Structural control on the directional amplification of seismic noise (Campo Imperatore, central Italy)

Abstract Seismic signals propagating across a fault may yield information on the internal structure of the fault zone. Here we have assessed the amplification of seismic noise (i.e., ambient vibrations generated by natural or anthropogenic disturbances) across the Vado di Corno Fault (Campo Imperatore, central Italy). The fault zone is considered as an exhumed analogue of the normal faults activated during the L'Aquila 2009 earthquake sequence. Detailed structural geological survey of the footwall block revealed that the fault zone is highly anisotropic and is affected by a complex network of faults and fractures with dominant WNW–ESE strike. We measured seismic noise with portable seismometers along a ∼500 m long transect perpendicular to the average fault strike. Seismic signals were processed calculating the horizontal-to-vertical spectral ratios and performing wavefield polarization analyses. We found a predominant NE–SW to NNE–SSW (i.e., ca. perpendicular to the average strike of the fault-fracture network) amplification of the horizontal component of the seismic waves. Numerical simulations of earthquake-induced ground motions ruled out the role of topography in controlling the polarization and the amplitude of the waves. Therefore, the higher seismic noise amplitude observed in the fault-perpendicular direction was related to the measured fracture network and the resulting stiffness anisotropy of the rock mass. These observations open new perspectives in using measures of ambient seismic noise, which are fast and inexpensive, to estimate the dominant orientation of fracture networks within fault zones

Horizontal polarization of ground motion in the Hayward fault zone

We investigate shear wave polarization in the Hayward fault zone near Niles Canyon, Fremont, CA. Waveforms of 12 earthquakes recorded by a seven-accelerometer seismic array around the fault are analysed to clarify directional site effects in the fault damage zone. The analysis is performed in the frequency domain through H/V spectral ratios with horizontal components rotated from 0◦ to 180◦, and in the time domain using the eigenvectors and eigenvalues of the covariance matrix method employing three component records. The near-fault ground motion tends to be polarized in the horizontal plane. At two on-fault stations where the local strike is N160◦, ground motion polarization is oriented N88 ± 19◦ and N83 ± 32◦, respectively. At a third on-fault station, the motion is more complex with horizontal polarization varying in different frequency bands. However, a polarization of N86 ± 7◦, similar to the results at the other two on-fault stations, is found in the frequency band 6–8 Hz. The predominantly high-angle polarization from the fault strike at the Hayward Fault is consistent with similar results at the Parkfield section of the San Andreas Fault and the Val d’Agri area (a Quaternary extensional basin) in Italy. In all these cases, comparisons of the observed polarization directions with models of fracture orientation based on the fault movement indicate that the dominant horizontal polarization is near-orthogonal to the orientation of the expected predominant cracking direction. The results help to develop improved connections between fault mechanics and near-fault ground motion

Unveiling a hidden fortification system at “Faraglioni” Middle Bronze Age Village of Ustica Island (Palermo, Italy) through ERT and GPR prospections

We carried out a geophysical research project in the Middle Bronze Age village of Ustica (Palermo, Sicily, Italy), named “Faraglioni Village” after the stack formations which detach from the coast north of the archaeological site. The investigation, which comprised Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) techniques, allowed us to discover the buried foundations of an outwork fortification system never evidenced by previous archaeological studies, only hypothesised from the observation of aerial photography and partially outcropping boulders, which align roughly parallel to the main defensive wall of the Village. Our geophysical prospection involved the entire 250 m-long arc of the outward village defensive wall, with the acquisition of eleven ERT profiles and 27 GPR scans. The techniques were selected based on both favourable logistics and methods applicability: ERT sections allowed us to trace a series of high-resistivity anomalies arranged to form an arc-shaped structure along the perimeter of the defensive wall. GPR investigation was localised in the most accommodating patch of terrain of the site, with the effort of intercepting clear enough sections of the target, to determine more accurately its shape, depth, and overall dimensions. Our discovery paves the way for new investigations, mainly aimed at defining the timing of construction of the fortification system, as well as the function of the remains of other architectural structures identified close to the wall, which could represent the target of further geophysical investigations

Placenta-derived cells for acute brain injury

Acute brain injury resulting from ischemic/hemorrhagic or traumatic damage is one of the leading causes of mortality and disability worldwide and is a significant burden to society. Neuroprotective options to counteract brain damage are very limited in stroke and traumatic brain injury (TBI). Given the multifaceted nature of acute brain injury and damage progression, several therapeutic targets may need to be addressed simultaneously to interfere with the evolution of the injury and improve the patient’s outcome. Stem cells are ideal candidates since they act on various mechanisms of protection and repair, improving structural and functional outcomes after experimental stroke or TBI. Stem cells isolated from placenta offer advantages due to their early embryonic origin, ease of procurement, and ethical acceptance. We analyzed the evidence for the beneficial effects of placenta-derived stem cells in acute brain injury, with the focus on experimental studies of TBI and stroke, the engineering strategies pursued to foster cell potential, and characterization of the bioactive molecules secreted by placental cells, known as their secretome, as an alternative cell-free strategy. Results from the clinical application of placenta-derived stem cells for acute brain injury and ongoing clinical trials are summarily discussed

Ground motion amplification at sites with pronounced topography: the controversial role of local geology

The topographic amplification of seismic waves has received an increasing interest in the last four decades following observations of large amplification on mountain tops (e.g. Davis and West, 1973; Griffiths and Bollinger, 1979; Çelebi, 1987; Umeda et al., 1987; Kawase and Aki, 1990; Ponti and Wells, 1991; Hartzell et al., 1994; Pedersen et al., 1994a; Chavez-Garcia et al., 1996). The recurrence and consistency of these observations has motivated much work both in terms of theoretical investigations and numerical simulations of the diffraction of seismic waves caused by the topography (e.g. Bouchon, 1973: Bard and Tucker, 1985; Géli et al., 1988; Anooshehpoor and Brune, 1989; Gaffet and Bouchon, 1989; Sanchez-Sesma and Campillo, 1991; Pedersen et al., 1994b; Le Brun et al., 1999; Paolucci, 2002). The simulations and the observations are often in qualitative agreement with the amplification at the topography top and for wavelengths comparable to the mountain width. The disagreement concerns the calculated amplification level that tends to underestimates observations. This discrepancy has suggested that other effects could be responsible of the amplification effect, as the geological setting, complicated incident wave field, more complex topography, etc (Bard and Chaljub, 2009). Beside strong amplification, topographic irregularities have been recognized to produce directional effect of resonance; the scattered wave field is polarized in a site-characteristic direction. Spudich et al. (1996) found that directional amplification occurs transversally to the hill major axis, as subsequently assessed by several other authors (e.g., Del Gaudio and Wasowski, 2007; Massa et al., 2010; Pischiutta et al., 2010). In the framework of a statistical study performed using stations of the Italian seismic network to check the recurrence of directional amplification effects, Pischiutta et al. (2010 and 2011) and Rovelli et al. (2011) investigated the relation between the direction of maximum amplification and the hill elongation at around 40 selected stations of the Italian seismic network. They found that only the 25% of stations showed an angular relation between directional amplification and the hill elongation ranging from 80 and 90 degrees. The same conclusions were reached by Burjanek et al. (2014a and 2014b) who investigated the relation between the S-wave velocity profiles and the amplification occurrence at 25-instrumented sites with complex topography in Switzerland and Japan. They stressed that the amplification was controlled primarily by the sub-surface velocity structure and they did not identify any link between the surface topography and the observed response at the studied 25 sites. Thus recent findings have suggested that large systematic amplifications at topographic sites cannot be explained by surface geometry only, and that although the effect of geometry is present, it cannot be simply decoupled from the site response. We think that directional amplification observed at sites with pronounced topography are often correlated with rock fractures. This feature has not considered adequately so far. Here we propose a model that could explain directional amplification. Similar effects have been recently observed (Marzorati et al., 2011) and associated to gravitational instabilities (Burjanek et al., 2010) as well as to fault damage zones (Falsaperla et al., 2010; Pischiutta et al., 2012 and 2013; Di Giulio et al., 2013). Pischiutta et al. (2014 and 2014) interpreted the strong polarization in terms of fracture fields that make the rock more compliant in the strike-transverse direction (Pischiutta et al., 2012 and 2014).UnpublishedBologna3T. Pericolosità sismica e contributo alla definizione del rischiorestricte

Orthogonal relation between wavefield polarization and fast <i>S</i> wave direction in the Val d'Agri region: An integrating method to investigate rock anisotropy

AbstractWavefield polarization is investigated using 200 seismograms recorded by a network of 20 stations installed on rock outcrops in the Val d'Agri region that hosts the largest oil fields in the southern Apennines (Italy). Polarization is assessed both in the frequency and time domains through the individual‐station horizontal‐to‐vertical spectral ratio and covariance‐matrix analysis, respectively. We find that most of the stations show a persistent horizontal polarization of waveforms, with a NE‐SW predominant trend. This direction is orthogonal to the general trend of Quaternary normal faults in the region and to the maximum horizontal stress related to the present extensional regime. According to previous studies in other areas, such a directional effect is interpreted as due to the presence of fault‐related fracture fields, polarization being orthogonal to their predominant direction. A comparison with S wave anisotropy inferred from shear wave splitting indicates an orthogonal relation between horizontal polarization and fast S wave direction. This suggests that wavefield polarization and fast velocity direction are effects of the same cause: The existence of an anisotropic medium represented by fractured rocks where shear wave velocity is larger in the crack‐parallel component and compliance is larger perpendicularly to the crack strike. The latter is responsible for the observed anisotropic pattern of amplitudes of horizontal ground motion in the study area

Ground motion amplification at sites with pronounced topography: the controversial role of local geology

The topographic amplification of seismic waves has received an increasing interest in the last four decades following observations of large amplification on mountain tops (e.g. Davis and West, 1973; Griffiths and Bollinger, 1979; Çelebi, 1987; Umeda et al., 1987; Kawase and Aki, 1990; Ponti and Wells, 1991; Hartzell et al., 1994; Pedersen et al., 1994a; Chavez-Garcia et al., 1996). The recurrence and consistency of these observations has motivated much work both in terms of theoretical investigations and numerical simulations of the diffraction of seismic waves caused by the topography (e.g. Bouchon, 1973: Bard and Tucker, 1985; Géli et al., 1988; Anooshehpoor and Brune, 1989; Gaffet and Bouchon, 1989; Sanchez-Sesma and Campillo, 1991; Pedersen et al., 1994b; Le Brun et al., 1999; Paolucci, 2002). The simulations and the observations are often in qualitative agreement with the amplification at the topography top and for wavelengths comparable to the mountain width. The disagreement concerns the calculated amplification level that tends to underestimates observations. This discrepancy has suggested that other effects could be responsible of the amplification effect, as the geological setting, complicated incident wave field, more complex topography, etc (Bard and Chaljub, 2009). Beside strong amplification, topographic irregularities have been recognized to produce directional effect of resonance; the scattered wave field is polarized in a site-characteristic direction. Spudich et al. (1996) found that directional amplification occurs transversally to the hill major axis, as subsequently assessed by several other authors (e.g., Del Gaudio and Wasowski, 2007; Massa et al., 2010; Pischiutta et al., 2010). In the framework of a statistical study performed using stations of the Italian seismic network to check the recurrence of directional amplification effects, Pischiutta et al. (2010 and 2011) and Rovelli et al. (2011) investigated the relation between the direction of maximum amplification and the hill elongation at around 40 selected stations of the Italian seismic network. They found that only the 25% of stations showed an angular relation between directional amplification and the hill elongation ranging from 80 and 90 degrees. The same conclusions were reached by Burjanek et al. (2014a and 2014b) who investigated the relation between the S-wave velocity profiles and the amplification occurrence at 25-instrumented sites with complex topography in Switzerland and Japan. They stressed that the amplification was controlled primarily by the sub-surface velocity structure and they did not identify any link between the surface topography and the observed response at the studied 25 sites. Thus recent findings have suggested that large systematic amplifications at topographic sites cannot be explained by surface geometry only, and that although the effect of geometry is present, it cannot be simply decoupled from the site response. We think that directional amplification observed at sites with pronounced topography are often correlated with rock fractures. This feature has not considered adequately so far. Here we propose a model that could explain directional amplification. Similar effects have been recently observed (Marzorati et al., 2011) and associated to gravitational instabilities (Burjanek et al., 2010) as well as to fault damage zones (Falsaperla et al., 2010; Pischiutta et al., 2012 and 2013; Di Giulio et al., 2013). Pischiutta et al. (2014 and 2014) interpreted the strong polarization in terms of fracture fields that make the rock more compliant in the strike-transverse direction (Pischiutta et al., 2012 and 2014)