scholarly journals Seismic Attenuation Anisotropy in the Southernmost Part of the Taupo Volcanic Zone, North Island, New Zealand

2022 ◽  
Author(s):  
◽  
Syuhada, Syuhada
Keyword(s):  
Frequency Dependence ◽  
Seismic Attenuation ◽  
Taupo Volcanic Zone ◽  
S Wave ◽  
Volcanic Zone ◽  
S Waves ◽  
Distance Range ◽  
Uncertainty Estimates ◽  
Q Model ◽  
Attenuation Anisotropy

<p>We investigate the mechanisms of seismic anisotropy and attenuation (1/Q) beneath the southernmost part of the Taupo Volcanic Zone (TVZ) by computing variations in S-wave attenuation factors with the direction of wave polarization. We rotate pairs of horizontal components in steps of 22.5◦ from 0◦ to 67.5◦ and into the radial and transverse directions to search for the optimal separation of the attenuation curves and thereby determine an anisotropy symmetry system. The frequency dependence of Q for the rotated S-waves is estimated by means of the non-parametric generalized inversion technique (GIT) of Castro et al. (1990) using shallow earthquakes (< 40 km depth) recorded by GeoNet within 100 km of Mt. Ruapehu. To analyze the effects on computed attenuation properties of source locations, we divide our dataset into two groups: a “TVZ” group containing earthquakes within the TVZ in a distance range of 5–55 km and a “non-TVZ” group containing earthquakes outside the TVZ in a distance range of 5–50 km. To measure Q, we compute the spectral amplitude decay with distance in terms of empirical functions at 20 separate frequencies in the frequency bands 2–10 Hz and 2– 12 Hz for the TVZ and non-TVZ datasets respectively. We construct homogeneous and two-layer Q models for the TVZ dataset based on characteristic features of the attenuation function, while for outside TVZ we only analyse a homogeneous Q model. The homogeneous Q models obtained for the two datasets indicate that S-waves are more attenuated within the TVZ than outside. The homogeneous Q model for the TVZ dataset reveals that the S-wave is anisotropic at high frequencies ( f > 6 Hz) along N–S/E– W directions with the relation QSE ( f ) = (6.15±1.22) f (1.73±0.12) and QSN ( f ) = (4.14± 1.26) f (2.06±0.14), while the non-TVZ dataset shows a weak frequency dependence of attenuation anisotropy at low frequencies in NE–SW/SE–NW directions giving the power law function QSNE ( f ) = (50.93±1.18) f (0.20±0.10) and QSSE ( f ) = (22.60±1.10) f (0.53±0.06). Here, the uncertainty estimates are 95% confidence intervals. To investigate the variation of attenuation anisotropy with depth within the TVZ, we first calculate Q along propagation paths (< 25 km, which corresponds to a maximum turning point depth of 9 km ) and then using paths of 25–55 km length. Small attenuation anisotropy with low attenuation in the N–S direction for the upper crust of TVZ may be related to heterogenous structure as reported by previous studies. Attenuation anisotropy in the northwest direction yielding lower attenuation inferred for the deeper crust suggests the presence of connected melt aligned with the extension direction of TVZ .</p>

scholarly journals Seismic Attenuation Anisotropy in the Southernmost Part of the Taupo Volcanic Zone, North Island, New Zealand

2022 ◽  
Author(s):  
◽  
Syuhada, Syuhada
Keyword(s):  
Frequency Dependence ◽  
Seismic Attenuation ◽  
Taupo Volcanic Zone ◽  
S Wave ◽  
Volcanic Zone ◽  
S Waves ◽  
Distance Range ◽  
Uncertainty Estimates ◽  
Q Model ◽  
Attenuation Anisotropy

<p>We investigate the mechanisms of seismic anisotropy and attenuation (1/Q) beneath the southernmost part of the Taupo Volcanic Zone (TVZ) by computing variations in S-wave attenuation factors with the direction of wave polarization. We rotate pairs of horizontal components in steps of 22.5◦ from 0◦ to 67.5◦ and into the radial and transverse directions to search for the optimal separation of the attenuation curves and thereby determine an anisotropy symmetry system. The frequency dependence of Q for the rotated S-waves is estimated by means of the non-parametric generalized inversion technique (GIT) of Castro et al. (1990) using shallow earthquakes (< 40 km depth) recorded by GeoNet within 100 km of Mt. Ruapehu. To analyze the effects on computed attenuation properties of source locations, we divide our dataset into two groups: a “TVZ” group containing earthquakes within the TVZ in a distance range of 5–55 km and a “non-TVZ” group containing earthquakes outside the TVZ in a distance range of 5–50 km. To measure Q, we compute the spectral amplitude decay with distance in terms of empirical functions at 20 separate frequencies in the frequency bands 2–10 Hz and 2– 12 Hz for the TVZ and non-TVZ datasets respectively. We construct homogeneous and two-layer Q models for the TVZ dataset based on characteristic features of the attenuation function, while for outside TVZ we only analyse a homogeneous Q model. The homogeneous Q models obtained for the two datasets indicate that S-waves are more attenuated within the TVZ than outside. The homogeneous Q model for the TVZ dataset reveals that the S-wave is anisotropic at high frequencies ( f > 6 Hz) along N–S/E– W directions with the relation QSE ( f ) = (6.15±1.22) f (1.73±0.12) and QSN ( f ) = (4.14± 1.26) f (2.06±0.14), while the non-TVZ dataset shows a weak frequency dependence of attenuation anisotropy at low frequencies in NE–SW/SE–NW directions giving the power law function QSNE ( f ) = (50.93±1.18) f (0.20±0.10) and QSSE ( f ) = (22.60±1.10) f (0.53±0.06). Here, the uncertainty estimates are 95% confidence intervals. To investigate the variation of attenuation anisotropy with depth within the TVZ, we first calculate Q along propagation paths (< 25 km, which corresponds to a maximum turning point depth of 9 km ) and then using paths of 25–55 km length. Small attenuation anisotropy with low attenuation in the N–S direction for the upper crust of TVZ may be related to heterogenous structure as reported by previous studies. Attenuation anisotropy in the northwest direction yielding lower attenuation inferred for the deeper crust suggests the presence of connected melt aligned with the extension direction of TVZ .</p>

Estimates of Q in central Asia as a function of frequency and depth using the coda of locally recorded earthquakes

1982 ◽  
Vol 72 (1) ◽  
pp. 129-149
Author(s):  
S. W. Roecker ◽  
B. Tucker ◽  
J. King ◽  
D. Hatzfeld
Keyword(s):  
Frequency Dependence ◽  
Central Asia ◽  
Wide Frequency Range ◽  
S Wave ◽  
Uppermost Mantle ◽  
Regional Heterogeneity ◽  
S Waves ◽  
Tectonic Processes ◽  
Hindu Kush ◽  
Attenuation Mechanism

abstract Digital recordings of microearthquake codas from shallow and intermediate depth earthquakes in the Hindu Kush region of Afghanistan were used to determine the attenuation factors of the S-wave coda (Qc) and primary S waves (Qβ). An anomalously rapid decay of the coda shortly after the S-wave arrival, observed also in a study of coda in central Asia by Rautian and Khalturin (1978), seems to be due primarily to depth-dependent variations in Qc. In particular, we deduce the average Qc in the crust and uppermost mantle (&lt;100-km depth) is approximately four times lower than the deeper mantle (&lt;400-km depth) over a wide frequency range (0.4 to 24 Hz). Further, while Qc generally increases with frequency at any depth, the degree of frequency dependence of Qc depends on depth. Except at the highest frequency studied here (∼48 Hz), the magnitude of Qc at a particular frequency increases with depth while its frequency dependence decreases. For similar depths, determinations of Qβ and Qc agree, suggesting a common wave composition and attenuation mechanism for S waves and codas. Comparison of these determinations of Qc in Afghanistan with those in other parts of the world shows that the degree of frequency dependence of Qc correlates with the expected regional heterogeneity. Such a correlation supports the prejudice that Qc is primarily influenced by scattering and suggests that tectonic processes such as folding and faulting are instrumental in creating scattering environments.

Effect of fluids and frequencies on Poisson’s ratio of sandstone samples

Geophysics ◽  
2016 ◽  
Vol 81 (2) ◽  
pp. D183-D195 ◽  
Author(s):  
Lucas Pimienta ◽  
Jérôme Fortin ◽  
Yves Guéguen
Keyword(s):  
Frequency Dependence ◽  
Poisson’S Ratio ◽  
Poisson's Ratio ◽  
Fluid Viscosity ◽  
S Wave ◽  
Standard Samples ◽  
Frequency Dependent ◽  
S Waves ◽  
Fontainebleau Sandstone ◽  
Dry Conditions

Poisson’s ratio [Formula: see text] is an important parameter when interpreting measured geophysical and seismic data. For an isotropic medium, it directly relates to the ratio of P- and S-wave velocities. We have measured [Formula: see text] as a function of pressure and frequency in fluid-saturated sandstones. The method of measuring [Formula: see text] was first tested as a function of pressure and frequency using standard samples. The phase shift [Formula: see text] between radial and axial strains was also measured. For all standard samples, such as the linear viscoelastic Plexiglas, the data indicated that [Formula: see text] correlated with [Formula: see text] and related to a dissipation on [Formula: see text]. Then, [Formula: see text] and [Formula: see text] were measured as a function of pressure and frequency for two dry and fluid-saturated Fontainebleau sandstone samples. Under dry conditions, no frequency dependence and very small pressure dependence were observed. Unusual behaviors were observed under fluid-saturated conditions. In particular, [Formula: see text] of one sample indicated a frequency-dependent bell-shaped dispersion under water and glycerin saturation that correlated with peaks in [Formula: see text]. Plotting the measurements as a function of apparent frequency (i.e., normalizing by the fluid viscosity) indicated a good fit between the water- and glycerin-saturated measurements. The bell-shaped dispersion in [Formula: see text] that was observed for one particular sandstone held for all effective pressures. These variations fully correlated with the peaks of [Formula: see text] observed. Our results can be interpreted using fluid flow and effective medium theories in the case of a porous microcracked rock. Drained/undrained and relaxed/unrelaxed transitions have frequency and magnitude of variations that are consistent with the measurements. The rock sample microcrack density strongly affects this frequency dependence. The inferred [Formula: see text] ratio at low effective pressures also indicates a large frequency-dependent bell-shaped dispersion. The parameter [Formula: see text] is a clear indicator of the frequency-dependent dissipation of [Formula: see text] and relates to the attenuation of P- and S-waves.

FAVORABLE STRUCTURAL SETTINGS FOR POTENTIAL GEOTHERMAL UPWELLINGS IN THE CENTRAL TAUPO VOLCANIC ZONE, NEW ZEALAND

2018 ◽  
Author(s):  
Natalie E. Wigger ◽  
◽  
James E. Faulds ◽  
Samuel J. Hampton ◽  
Josh W. Borella ◽  
...  
Keyword(s):  
New Zealand ◽  
Taupo Volcanic Zone ◽  
Volcanic Zone

scholarly journals Responses of dipole-source reflected shear waves in acoustically slow formations

2019 ◽  
Author(s):  
Hao Wang ◽  
Ning Li ◽  
Caizhi Wang ◽  
Hongliang Wu ◽  
Peng Liu ◽  
...  
Keyword(s):  
Finite Difference Time Domain ◽  
Dipole Source ◽  
Sh Wave ◽  
S Wave ◽  
High Fracture ◽  
S Waves ◽  
Angle Fracture ◽  
Dip Angle ◽  
Difference Time

Abstract In the process of dipole-source acoustic far-detection logging, the azimuth of the fracture outside the borehole can be determined with the assumption that the SH–SH wave is stronger than the SV–SV wave. However, in slow formations, the considerable borehole modulation highly complicates the dipole-source radiation of SH and SV waves. A 3D finite-difference time-domain method is used to investigate the responses of the dipole-source reflected shear wave (S–S) in slow formations and explain the relationships between the azimuth characteristics of the S–S wave and the source–receiver offset and the dip angle of the fracture outside the borehole. Results indicate that the SH–SH and SV–SV waves cannot be effectively distinguished by amplitude at some offset ranges under low- and high-fracture dip angle conditions, and the offset ranges are related to formation properties and fracture dip angle. In these cases, the fracture azimuth determined by the amplitude of the S–S wave not only has a $180^\circ $ uncertainty but may also have a $90^\circ $ difference from the actual value. Under these situations, the P–P, S–P and S–S waves can be combined to solve the problem of the $90^\circ $ difference in the azimuth determination of fractures outside the borehole, especially for a low-dip-angle fracture.

scholarly journals Physical property relationships of the Rotokawa Andesite, a significant geothermal reservoir rock in the Taupo Volcanic Zone, New Zealand

2014 ◽  
Vol 2 (1) ◽  
Author(s):  
Paul A Siratovich ◽  
Michael J Heap ◽  
Marlène C Villenueve ◽  
James W Cole ◽  
Thierry Reuschlé
Keyword(s):  
New Zealand ◽  
Physical Property ◽  
Geothermal Reservoir ◽  
Reservoir Rock ◽  
Taupo Volcanic Zone ◽  
Volcanic Zone
Sign in / Sign up

Export Citation Format

Share Document