42 research outputs found
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 0â—¦ to 180â—¦, 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
Label-free monitoring of tissue biochemistry following traumatic brain injury using Raman spectroscopy.
Traumatic brain injury (TBI) constitutes a major cause of death and long-term disability. At present, we lack methods to non-invasively track tissue biochemistry and hence select appropriate interventions for patients. We hypothesized that detailed label-free vibrational chemical analysis of focal TBI could provide such information. We assessed the early spatial and temporal changes in tissue biochemistry that are associated with brain injury in mice. Numerous differences were observed in the spectra of the contusion core and pericontusional tissue between 2 and 7 days. For example, a strong signal from haem was seen in the contusion core at 2 days due to haemorrhage, which subsequently resolved. More importantly, elevated cholesterol levels were demonstrated by 7 days, which may be a marker of important cell repair processes. Principal component analysis revealed an early 'acute' component dominated by haemorrhage and a delayed component reflecting changes in protein and lipid composition. Notably we demonstrated changes in Raman signature with time even in the contralateral hemisphere when compared to sham control mice. Raman spectroscopy therefore shows promise as a probe that is sensitive to important pathobiological processes in TBI and could be applied in future both in the experimental setting, as well as in the clinic
Wavefield Polarization in Fault Zones of the Western Flank of Mt. Etna: Observations and Fracture Orientation Modelling
Ambient noise measurements performed on the
western flank of Mt. Etna are analyzed to infer the occurrence of
directional amplification effects in fault zones. The data were
recorded along short (\500 m) profiles crossing the Ragalna Fault
System. Ambient noise records were processed to compute the
horizontal-to-vertical noise spectral ratio as a function of frequency
and direction of motion. Wavefield polarization was investigated in
the time–frequency domain as well. Peaks of the spectral ratios
generally fall in the frequency band 1.0–6.0 Hz pointing out
directional amplifications that are also confirmed by the results of
the time–frequency analysis, the largest amplification occurring
with high angle to the fault strike. A variation of the frequency of
the spectral peak is observed between the two sides of the fault,
possibly related to a damage fault asymmetry. Measurements
performed several kilometers away from the fault zone do not show
behavior that is as systematic as in the fault zone, and this suggests
that the observed directional effects can be ascribed to the fault
fabric. We relate the polarization effect to compliance anisotropy in
the fault zone, where the presence of predominantly oriented
fractures makes the normal component of ground motion larger
than the transversal one. In order to test the direction and the type
of fractures that are expected in the fault zone, we modeled the
brittle deformation pattern of the investigated fault. Theoretical
results are in good agreement with field observations of the fracture
strike.Published3083–30974T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa
Directional site effects in a non-volcanic gas emission area (Mefite d’Ansanto, southern Italy): Evidence of a local transfer fault transversal to large NW–SE extensional faults?
""The technique of the wavefield polarization is applied to ambient vibrations recorded in the Mefite d’Ansanto area, an important non-volcanic natural emission of low temperature CO2 enriched gases. Twenty-five measurements were performed in the study area, eleven near the emission site and the other fourteen in different sites within an area of 5 km. Polarization is assessed both in the frequency and time domain through the individual-station horizontal-to-vertical spectral ratio and covariance-matrix analysis, respectively. We find a significant tendency of ground motion in the gas emission area to be polarized in the horizontal plane, with a N115° predominant trend. This polarization tends to disappear while moving far from the site. According to previous papers in other study areas, such a directional effect is likely caused by fault-induced fractures and tends to be orthogonal to the fracture strike. The predominant NW–SE regional faulting does not fit the N115° polarization direction. To explain observations, we propose an interpretation in terms of a NE–SW oriented, local transfer fault as inferred from the lineament analysis. The intersection of the damage zone of this fault with the regional NW–SE normal fault system could easily be the responsible for the gas emissions since it favors a locally increased crustal weakness."
Orthogonal relation between wavefield polarization and fast S wave direction in the Val d’Agri region: An integrating method to investigate rock anisotropy
Wavefield 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
Directional resonance variations across the Pernicana Fault, Mt Etna, in relation to brittle deformation fields
The Pernicana Fault (PF) is the main structural element of Mt Etna and the northern boundary
of a section sliding to the southeast. Observed ground motion records in the damage zone of
the PF show strong variations of directional resonance in the horizontal plane. The observed
resonance directions exhibit an abrupt rotation of azimuth by about 30â—¦ across the fault,
varying from N166â—¦ on the north side to N139â—¦ on the south. We interpret the directional
resonance observations in terms of changes in the kinematics and deformation fields on the
opposite sides of the fault. The northern side is affected primarily by the left-lateral strike-slip
movement, whereas the southern side, that is subjected also to sliding, is under a dominant
extensional stress regime. Brittle deformation models based on the observed kinematic field
predict different sets of fractures on the opposite sides of the fault: synthetic cleavages and
extensional fractures are expected to dominate in the northern and southern sides, respectively.
These two fracture fields have different orientations (N74â—¦ and N42â—¦, respectively) and both
show a near-orthogonal relation (∼88◦ in the northern sector and ∼83◦ to the south) with the
azimuth of the observed directional resonance. We conclude that the direction of the largest
resonance motions is sensitive to and has transversal relationship with the dominant fracture
orientation. The directional amplification is inferred to be produced by stiffness anisotropy of
the fault damage zone, with larger seismic motions normal to the fractures.Published986–9963T. Pericolosità sismica e contributo alla definizione del rischioJCR Journalrestricte
Directional resonance variations across the Pernicana Fault, Mt Etna, in relation to brittle deformation fields
The Pernicana Fault (PF) is the main structural element of Mt Etna and the northern boundary
of a section sliding to the southeast. Observed ground motion records in the damage zone of
the PF show strong variations of directional resonance in the horizontal plane. The observed
resonance directions exhibit an abrupt rotation of azimuth by about 30â—¦ across the fault,
varying from N166â—¦ on the north side to N139â—¦ on the south. We interpret the directional
resonance observations in terms of changes in the kinematics and deformation fields on the
opposite sides of the fault. The northern side is affected primarily by the left-lateral strike-slip
movement, whereas the southern side, that is subjected also to sliding, is under a dominant
extensional stress regime. Brittle deformation models based on the observed kinematic field
predict different sets of fractures on the opposite sides of the fault: synthetic cleavages and
extensional fractures are expected to dominate in the northern and southern sides, respectively.
These two fracture fields have different orientations (N74â—¦ and N42â—¦, respectively) and both
show a near-orthogonal relation (∼88◦ in the northern sector and ∼83◦ to the south) with the
azimuth of the observed directional resonance. We conclude that the direction of the largest
resonance motions is sensitive to and has transversal relationship with the dominant fracture
orientation. The directional amplification is inferred to be produced by stiffness anisotropy of
the fault damage zone, with larger seismic motions normal to the fractures