were induced by a dramatic reduction in the fault’s shear strength due to a pulse of pore fluid pressure released after the L’Aquila main earthquake. Here we model the time evolution of the pore fluid pressure northward from the main hypocenter. We show that, during the sequence, the Campotosto fault failed in multiple episodes, when the specific patches/asperities underwent fluid pressure-related strength reductions of 7–10 MPa. Although such drops in strength are very large in amplitude, the contribution of other weakening mechanisms (perturbations of the Coulomb shear stress, and/or dynamic stresses induced by passing seismic waves) cannot be ruled out by our observations. However, the Coulomb shear stress variations either had negative amplitudes down to 0.2 MPa (i.e., tended to inhibit further seismic activity), or had very small positive amplitudes (〈0.05 MPa). Paleoseismological evidence supports the hypothesis that larger events (MW 6.5–7) have occurred on the Paganica fault [EMERGEO Working Group, 2009], whereas Lucente et al. [2010] concluded that an important migration of pore fluids characterized the preparatory phase of the L’Aquila main shock. Consequently, the MW 6.13 L’Aquila earthquake may be analogous, at a larger scale, to one of the three Campotosto largest aftershocks. The complex behavior observed for the L’Aquila-Campotosto fault system seems to be common to other seismogenic structures in the Central Apennines (e.g., the Umbria-Marche fault system), and need to be taken into consideration for the assessment of seismic hazard.
    Description: Published
    Description: B05302
    Description: 3.1. Fisica dei terremoti
    Description: JCR Journal
    Description: restricted
    Keywords: 2009 La'Aquila sequence ; pore fluid pressure diffusion ; seismic hazard ; 04. Solid Earth::04.06. Seismology::04.06.99. General or miscellaneous
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
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  • 8
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    The 2012 Ferrara seismic sequence: Regional crustal structure, earthquake sources, and seismic hazard (2015)
    American Geophysical Union
    Details
    Publication Date: 2017-04-04
    Description: Inadequate seismic design codes can be dangerous, particularly when they underestimate the true hazard. In this study we use data from a sequence of moderate-sized earthquakes in northeast Italy to validate and test a regional wave propagation model which, in turn, is used to under- stand some weaknesses of the current design spectra. Our velocity model, while regionalized and somewhat ad hoc, is consistent with geophysical observations and the local geology. In the 0.02–0.1 Hz band, this model is validated by using it to calculate moment tensor solutions of 20 earth- quakes (5.6 MW 3.2) in the 2012 Ferrara, Italy, seismic sequence. The seismic spectra observed for the relatively small main shock significantly exceeded the design spectra to be used in the area for critical structures. Observations and synthetics reveal that the ground motions are dominated by long-duration surface waves, which, apparently, the design codes do not adequately anticipate. In light of our results, the present seismic hazard assessment in the entire Pianura Padana, including the city of Milan, needs to be re-evaluated. Citation: Malagnini, L., R. B. Herrmann, I. Munafò, M. Buttinelli, M. Anselmi, A. Akinci, and E. Boschi (2012), The 2012 Ferrara seismic sequence: Regional crustal structure, earthquake sources, and seismic hazard, Geophys. Res. Lett., 39, L19302, doi:10.1029/ 2012GL053214.
    Description: Published
    Description: L19302
    Description: 4T. Fisica dei terremoti e scenari cosismici
    Description: JCR Journal
    Description: open
    Keywords: earthquake sources, seismic hazard ; 04. Solid Earth::04.06. Seismology::04.06.03. Earthquake source and dynamics
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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  • 9
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    Characterization of earthquake-induced ground motion from the L’Aquila seismic sequence of 2009, Italy (2011)
    willey blackwell
    Details
    Publication Date: 2017-04-04
    Description: Based only on weak-motion data, we carried out a combined study on region-specific source scaling and crustal attenuation in the Central Apennines (Italy). Our goal was to obtain a reappraisal of the existing predictive relationships for the ground motion, and to test them against the strong-motion data [peak ground acceleration (PGA), peak ground velocity (PGV) and spectral acceleration (SA)] gathered during the Mw 6.15 L’Aquila earthquake (2009 April 6, 01:32 UTC). The L’Aquila main shockwas not part of the predictive study, and the validation test was an extrapolation to one magnitude unit above the largest earthquake of the calibration data set. The regional attenuation was determined through a set of regressions on a data set of 12 777 high-quality, high-gain waveforms with excellent S/N ratios (4259 vertical and 8518 horizontal time histories). Seismograms were selected from the recordings of 170 foreshocks and aftershocks of the sequence (the complete set of all earthquakes with ML ≥ 3.0, from 2008 October 1 to 2010 May 10). All waveforms were downloaded from the ISIDe web page (http://iside.rm.ingv.it/iside/standard/index.jsp), a web site maintained by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). Weak-motion data were used to obtain a moment tensor solution, as well as a coda-based moment-rate source spectrum, for each one of the 170 events of the L’Aquila sequence (2.8 ≤ Mw ≤ 6.15). Source spectra were used to verify the good agreement with the source scaling of the Colfiorito seismic sequence of 1997–1998 recently described by Malagnini et al. (2008). Finally, results on source excitation and crustal attenuationwere used to produce the absolute site terms for the 23 stations located within ∼80 km of the epicentral area. The complete set of spectral corrections (crustal attenuation and absolute site effects) was used to implement a fast and accurate tool for the automatic computation of moment magnitudes in the Central Apennines.
    Description: Published
    Description: 325-337
    Description: 4.1. Metodologie sismologiche per l'ingegneria sismica
    Description: JCR Journal
    Description: reserved
    Keywords: Earthquake ground motions ; central Italy ; 04. Solid Earth::04.06. Seismology::04.06.99. General or miscellaneous
    Repository Name: Istituto Nazionale di Geofisica e Vulcanologia (INGV)
    Type: article
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