## Tesi etd-06272018-193121 |

Tipo di tesi

Tesi di laurea magistrale

Autore

GAVIANO, SONJA

URN

etd-06272018-193121

Titolo

The shallow, shear-wave velocity structure beneath the VIRGO gravitational antenna from
dense-array recordings of regional earthquakes

Struttura

SCIENZE DELLA TERRA

Corso di studi

GEOFISICA DI ESPLORAZIONE E APPLICATA

Commissione

**relatore**Saccorotti, Gilberto

Parole chiave

- Geophysics
- natural seismic sources
- rotation measures
- Rayleigh wave dispersion curve
- 2D tomography
- SPAC
- Beamforming
- Plane Wave Fitting
- shear wave profile

Data inizio appello

13/07/2018;

Consultabilità

parziale

Data di rilascio

13/07/2021

Riassunto analitico

This work aims at investigating the propagation of seismic waves in the vicinity of the VIRGO

gravitational-wave detector (Cascina, Pisa) and to infer a shallow shear-wave velocity structure

from the dispersion characteristics of Rayleigh waves.

For the study, I used earthquake and noise

recordings from a temporary, 50-station array deployed in the vicinity of VIRGO, whose data have

been made available by the EGO (European Gravitational Wave Observatory) consortium. In order to

obtain the general kinematic properties of the earthquakes wavefields, I first applied a

deterministic approach based on the common-waveform model of the signal, namely a frequency-domain

Beam-Forming. This method allow measuring the propagation direction and horizontal slowness of the

incoming signal; it was applied to short time windows sliding along the array seismograms, using

different subarrays whose apertures were selected in order to match different frequency bands.

As expected, the early portions of the earthquake signals are dominated by waves propagating from the

epicentral region; apparent velocities of the P- and S-wave arrivals are around 8km/s and 3km/s,

respectively. For the Rayleigh-wave arrivals, velocities range between 0.5 Km/s and 5 km/s, indicating

interferences of other wavetypes and / or propagation effects (reflections, refractions) associated

with the local geological and topographical complexities. For these same time intervals, the propagation

directions are scattered throughout a wide angular interval. These results suggest therefore that

deterministic methods are not appropriate for estimating Rayleigh waves phase velocity.

Thanks to array density and geophones sensitivity, and assuming that the gradient of the displacement

is constant throughout the array, I then attempted the estimation of the solid-block ground rotations w_t

around an axis parallel to the surface, which is related to Rayleigh-waves phase velocity.

By calculating w_t from properly bandpass filtered seismograms spanning the Rayleigh-wave arrivals from

different subarray configurations, Rayleigh wave phase velocity dispersion data can be obtained.

This analysis requires that the wavelength is about 5 times the array extent along the wave propagation

direction. In order to obtain phase velocity data over a range of frequencies, I therefore extended

the analyses over a set of subarrays of different aperture.

From the scaled average of the displacement – rotation ratios I then obtained phase velocity estimates

which finally allowed deriving a dispersion relationship.

If the wavefield is stochastic and stationary in both space and time, an indipendent Rayleigh wave phase

velocity dispersion curve can also be obtained through the stochastic method SPAC.

The average of the dispersion curves obtained by SPAC is inverted to obtain a shear wave velocity

profile using a perturbational approach.

For each frequency and for each station pair, the ratio between inter-station distance and the

Rayleigh-wave phase velocity provides an estimate of the corresponding traveltime. The final part

of the work consists of the tomographic inversion of these traveltimes in order to obtain 2D Rayleigh

wave phase velocity maps, that highlight ground inhomogeneity of the area at that particular frequency.

Further developments will involve a through assessment of uncertainties in the estimates of Rayleigh

wave phase velocities, in order to better constrain tomography results.

gravitational-wave detector (Cascina, Pisa) and to infer a shallow shear-wave velocity structure

from the dispersion characteristics of Rayleigh waves.

For the study, I used earthquake and noise

recordings from a temporary, 50-station array deployed in the vicinity of VIRGO, whose data have

been made available by the EGO (European Gravitational Wave Observatory) consortium. In order to

obtain the general kinematic properties of the earthquakes wavefields, I first applied a

deterministic approach based on the common-waveform model of the signal, namely a frequency-domain

Beam-Forming. This method allow measuring the propagation direction and horizontal slowness of the

incoming signal; it was applied to short time windows sliding along the array seismograms, using

different subarrays whose apertures were selected in order to match different frequency bands.

As expected, the early portions of the earthquake signals are dominated by waves propagating from the

epicentral region; apparent velocities of the P- and S-wave arrivals are around 8km/s and 3km/s,

respectively. For the Rayleigh-wave arrivals, velocities range between 0.5 Km/s and 5 km/s, indicating

interferences of other wavetypes and / or propagation effects (reflections, refractions) associated

with the local geological and topographical complexities. For these same time intervals, the propagation

directions are scattered throughout a wide angular interval. These results suggest therefore that

deterministic methods are not appropriate for estimating Rayleigh waves phase velocity.

Thanks to array density and geophones sensitivity, and assuming that the gradient of the displacement

is constant throughout the array, I then attempted the estimation of the solid-block ground rotations w_t

around an axis parallel to the surface, which is related to Rayleigh-waves phase velocity.

By calculating w_t from properly bandpass filtered seismograms spanning the Rayleigh-wave arrivals from

different subarray configurations, Rayleigh wave phase velocity dispersion data can be obtained.

This analysis requires that the wavelength is about 5 times the array extent along the wave propagation

direction. In order to obtain phase velocity data over a range of frequencies, I therefore extended

the analyses over a set of subarrays of different aperture.

From the scaled average of the displacement – rotation ratios I then obtained phase velocity estimates

which finally allowed deriving a dispersion relationship.

If the wavefield is stochastic and stationary in both space and time, an indipendent Rayleigh wave phase

velocity dispersion curve can also be obtained through the stochastic method SPAC.

The average of the dispersion curves obtained by SPAC is inverted to obtain a shear wave velocity

profile using a perturbational approach.

For each frequency and for each station pair, the ratio between inter-station distance and the

Rayleigh-wave phase velocity provides an estimate of the corresponding traveltime. The final part

of the work consists of the tomographic inversion of these traveltimes in order to obtain 2D Rayleigh

wave phase velocity maps, that highlight ground inhomogeneity of the area at that particular frequency.

Further developments will involve a through assessment of uncertainties in the estimates of Rayleigh

wave phase velocities, in order to better constrain tomography results.

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