scholarly journals Methods and Computational Techniques for Investigating and Monitoring Seismic Velocities in the Earth's Crust

2021 ◽  
Author(s):  
◽  
Adrian Shelley
Keyword(s):  
Shear Wave ◽  
Cross Correlation ◽  
Seismic Velocity ◽  
Crack Density ◽  
Shear Wave Splitting ◽  
Noise Interferometry ◽  
Stress Changes ◽  
Wave Splitting ◽  
Velocity Variations ◽  
Velocity Changes

<p>This thesis is concerned with scrutinising the source, distribution and detectability of seismic velocity phenomena that may be used as proxies to study conditions in the crust. Specifically, we develop modelling techniques in order to analyse the directional variation of seismic wave speed in the crust and test them at Mt. Asama in Japan and Canterbury, New Zealand. We also implement both active source and noise interferometry to identify velocity variations at Mt. Ruapehu, New Zealand.  Observations of temporal variation of anisotropic seismic velocity parameters at Asama volcano in Japan indicate that there is some process (or processes) affecting anisotropy, attributed to closure of microcracks in the rock as it is subjected to volcanic stress in the crust. To test this assertion, a 3D numerical model is created incorporating volcanic stress, ray tracing and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model. Anisotropy is calculated in two ways; by considering a basic scenario where crack density is uniform and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress. We find that the approach is sensitive to crack density, crack compliance, and the regional stress field. In the case of dry crack closure, modelled stress conditions produce a much smaller degree of anisotropy than indicated by measurements. We propose that the source of anisotropy changes at Asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes, such as the movement of pore fluid.  We develop a generalised anisotropy inversion model based on the linearised, iterative least-squares inversion technique of Abt and Fischer [2008]. The model is streamlined for use with results from the MFAST automatic shear wave splitting software [Savage et al., 2010]. The method iteratively solves for the best fitting magnitude and orientation of anisotropy in each element of the model space using numerically calculated partial derivatives. The inversion is applied to the Canterbury plains in the region surrounding the Greendale fault, using shear-wave splitting data from the 2010 Darfield earthquake sequence. Crustal anisotropy is resolved down to a depth of 20 km at a spatial resolution of 5 km, with good resolution near the Greendale fault. We identify a lateral variation in anisotropy strength across the Greendale fault, possibly associated with post-seismic stress changes.  We perform active source and noise interferometry at Ruapehu in order to investigate potential seismic velocity changes and assess their use as a possible eruption forecasting method. Six co-located 100 kg ammonium nitrate fuel oil explosives were set off serially at Lake Moawhango, situated approximately 20 km south-east of Mount Ruapehu. Two methods of interferometry, using moving window cross correlation in the time and frequency domains, respectively, were applied to the recorded signal from each explosion pair in order to determine velocity changes from the signal coda waves. We identify possible diurnal velocity variations of ~ 0:7% associated with strain caused by the solid Earth tide. Synthetic testing of velocity variation recoverability was also performed using both methods. Interferometry of noise cross-correlations during the period was also performed using moving window cross correlation in the frequency domain. Analysis of velocity variations in the ZZ, RR and TT component pairs show little coherency. This, combined with results from synthetic testing that show that the frequency domain interferometry technique employed is unstable above velocity variations of 0.1%, indicate that the method may not be suitable for determining velocity variations at Ruapehu.</p>

scholarly journals Methods and Computational Techniques for Investigating and Monitoring Seismic Velocities in the Earth's Crust

2021 ◽  
Author(s):  
◽  
Adrian Shelley
Keyword(s):  
Shear Wave ◽  
Cross Correlation ◽  
Seismic Velocity ◽  
Crack Density ◽  
Shear Wave Splitting ◽  
Noise Interferometry ◽  
Stress Changes ◽  
Wave Splitting ◽  
Velocity Variations ◽  
Velocity Changes

<p>This thesis is concerned with scrutinising the source, distribution and detectability of seismic velocity phenomena that may be used as proxies to study conditions in the crust. Specifically, we develop modelling techniques in order to analyse the directional variation of seismic wave speed in the crust and test them at Mt. Asama in Japan and Canterbury, New Zealand. We also implement both active source and noise interferometry to identify velocity variations at Mt. Ruapehu, New Zealand.  Observations of temporal variation of anisotropic seismic velocity parameters at Asama volcano in Japan indicate that there is some process (or processes) affecting anisotropy, attributed to closure of microcracks in the rock as it is subjected to volcanic stress in the crust. To test this assertion, a 3D numerical model is created incorporating volcanic stress, ray tracing and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model. Anisotropy is calculated in two ways; by considering a basic scenario where crack density is uniform and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress. We find that the approach is sensitive to crack density, crack compliance, and the regional stress field. In the case of dry crack closure, modelled stress conditions produce a much smaller degree of anisotropy than indicated by measurements. We propose that the source of anisotropy changes at Asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes, such as the movement of pore fluid.  We develop a generalised anisotropy inversion model based on the linearised, iterative least-squares inversion technique of Abt and Fischer [2008]. The model is streamlined for use with results from the MFAST automatic shear wave splitting software [Savage et al., 2010]. The method iteratively solves for the best fitting magnitude and orientation of anisotropy in each element of the model space using numerically calculated partial derivatives. The inversion is applied to the Canterbury plains in the region surrounding the Greendale fault, using shear-wave splitting data from the 2010 Darfield earthquake sequence. Crustal anisotropy is resolved down to a depth of 20 km at a spatial resolution of 5 km, with good resolution near the Greendale fault. We identify a lateral variation in anisotropy strength across the Greendale fault, possibly associated with post-seismic stress changes.  We perform active source and noise interferometry at Ruapehu in order to investigate potential seismic velocity changes and assess their use as a possible eruption forecasting method. Six co-located 100 kg ammonium nitrate fuel oil explosives were set off serially at Lake Moawhango, situated approximately 20 km south-east of Mount Ruapehu. Two methods of interferometry, using moving window cross correlation in the time and frequency domains, respectively, were applied to the recorded signal from each explosion pair in order to determine velocity changes from the signal coda waves. We identify possible diurnal velocity variations of ~ 0:7% associated with strain caused by the solid Earth tide. Synthetic testing of velocity variation recoverability was also performed using both methods. Interferometry of noise cross-correlations during the period was also performed using moving window cross correlation in the frequency domain. Analysis of velocity variations in the ZZ, RR and TT component pairs show little coherency. This, combined with results from synthetic testing that show that the frequency domain interferometry technique employed is unstable above velocity variations of 0.1%, indicate that the method may not be suitable for determining velocity variations at Ruapehu.</p>

Ultrasonic velocity and shear‐wave splitting behavior of a Colton sandstone under a changing triaxial stress

Geophysics ◽  
1999 ◽  
Vol 64 (5) ◽  
pp. 1603-1607 ◽  
Author(s):  
Menno W. P. Dillen ◽  
Helma M. A. Cruts ◽  
Jeroen Groenenboom ◽  
Jacob T. Fokkema ◽  
Adri J. W. Duijndam
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Wave Mode ◽  
Ultrasonic Velocities ◽  
Compressional Waves ◽  
Stress Changes ◽  
Wave Splitting ◽  
Stress Directions ◽  
Velocity Changes ◽  
Induced Changes

Ultrasonic experiments on a dry Colton sandstone placed in a triaxial pressure machine show that effective stress changes lead to distinct anisotropic velocity changes in compressional waves and shear waves. The stress imprint can be recognized from the associated velocity pattern by relating the velocities to the three normal stress directions. The ultrasonic velocities indicate that the sensitivity of the different waves to stress predominantly depends on stresses applied in the polarization and propagation directions of the particular wave mode. Also, stress‐induced changes in shear‐wave splitting are observed.

scholarly journals Seismic velocity changes at White Island volcano, New Zealand, using ten years of ambient noise interferometry

2021 ◽  
Author(s):  
◽  
Alexander Yates
Keyword(s):  
Ambient Noise ◽  
Seismic Velocity ◽  
Environmental Influence ◽  
Seismic Stations ◽  
Noise Interferometry ◽  
Stress Changes ◽  
Volcanic Processes ◽  
Seismic Velocity Changes ◽  
Velocity Changes ◽  
White Island

<p>Seismic velocity changes at volcanoes carry information about stresses present within hydrothermal and magmatic systems. In this thesis, temporal velocity changes are measured at White Island volcano using ambient noise interferometry between 2007–2017. This period contains multiple well-documented eruptions starting in 2012, following an inactive period that extends back over a decade. Three primary objectives are identified: (1) investigate what seismic velocity changes can tell us about dynamic changes beneath the volcano, (2) investigate non-volcanic sources and their possible influence on interpretations, and (3) consider the potential for real-time monitoring using ambient-noise. These objectives extend beyond White Island volcano, with implications for ambient noise monitoring of volcanoes globally.  Two different approaches are used to measure velocity changes at White Island. The first involves cross-correlating noise recorded by pairs of seismic stations. Velocity changes are sought by averaging changes recorded across ten station-pairs that consist of an onshore station and a station on the volcano. The second approach involves cross-correlating the different components of individual seismic stations. This represents a less traditional approach to monitoring volcanoes, but is well-suited to White Island which has one permanent station active throughout eruptive activity. Single seismic stations located onshore are also processed to investigate background regional changes.  Two periods of long-term velocity increases are detected at the volcano. The first occurs during a highly active period in 2012–2013 and the second occurs in the months preceding an explosive eruption in April 2016. Comparison with velocities recorded by onshore stations suggest a meteorological source for these changes is unlikely. Velocity increases are therefore interpreted to reflect cracks closing under increased pressures beneath the volcano. Similarly, a rapid decline in the velocity within 2–3 months of the April 2016 eruption is interpreted to reflect depressurization of the system.  In addition to volcanic sources, we also find clear evidence of non-volcanic processes influencing velocity changes at the volcano. Two clear co-seismic velocity decreases of approximately 0.05–0.1% are associated with a Mw 5.2 earthquake in 2008 — within 10 km of the volcano — and the Mw 7.1 East Cape earthquake in 2016. The East Cape earthquake — located 200 km away from the volcano — produces significant velocity decreases over a large region, as detected by stations onshore and on White Island. This likely reflects dynamic stress changes as a result of passing seismic waves, with an eruption two weeks later interpreted here to have been triggered by this event. Finally, we identify similarities between annual variations recorded by onshore stations and changes at the volcano, suggesting an environmental influence. Velocity changes at White Island therefore represent a complex interaction of volcanic and non-volcanic processes, highlighting the need for improved understanding of external sources of change to accurately detect short-term eruptive precursors.</p>

Erratum to Upper-Crustal Seismic Anisotropy in the Cantabrian Mountains (North Spain) from Shear-Wave Splitting and Ambient Noise Interferometry Analysis

2020 ◽  
Vol 92 (1) ◽  
pp. 613-613
Author(s):  
Jorge Acevedo ◽  
Gabriela Fernández-Viejo ◽  
Sergio Llana-Fúnez ◽  
Carlos López-Fernández ◽  
Javier Olona
Keyword(s):  
Shear Wave ◽  
Ambient Noise ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Cantabrian Mountains ◽  
Noise Interferometry ◽  
Wave Splitting

3-D seismic modeling for cracked media: Shear‐wave splitting at zero‐offset

Geophysics ◽  
2000 ◽  
Vol 65 (1) ◽  
pp. 211-221 ◽  
Author(s):  
Jaime Ramos‐Martínez ◽  
Andrey A. Ortega ◽  
George A. McMechan
Keyword(s):  
Shear Wave ◽  
Effective Properties ◽  
Transverse Isotropy ◽  
Shear Waves ◽  
Crack Density ◽  
Shear Wave Splitting ◽  
Carbonate Reservoir ◽  
Vertical Crack ◽  
Wave Splitting ◽  
Zero Offset

Splitting of zero‐offset reflected shear‐waves is measured directly from three‐component finite‐difference synthetic seismograms for media with intersecting vertical crack systems. Splitting is simulated numerically (by finite differencing) as a function of crack density, aspect ratio, fluid content, bulk density, and the angle between the crack systems. The type of anisotropy symmetry in media containing two intersecting vertical crack systems depends on the angular relation between the cracks and their relative crack densities, and it may be horizontal transverse isotropy (HTI), tetragonal, orthorhombic, or monoclinic. The transition from one symmetry to another is visible in the splitting behavior. The polarities of the reflected quasi‐shear waves polarized perpendicular and parallel to the source particle motion distinguish between HTI and orthorhombic media. The dependence of the measured amount of splitting on crack density for HTI symmetry is consistent with that predicted theoretically by the shear‐wave splitting factor. In orthorhombic media (with two orthogonal crack systems), a linear increase is observed in splitting when the difference between crack densities of the two orthogonal crack systems increases. Splitting decreases nonlinearly with the intersection angle between the two crack systems from 0° to 90°. Surface and VSP seismograms are simulated for a model with several flat homogeneous layers, each containing vertical cracks with the same and with different orientations. When the crack orientation varies with depth, previously split shear waves are split again at each interface, leading to complicated records, even for simple models. Isotropic and anisotropic three‐component S-wave zero‐offset sections are synthesized for a zero‐offset survey line over a 2.5-D model of a carbonate reservoir with a complicated geometry and two intersecting, dipping crack sets. The polarization direction of the fast shear wave, propagating obliquely through the cracked reservoir, is predicted by theoretical approximations for effective properties of anisotropic media with two nonorthogonal intersecting crack sets.

Crustal anisotropy from shear-wave splitting of local earthquakes in the Garhwal Lesser Himalaya

2019 ◽  
Vol 219 (3) ◽  
pp. 2013-2033
Author(s):  
Jyotima Kanaujia ◽  
Supriyo Mitra ◽  
S C Gupta ◽  
M L Sharma
Keyword(s):  
Shear Wave ◽  
Crack Density ◽  
Shear Wave Splitting ◽  
Homogeneous Distribution ◽  
Lesser Himalaya ◽  
Crustal Anisotropy ◽  
Axis Orientation ◽  
Composite Effect ◽  
Wave Splitting ◽  
The Mean

SUMMARY Crustal anisotropy of the Garhwal Lesser Himalaya has been studied using local earthquake data from the Tehri seismic network. Earthquakes with magnitude (mL) up to 3, which occurred between January 2008 to December 2010, have been used for the shear wave splitting (SWS) analysis. SWS measurements have been done for steeply incident ray paths (ic ≤ 45°) to estimate the anisotropy fast axis orientation (ϕ) and the delay time (∂t). A total of 241 waveforms have been analysed, which yielded 209 splitting measurements, and 32 null results. The analysis reveals spatial and depth variation of ϕ and ∂t, suggesting complex anisotropic structure beneath the Garhwal Lesser Himalaya. The mean ∂t is estimated to be 0.07 ± 0.065 s with a mean depth normalized ∂t of 0.005 s km–1. We present the ϕ and Vs per cent anisotropy results by segregating these as a function of depth, for earthquakes originating above and below the Main Himalayan Thrust (MHT); and spatially, for stations located in the Outer Lesser Himalaya (OLH) and the Inner Lesser Himalaya (ILH). Earthquakes above the MHT sample only the Himalayan wedge, while those below the MHT sample both the underthrust Indian crust and the Himalayan wedge. Within the Himalayan wedge, for both OLH and ILH, the mean ϕ is oriented NE–SW, in the direction of maximum horizontal compressive stress axis (SHmax). This anisotropy is possibly due to stress-aligned microcracks controlled by the local stress pattern within the Himalayan wedge. The mean of normalized ∂t for all events originating within the Himalaya is 0.006 s km–1, which yields a Vs per cent anisotropy of ∼2.28 per cent. Assuming a homogeneous distribution of stress-aligned microcracks we compute a crack density of ∼0.0228 for the Garhwal Lesser Himalaya. At stations close to the regional fault systems, the mean ϕ is subparallel to the strike of the faults, and the anisotropy, locally, appears to be structure-related. For earthquakes originating below the MHT, in OLH, the mean ϕ orientation matches those from the Himalayan wedge and the normalized ∂t decreases with depth. This suggests depth localization of the anisotropy, primarily present within the Himalayan wedge. In the ILH, we observe large variations in the mean ϕ orientation and larger values of ∂t close to the regional fault/thrust systems. This is possibly a composite effect of the structure-related shallow crustal anisotropy and the frozen anisotropy of the underthrusting Indian crust. However, these cannot be segregated in this study.

Upper-Crustal Seismic Anisotropy in the Cantabrian Mountains (North Spain) from Shear-Wave Splitting and Ambient Noise Interferometry Analysis

2020 ◽  
Vol 92 (1) ◽  
pp. 421-436 ◽  
Author(s):  
Jorge Acevedo ◽  
Gabriela Fernández-Viejo ◽  
Sergio Llana-Fúnez ◽  
Carlos López-Fernández ◽  
Javier Olona
Keyword(s):  
Shear Wave ◽  
Ambient Noise ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Clear Correlation ◽  
Cantabrian Mountains ◽  
Noise Interferometry ◽  
Delay Times ◽  
Wave Splitting ◽  
Fast Polarization

Abstract The upper-crustal anisotropy of the Cantabrian Mountains (North Spain) has been investigated using two independent but complementary methodologies: (a) shear-wave splitting and (b) ambient seismic noise interferometry. For this purpose, we have processed and compared seismic data from two networks with different scales and recording periods. The shear-wave splitting results show delay times between 0.06 and 0.23 s and spatially variable fast-polarization directions. We calculate that the anisotropic layer has a maximum effective thickness of around 7.5 km and an average anisotropy magnitude of between 4% and 8%. Consistently, our ambient noise observations point to an anisotropy magnitude between 4% and 9% in the first 10 km of the crust. Our results show a clear correlation between the fast directions from both methods and the orientations of the local faults, suggesting that the anisotropy is mainly controlled by the structures. Furthermore, in the west of the study area, fast-polarization directions tend to align parallel to the Variscan fabric in the crust, whereas to the east, in which the Alpine imprint is stronger, many fast directions are aligned parallel to east–west-oriented Alpine features.

scholarly journals Determination of Shear Wave Splitting Parameters in 2018 Lombok Earthquake Using Rotation Correlation Method: Preliminary Result

2021 ◽  
Vol 873 (1) ◽  
pp. 012101
Author(s):  
Annisa Trisnia Sasmi ◽  
Andri Dian Nugraha ◽  
Muzli Muzli ◽  
Sri Widiyantoro ◽  
Zulfakriza Zulfakriza ◽  
...  
Keyword(s):  
Shear Wave ◽  
Cross Correlation ◽  
Arrival Time ◽  
Seismic Anisotropy ◽  
Rotation Angle ◽  
Correlation Method ◽  
Shear Wave Splitting ◽  
Waveform Data ◽  
Wave Splitting

Abstract Shear-wave splitting (SWS), or the propagation of two independent shear waves, can be used as an indicator of seismic anisotropy. In this study, we utilize this concept using aftershock data of the 2018 Lombok earthquake which had been acquired in period of August 4 – September 9, 2018. The goal of this research is to better understand the crack distribution related to the rupture zone of the 2018 Lombok earthquake. After applying instrument correction to the data, the waveform data were then windowed in each P and S arrival time. To determine the SWS parameters, we performed rotation in each horizontal seismogram components. The horizontal components were rotated from azimuth 0° to 180° with an increment of 1°. Cross-correlation coefficient (CCC) was determined for each rotation angle. The polarization direction and the SWS delay time were chosen from the parameters shown in the highest value of CCC.

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