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  • 1
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    Complex normal faulting in the Apennines thrust-and-fold belt: the 1997 seismic sequence in central Italy (2004)
    In:  Bulletin of the Seismological Society of America, Taipei, Elsevier, vol. 94, no. 1, pp. 99-116, pp. L07301, (ISBN: 0-12-018847-3)
    Details
    Publication Date: 2004
    Keywords: Seismicity ; Tectonics ; BSSA
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    BIBLIOGRAPHY SEISMOLOGY
  • 2
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    Remotely triggered micro-earthquakes in the Larderello-Travale Geothermal Field (Italy) following the 2012 May 20, Mw 5.9 Po-plain earthquake. (2013)
    Wiley
    Details
    Publication Date: 2013-02-23
    Description: [1]  We report observations of remotely-triggered earthquakes at the Larderello-Travale Geothermal Field (Italy), following the Mw = 5.9 Po-Plain earthquake on May 20, 2012. Four distinct triggered events are recognized within a short (~25 s) time interval accompanying the sweeping of ~10s Rayleigh waves. Triggered sources are clustered at depths in between 4 km and 6 km. The magnitude and distance of the mainshock agrees well with the triggering threshold previously proposed for The Geysers, California. For three out of four earthquakes, the Rayleigh wave dynamic stresses are mostly associated with extensional vertical (s zz ) and shear (s xz ) components, which range up to 5 KPa. Once considering the structural setting of the area, the most likely triggering mechanism involves the rupture of normal faults which are kept close to failure by high-pressure crustal fluids.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 3
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    Radiography of a normal fault system by 64,000 high-precision earthquake locations: the 2009 L'Aquila (central Italy) case study. (2013)
    Wiley
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    Publication Date: 2013-02-16
    Description: [1]  We studied the anatomy of the fault system where the 2009 L'Aquila earthquake (M W 6.1) nucleated by means of ~64 k high-precision earthquake locations spanning one year. Data were analyzed by combining an automatic picking procedure for P- and S-waves, together with cross-correlation and double-difference location methods reaching a completeness magnitude for the catalogue equal to 0.7 including 425 clusters of similar earthquakes. [2]  The fault system is composed by two major faults: the high angle L'Aquila fault and the listric Campotosto fault, both located in the first 10 km of the upper crust. We detect an extraordinary degree of detail in the anatomy of the single fault segments resembling the degree of complexity observed by field geologists on fault outcrops. We observe multiple antithetic and synthetic fault segments tens of meters long in both the hanging-wall and footwall along with bends and cross fault intersections along the main fault and fault splays. [3]  The width of the L'Aquila fault zone varies along strike from 0.3 km where the fault exhibits the simplest geometry and experienced peaks in the slip distribution, up to 1.5 km at the fault tips with an increase in the geometrical complexity. These characteristics, similar to damage zone properties of natural faults, underline the key role of aftershocks in fault growth and coseismic rupture propagation processes. Additionally, we interpret the persistent nucleation of similar events at the seismicity cut-off depth as the presence of a rheological (i.e. creeping) discontinuity explaining how normal faults detach at depth.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 4
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    Fault zone properties affecting the rupture evolution of the 2009 (Mw 6.1) L'Aquila earthquake (central Italy): Insights from seismic tomography (2011)
    Wiley
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    Publication Date: 2011-05-28
    Description: We have inverted P- and S-wave travel times from seismograms recorded by a dense local network to infer the velocity structure in the crustal volume where the April 6th 2009 main shock nucleated. The goal is to image local variations of P-wave velocity and Poisson ratio along the main shock fault zone for interpreting the complexity of the rupture history. The initial stages of the mainshock rupture are characterized by an emergent phase (EP) followed by an impulsive phase (IP) 0.87 s later. The EP phase is located in a very high VP and relatively low Poisson ratio (ν) region. The IP phase marks the beginning of the large moment release and is located outside the low ν volume. The comparison between the spatial variations of VP and Poisson ratio within the main shock nucleation volume inferred in this study with the rupture history imaged by inverting geophysical data allows us to interpret the delayed along-strike propagation in terms of heterogeneity of lithology and material properties.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 5
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    The anatomy of the 2009 L'Aquila normal fault system (central Italy) imaged by high resolution foreshock and aftershock locations (2011)
    Wiley
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    Publication Date: 2011-12-20
    Description: On 6 April (01:32 UTC) 2009 a MW 6.1 normal faulting earthquake struck the axial area of the Abruzzo region in central Italy. We study the geometry of fault segments using high resolution foreshock and aftershock locations. Two main SW dipping segments, the L'Aquila and Campotosto faults, forming an en echelon system 40 km long (NW trending). The 16 km long L'Aquila fault shows a planar geometry with constant dip (∼48°) through the entire upper crust down to 10 km depth. The Campotosto fault activated by three events with 5.0 ≤ MW ≤ 5.2 shows a striking listric geometry, composed by planar segments with different dips along depth rather than a smoothly curving single fault surface. The investigation of the spatiotemporal evolution of foreshock activity within the crustal volume where the subsequent L'Aquila main shock nucleated allows us to image the progressive activation of the main fault plane. From the beginning of 2009 the foreshocks activated the deepest portion of the fault until a week before the main shock, when the largest foreshock (MW 4.0) triggered a minor antithetic segment. Seismicity jumped back to the main plane a few hours before the main shock. Secondary synthetic and antithetic fault segments are present both on the hanging and footwall of the system. The stress tensor obtained by inverting focal mechanisms of the largest events reveals a NE trending extension and the majority of the aftershocks are kinematically consistent. Deviations from the dominant extensional strain pattern are observed for those earthquakes activating minor structures.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 6
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    A detailed analysis of wastewater induced seismicity in the Val d'Agri oil field (Italy) (2015)
    Wiley
    Details
    Publication Date: 2015-03-25
    Description: The Val d'Agri basin in the Apennines seismic belt hosts the largest oil field in onshore Europe. High-quality recordings from a temporary dense network unravel a swarm of 111 small-magnitude events (M L ≤ 1.8) occurred in June 2006 during the first stage of wastewater injection into a high-rate well. High-precision relative locations define a pre-existing blind fault located 1 km below the well inside fractured and saturated carbonates where wastewater is re-injected. Seismicity begins 3 hours after the initiation of injection. The seismicity rate strictly correlates with injection curves and temporal variations of elastic and anisotropic parameters. Seismicity is induced by rapid communication of pore pressure perturbations along a high permeability fault-zone favorably oriented with respect to the local extensional stress field. Our accurate 3-D locations of 219 events (M L ≤ 2.2) detected by the local operator network after June 2006 concentrate on the pre-existing fault measuring 5.5-km along dip. Over the following 7½ years the seismicity rate correlates with short-term increases in injection pressure.
    Print ISSN: 0094-8276
    Electronic ISSN: 1944-8007
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 7
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    Reservoir structure and wastewater induced seismicity at the Val d'Agri oilfield (Italy) shown by three-dimensional Vp and Vp/Vs local earthquakes tomography (2017)
    Wiley
    Details
    Publication Date: 2017-10-10
    Description: Wastewater injection into a high-rate well in the Val d'Agri oilfield, the largest in onshore Europe, has induced swarm microseismicity since the initiation of disposal in 2006. To investigate the reservoir structure and to track seismicity we performed a high-spatial resolution local earthquake tomography using 1281 natural and induced earthquakes recorded by local networks. The properties of the carbonate reservoir (rock fracturing, pore fluid pressure) and inherited faults control the occurrence and spatiotemporal distribution of seismicity. A low-Vp, high-Vp/Vs region under the well represents a fluid saturated fault zone ruptured by induced seismicity. High-Vp, high-Vp/Vs bumps match reservoir culminations indicating saturated liquid-bearing zones, whereas a very-low Vp, low-Vp/Vs anomaly might represent a strongly fractured and depleted zone characterized by significant fluid withdrawal. The comprehensive picture of the injection-linked seismicity obtained by integrating reservoir-scale tomography, high-precision earthquake locations, geophysical and injection data suggests that the driving mechanism is the channelling of pore pressure perturbations through a high permeable fault damage zone within the reservoir. The damage zone surrounds a Pliocene reverse fault optimally oriented in the current extensional stress field. The ruptured damage zone measures 2 km along strike, 3 km along dip and is confined between low permeability ductile formations. Injection pressure is the primary parameter controlling seismicity rate. Our study underlines that local earthquake tomography also using wastewater induced seismicity can give useful insights into the physical mechanism leading to these earthquakes.
    Print ISSN: 0148-0227
    Topics: Geosciences , Physics
    Published by Wiley on behalf of American Geophysical Union (AGU).
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  • 8
    Electronic Resource
    Electronic Resource
    Spatio-temporal distribution of seismic activity during the Umbria-Marche crisis, 1997 (2000)
    Springer
    Journal of seismology 4 (2000), S. 377-386 
    Details
    ISSN: 1573-157X
    Keywords: Seismicity ; Central Italy ; Umbria-Marche ; Aftershock sequence ; seismic crisis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract We present the spatio-temporal distribution of more than 2000 earthquakesthat occurred during the Umbria-Marche seismic crisis, between September 26and November 3, 1997. This distribution was obtained from recordings of atemporary network that was installed after the occurrence of the first two largest shocks (Mw =, 5.7, Mw = 6.0) of September 26. This network wascomposed of 27 digital 3-components stations densely distributed in theepicentral area. The aftershock distribution covers a region of about 40 km long and about2 km wide along the NW-SE central Apennines chain. The activity is shallow,mostly located at less than 9 km depth. We distinguished three main zonesof different seismic activity from NW to SE. The central zone, that containsthe hypocenter of four earthquakes of magnitude larger than 5, was the moreactive and the more complex one. Sections at depth identify 40–50°dipping structures that agree well with the moment tensor focalmechanisms results. The clustering and the migration of seismicity from NW to SE and the generalfeatures are imaged by aftershock distribution both horizontally and at depth.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1026568419411
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  • 9
    Electronic Resource
    Electronic Resource
    Foreshock sequence of September 26th, 1997 Umbria-Marche earthquakes (2000)
    Springer
    Journal of seismology 4 (2000), S. 387-399 
    Details
    ISSN: 1573-157X
    Keywords: dilatancy model ; foreshock ; Vp/Vs variation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Physics
    Notes: Abstract On September 3rd (22:07 UTC), 1997 a small earthquake with Mw=4.54 started the foreshocks sequence (≈1500 events with ML 〈3.1) of the September 26th seismic sequence. Two days after, three seismic stations of the University of Camerino were installed around the macroseismic epicenter of the foreshock. We present in this paper the location of foreshocks (with 2.1〈ML〈3.3) which occurred between September 3rd and 26th. Foreshocks location, with horizontal (ERH) and vertical (ERZ) error less than 1.5 km, define an area ≈4 km large. Foreshocks have been localized between the epicenters of the two major events of September 26th, which occurred at 00:33 UTC with Mw=5.6 and at 09:40 UTC with Mw=6.0 (Amato et al., 1998; Ekström et al., 1998). In a vertical cross-section, hypocenters show a low angle (≈30°) structure with SW dip-direction. Focal mechanisms for three of the major events show dip-slip fault solutions with strike direction of about N130, in agreement with the CMT solutions of September 3rd and September 26th earthquakes (Ekström et al., 1998). Data recorded at two stations Popola (POP) e Capodacqua (CPQ) located on the rupture area of the September 26th faults, allowed us to calculate a mean Vp/Vs ratio of 1.84±0.03 for the foreshock. This value is lower than the Vp/Vs ratio of 1.89±0.02 calculated for the aftershock sequence occurred in the same area. Besides, the Vp/Vs ratio during the foreshocks sequence is not stable in time but it seems to increase approaching September 26th. After September 26th mainshocks, this value tends to stabilize around a higher value of 1.89. Following the dilatancy model, we suggest that the relative low Vp/Vs ratio before the main shocks could indicate the presence of fluid in the focal volume. The presence of fluids could have increased the effective stress on the fault plane and could be responsible for the long foreshock activity before the two main earthquakes of September 26th. Therefore, we suggest that this foreshock activity could have also contributed to reduce the friction along the September 26th fault plane, breaking the active structure in two smaller segments. In this hypothesis, foreshock activity could have drastically contributed to mitigate the seismic potential of the Colfiorito's active structure.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1023/A:1026508425230
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  • 10
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    Shear wave splitting of the 2009 L'Aquila seismic sequence: fluid saturated microcracks and crustal fractures in the Abruzzi region (Central Apennines, Italy) (2016)
    Oxford University Press
    Details
    Publication Date: 2016-02-05
    Description: The Abruzzi region is located in the Central Apennines Neogene fold-and-thrust belt and has one of the highest seismogenic potential in Italy, with high and diffuse crustal seismicity related to NE–SW oriented extension. In this study, we investigate the detailed spatial variation in shear wave splitting providing high-resolution anisotropic structure beneath the L'Aquila region. To accomplish this, we performed a systematic analysis of crustal anisotropic parameters: fast polarization direction ( ) and delay time ( t ). We benefit from the dense coverage of seismic stations operating in the area and from a catalogue of several accurate earthquake locations of the 2009 L'Aquila seismic sequence, related to the M w 6.1 2009 L'Aquila main shock, to describe in detail the geometry of the anisotropic volume around the active faults that ruptured. The spatial variations both in and t suggest a complex anisotropic structure beneath the region caused by a combination of both structural- and stress-induced mechanisms. The average is NNW–SSE oriented (N141°), showing clear similarity both with the local fault strike and the SH max . In the central part of the study area fast axes are oriented NW–SE, while moving towards the northeastern and northwestern sectors the fast directions clearly diverge from the general trend of NW–SE and rotate accordingly to the local fault strikes. The above-mentioned fault-parallel distribution suggests that the observed anisotropy is mostly controlled by the local fault-related structure. Toward the southeast fast directions become orthogonal both to strike of the local mapped faults and to the SH max . Here, are predominantly oriented NE–SW; we interpret this orientation as due to the presence of a highly fractured and overpressurized rock volume which should be responsible of the 90° flips in and the increase in t . Another possible mechanism for NE–SW orientation of in the southeastern sector could be ascribed to the presence of a buried, deep NE–SW oriented fault system. t , both unnormalized and normalized, does not show any clear evidence of increasing with increasing depth indicating that the anisotropy is confined primarily to the shallower crustal layers (~10 km depth). Interpolating t show that higher values are found at the edges of the main patches of the rupture related to the 2009 main shock, while lower values are limited in the central part of the fault plane, where the coseismic slip was higher. We infer that in the areas surrounding the ruptured region, lateral variations in material properties caused overpressurized fluid conditions, while within the main shock ruptured area, high energy released produced an open crack system such that overpressurization was not possible.
    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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