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Munafò, Irene
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Munafò, Irene
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- PublicationRestrictedControl of pore fluid pressure diffusion on fault failure mode:Insights from the 2009 L’Aquila seismic sequence(2012)
; ; ; ; ; ;Malagnini, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Lucente, F.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;De Gori, P.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione CNT, Roma, Italia ;Akinci, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Munafò, I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; 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.744 39 - PublicationRestrictedGradual fault weakening with seismic slip: inferences from the seismic sequences of L'Aquila, 2009 and Northridge, 1994(2014)
; ; ; ; ; ; ;Malagnini, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Munafo', I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Cocco, M.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;NIelsen, S.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Mayeda, K. M.; UC Berkeley ;Boschi, E.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione AC, Roma, Italia; ; ; ; ; We estimate seismological fracture energies from two subsets of events selected from the seismic sequences of L’Aquila (2009), and Northridge (1994): 57 and 16 selected events, respectively, including the main shocks. Following ABERCROMBIE and RICE (2005), we postulate that fracture energy (G) represents the post-failure integral of the dynamic weakening curve, which is described by the evolution of shear traction as a function of slip. Following a direct-wave approach, we compute mainshock-/aftershock-source spectral ratios, and analyze them using the approach proposed by MALAGNINI et al. (this issue, 2014) to infer corner frequencies and seismic moment. Our estimates of source parameters (including fracture energies) are based on best-fit grid searches performed over empirical source spectral ratios. We quantify the source scaling of spectra from small and large earthquakes by using the MDAC formulation of WALTER and TAYLOR (2001). The source parameters presented in this paper must be considered as point source estimates representing averages calculated over specific ruptured portions of the fault area. In order to constrain the scaling of fracture energy with coseismic slip, we investigate two different slip-weakening functions to model the shear traction as a function of slip: (i) a power law, as suggested by ABERCROMBIE and RICE (2005), and (ii) an exponential decay. Our results show that the exponential decay of stress on the fault allows a good fit between measured and predicted fracture energies, both for the main events and for their aftershocks, regardless of the significant differences in the energy budgets between the large (main) and small earthquakes (aftershocks). Using the power-law slip-weakening function would lead us to a very different situation: in our two investigated sequences, if the aftershock scaling is extrapolated to events with large slips, a power law (a la Abercrombie and Rice) would predict unrealistically large stress drops for large, main earthquakes. We conclude that the exponential stress evolution law has the advantage of avoiding unrealistic stress drops and unbounded fracture energies at large slip values, while still describing the abrupt shear-stress degradation observed in high-velocity laboratory experiments (e.g., DI TORO et al., 2011).294 22 - PublicationOpen AccessSource characteristic of 2000 small earthquakes nucleating on the Alto Tiberina fault system (Central Italy)(2014-12-15)
; ; ; ; ; ; ; ; ; ; ; The Alto Tiberina Fault (ATF) is a 60 km long east-dipping low-angle normal fault, located in a sector of the Northern Apennines (Italy) undergoing active extension since the Quaternary (Chiaraluce et al. 2007). The ATF has been imaged by analyzing the active source seismic reflection profiles, and the instrumentally recorded persistent background seismicity. The present study is an attempt to separate the contributions of source, site, and crustal attenuation, in order to focus on the mechanics of the ATF, as well as the syn- and antithetic structrure related on the ATF hanging-wall (i.e. Gubbio fault and Umbria Valley fault). In order to compute source spectra, we perform a set of regression over the seismograms of ~ 400 small earthquakes (0.5 < ML < 3.0) recorded between 2010 and 2014 at 50 permanent seismic stations deployed in the framework of the Alto Tiberina Near Fault Observatory project (TABOO; Chiaraluce et al., 2014), three of which located in shallow boreholes. Because we deal with some very small earthquakes, we maximize the signal to noise ratio (SNR) with a technique based on the analysis of peak values of bandpass-filtered time histories, in addition to the same processing performed on Fourier amplitudes. We rely on Random Vibration Theory (RVT, Cartwright and Longuet-Higgins, 1956) to completely switch from peak values in the time domain to Fourier spectral amplitudes. So far, highly accurate, stable source spectra have been used to compute moment magnitudes (Mw) of all the events in the present data set, whereas in future developments the same data will be used to gain insights into the underlying mechanics of faulting and the earthquake processes.84 19 - PublicationOpen AccessPractical Issues in Monitoring a Hydrocarbon Cultivation Activity in Italy: The Pilot Project at the Cavone Oil Field(2021-11)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; In this paper we describe the results of an experimental implementation of the recent guidelines issued by the Italian regulatory body for monitoring hydrocarbon production activities. In particular, we report about the pilot study on seismic, deformation, and pore pressure monitoring of the Mirandola hydrocarbon cultivation facility in Northern Italy. This site hosts the Cavone oil field that was speculated of possibly influencing the 2012 ML 5.8 Mirandola earthquake source. According to the guidelines, the monitoring center should analyse geophysical measurements related to seismicity, crustal deformation and pore pressure in quasi real-time (within 24–48 h). A traffic light system would then be used to regulate underground operations in case of detecting significant earthquakes (i.e., events with size and location included in critical ranges). For these 2-year period of guidelines experimentation, we analysed all different kinds of available data, and we tested the existence of possible relationship between their temporal trends. Despite the short time window and the scarce quantity of data collected, we performed the required analysis and extracted as much meaningful and statistically reliable information from the data. We discuss here the most important observations drawn from the monitoring results, and highlight the lessons learned by describing practical issues and limitations that we have encountered in carrying out the tasks as defined in the guidelines. Our main goal is to contribute to the discussion about how to better monitor the geophysical impact of this kind of anthropogenic activity. We point out the importance of a wider seismic network but, mostly, of borehole sensors to improve microseismic detection capabilities. Moreover, the lack of an assessment of background seismicity in an unperturbed situation -due to long life extraction activities- makes it difficult to get a proper picture of natural background seismic activity, which would be instead an essential reference information for a tectonically-active regions, such as Northern Italy.1137 46 - PublicationOpen AccessCharacteristics of high frequency ground motions in the Maule region (Chile), obtained from aftershocks of the 2010 Mw 8.8 earthquake(2013-11-19)
; ; ; ;Munafò, I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Malagnini, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Herrmann, R. B.; Saint Louis University, St. Louis, MO, United States; ; 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.127 276 - PublicationRestrictedOn the relationship between ML and MW in a broad range: an example from the Apennines, ItalyBeing tied to a physical quantity, moment magnitude (Mw) should be the reference estimate of earthquake size and used whenever possible. Local magnitude (ML) represents a simple alternative for a reliable estimate of size, its best use being either for quick outcomes or when the computation of Mw is difficult (e.g., for small earthquakes). However, ML and Mw are profoundly different and not interchangeable. Here, we analyze a large set of 1509 ML–Mw data points from earthquakes of the central and northern Apennines (CNA), quantify the empirical scaling, and look for features of global validity. Our data set is made of 449 unpublishedMws from moment tensor solutions of events from the Amatrice-Visso-Norcia (AVN) sequence, 170 published Mws from moment tensor solutions of events from the L’Aquila seismic sequence (2009), and 890 published ML–Mw data points from earthquakes of the Altotiberina fault (ATF, 2010–2014; Mws from spectral correction). We integrate our empirical data set by computing the local magnitudes of the events from the AVN and L’Aquila sequences. Our analysis of CNA earthquakes shows that, for earthquakes up to a crossover magnitude MLco ≈ 4:3: Mw = 2/3 ML + C′; C′ = 1.14. Moreover, for earthquakes with ML > MLco, up to ML 6.5, our data suggest Mw = bML + C′′; b = 1.28; C′′= −1:50, in which b depends on the combined effects of source scaling and crustal attenuation, and C′ and C″ on regional attenuation (G (r) , Q (f) , κ0), focal depth, and rigidity at source. Finally, a synthetic study calibrated on the crustal attenuation and the source characteristics of the AVN data set reproduces the observed scaling between ML and Mw, predicting that MLs in the analyzed region saturate above ML ∼ 6:5. Smooth transitions are predicted between the different regimes.
212 201 - PublicationOpen AccessAssessing the nature of stochastic uncertainties for ground motion predictions: the Apennines, Italy.(2019-04)
; ; ; ; ; ; ; In this study we aim to assessing the nature of stochastic uncertainties in ground-motion predictions, by including the variability of region-specific crustal attenuation in time and space in the Central/Northern Apennines (Italy), using the events occurred during 2016-2017 earthquake sequence. Spectral characteristics of excitation, attenuation and duration of ground motion are derived through a regression analysis of the peak ground velocities in the frequency range of 0.25–22 Hz. Regressions are carried out over thousands time windows before and after the Amatrice (M6.0), Visso (M5.9) and Norcia (M6.3) earthquakes, in order to evaluate the fluctuations in seismic wave attenuation induced by the largest mainshocks of the seismic sequence. Propagation terms are modeled using random vibration theory, through a grid search over the attenuation parameters. Here we show that crustal attenuation is strongly affected by transients triggered by the main events, and quantify the impact of the seismic wave attenuation variability on the ground-motion hazard in the Central/Northern Apennines. We also determine the effect of spatial variability of crustal attenuation and its contribution to stochastic uncertainties in ground motion predictions.58 11 - PublicationRestrictedMws of Seismic Sources udder Thick sedimentsMoment 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 Mw ∼ 5.6, even though their enhanced spectral level from 0.1 to 0.4 Hz is consistent with Mw ∼ 6.0.
169 4 - PublicationOpen AccessRegional Moment Tensors of the 2009 L'Aquila Earthquake Sequence(2011)
; ; ; ;Herrmann, R.; Saint Louis University ;Malagnini, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Munafo, I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;; 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.285 2221 - PublicationRestrictedCharacterization of earthquake-induced ground motion from the L’Aquila seismic sequence of 2009, Italy(2011-01)
; ; ; ; ; ; ;Malagnini, L.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Akinci, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Mayeda, K.; Berkeley Seismological Observatory, University of California, Berkeley, CA, USA ;Munafo, I.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia ;Herrmann, R. B.; Department of Earth and Atmospheric Sciences of Saint Louis University, St. Louis, MO, USA ;Mercuri, A.; Istituto Nazionale di Geofisica e Vulcanologia, Sezione Roma1, Roma, Italia; ; ; ; ;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.413 39
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