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  • 1
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    Making Reliable Shear-Wave Splitting Measurements (2013)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2013-10-01
    Description: Shear-wave splitting (SWS) analysis using SKS , SKKS , and PKS (hereafter collectively called XKS ) phases is one of the most commonly used techniques in structural seismology. In spite of the apparent simplicity in performing SWS measurements, large discrepancies in published SWS parameters (fast direction and splitting time) suggest that a significant portion of splitting parameters has been incorrectly determined. Here, based on the popularly used minimization of transverse energy technique, we present a procedure that combines automatic data processing and careful manual screening, which includes adjusting the XKS window used for splitting analysis, modifying band-pass filtering corner frequencies, and verifying and (if necessary) changing the quality ranking of the measurements. Using real and synthetic data, we discuss causes and diagnostics of a number of common problems in performing SWS analysis, and suggest possible remedies. Those problems include noise in the XKS window being mistaken as signal, non- XKS seismic arrivals in the XKS window, excessive use of null ranking, measurements from misoriented sensors and from sensors with mechanical problems, and inappropriate dismissal of usable measurements.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 2
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    Delineating Shallow S-Wave Velocity Structure Using Multiple Ambient-Noise Surface-Wave Methods: An Example from Western Junggar, China (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-04-07
    Description: Aiming for a geophysical survey to around 1.5 km depth, we conduct a passive-source surface-wave survey along a 70-km-long line on the western side of the Junggar basin in northwest China. We use frequency–time analysis and spatial autocorrelation methods to extract surface-wave dispersion curves from ambient-noise data of station pairs, as well as ambient-noise multichannel analysis of surface waves from multistations. Dispersion curves corresponding to the same positions obtained by these three methods independently are comparable, which shows the validity of using these methods. After inverting more than 400 dispersion curves and aligning their results together, we construct a 2D shear S -wave velocity image around 1.5 km deep and 60 km long. A less than 6% difference between the inverted S -wave velocity profile and the borehole data in situ shows the relatively high accuracy of our method. The 2D S -wave velocity image clearly delineates the main geologic structures, including transition of landform, intrusion of rock mass, and shape of bedrock beneath the survey line.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 3
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    A Uniform Database of Teleseismic Shear-Wave Splitting Measurements for the Western and Central United States: December 2014 Update (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-02-25
    Description: We present a new version of a shear-wave splitting (SWS) database for the western and central United States (WCUS) using broadband seismic data recorded up to the end of 2014 to update a previous version that used data recorded prior to the end of 2012, when the USArray Transportable Array stations were still recording in the easternmost region of the WCUS. A total of 7452 pairs of additional measurements recorded by 1202 digital broadband seismic stations are obtained, and all the measurements in the previous database are rechecked. The resulting uniform SWS database contains a total of 23,448 pairs of well-defined SKS , SKKS , and PKS splitting parameters. Relative to the previous version of the database, the additional measurements notably improved the spatial and azimuthal coverages of the measurements, providing an improved dataset for constraining geodynamic models related to lithospheric deformation and asthenospheric flow, as well as for complex anisotropy recognition and characterization.
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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  • 4
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    Estimation of the Depth of Anisotropy Using Spatial Coherency of Shear-Wave Splitting Parameters (2011)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2011-10-01
    Description: Shear-wave splitting (SWS) analyses are essential in understanding the structure and dynamics of the Earth's deep interior. While splitting measurements have excellent horizontal resolution relative to other anisotropy-measuring techniques, their vertical resolution is low due to the steep incidence angle of the seismic phases used by the analyses. Here, using synthetic and real data, we present and test a simple approach to estimate the optimal depth of anisotropy by measuring the spatial coherency of the splitting parameters. The approach searches for the optimal depth by computing a spatial variation factor. Tests using synthetic SWS data produced with varying number of events, number of stations, and levels of noise suggest that the approach can satisfactorily find the depth of the source of anisotropy. Successful application of the depth-estimation procedure requires well-defined splitting parameters obtained from a multistation network and multiple events from a decent back-azimuthal range. It also requires significant and smooth spatial variations of anisotropy with horizontal axis of symmetry within a single layer of anisotropy. We applied the approach to 448 pairs of splitting measurements obtained at about 50 stations on the Ethiopian Plateau and found an optimal depth of anisotropy of about 300 km, suggesting an asthenospheric origin of the observed anisotropy.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 5
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    Delineating Shallow S-Wave Velocity Structure Using Multiple Ambient-Noise Surface-Wave Methods: An Example from Western Junggar, China [Article] (2016)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2016-04-15
    Description: Aiming for a geophysical survey to around 1.5 km depth, we conduct a passive-source surface-wave survey along a 70-km-long line on the western side of the Junggar basin in northwest China. We use frequency–time analysis and spatial autocorrelation methods to extract surface-wave dispersion curves from ambient-noise data of station pairs, as well as ambient-noise multichannel analysis of surface waves from multistations. Dispersion curves corresponding to the same positions obtained by these three methods independently are comparable, which shows the validity of using these methods. After inverting more than 400 dispersion curves and aligning their results together, we construct a 2D shear S -wave velocity image around 1.5 km deep and 60 km long. A less than 6% difference between the inverted S -wave velocity profile and the borehole data in situ shows the relatively high accuracy of our method. The 2D S -wave velocity image clearly delineates the main geologic structures, including transition of landform, intrusion of rock mass, and shape of bedrock beneath the survey line.
    Print ISSN: 0037-1106
    Electronic ISSN: 1943-3573
    Topics: Geosciences , Physics
    Published by Seismological Society of America (SSA)
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  • 6
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    Preface to the Focus Section on Cascadia Initiative Preliminary Results (2015)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2015-09-11
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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  • 7
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    A Preliminary Full-Wave Ambient-Noise Tomography Model Spanning from the Juan de Fuca and Gorda Spreading Centers to the Cascadia Volcanic Arc (2015)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2015-09-11
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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  • 8
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    Quality Analysis of High‐Frequency Air‐Gun Shot Seismic Recordings in the Juan de Fuca Plate (2018)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2018
    Description: 〈span〉〈div〉ABSTRACT〈/div〉The goal of this study is to extract high‐frequency seismic waveforms recorded by both offshore and onshore instrumentations and to quantitatively evaluate the data quality in terms of source‐to‐receiver distance, source parameters, water depth, and sediment thickness. There were two active‐source seismic experiments within the Juan de Fuca plate, MGL1211 and MGL1212, during the first year deployment of the Cascadia Initiative Amphibious Array. In total, we chose 114 ocean‐bottom seismometers and 211 inland stations located around the experiment region. The common receiver gathering analysis shows that most offshore stations located in deep‐water record clear first 〈span〉P〈/span〉‐wave arrivals up to 150 km away from the air‐gun shot. The first arrival transits from crustal phase 〈span〉Pg〈/span〉 to head wave 〈span〉Pn〈/span〉 at a source‐to‐receiver distance of 25–40 km. For stations located at shallow water, the seismic recordings appear much noisier compared with those of deep‐water stations. Only five inland stations record clear air‐gun shot signals up to 200 km away from the source. The signal‐to‐noise ratio of the first 〈span〉P〈/span〉 arrivals generally decreases from a few hundred down to a single number with increasing source‐to‐receiver distance, from deep (∼3000  m) to shallow water (∼100  m), with increasing sediment thickness and decreasing air‐gun shot spacing and time interval. On average, the ratio appears 5–10 times lower at inland stations than at offshore stations. The sediment thickness estimated in this study demonstrates significant along‐strike variations, which is up to 5 km offshore Oregon and 3.5 km offshore Washington. The coincidence of thick sediments with seismicity suggests the importance of sediments on the coupling of the plate interface.〈/span〉
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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  • 9
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    Quality Analysis of High‐Frequency Air‐Gun Shot Seismic Recordings in the Juan de Fuca Plate (2018)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2018
    Description: 〈span〉〈div〉ABSTRACT〈/div〉The goal of this study is to extract high‐frequency seismic waveforms recorded by both offshore and onshore instrumentations and to quantitatively evaluate the data quality in terms of source‐to‐receiver distance, source parameters, water depth, and sediment thickness. There were two active‐source seismic experiments within the Juan de Fuca plate, MGL1211 and MGL1212, during the first year deployment of the Cascadia Initiative Amphibious Array. In total, we chose 114 ocean‐bottom seismometers and 211 inland stations located around the experiment region. The common receiver gathering analysis shows that most offshore stations located in deep‐water record clear first 〈span〉P〈/span〉‐wave arrivals up to 150 km away from the air‐gun shot. The first arrival transits from crustal phase 〈span〉Pg〈/span〉 to head wave 〈span〉Pn〈/span〉 at a source‐to‐receiver distance of 25–40 km. For stations located at shallow water, the seismic recordings appear much noisier compared with those of deep‐water stations. Only five inland stations record clear air‐gun shot signals up to 200 km away from the source. The signal‐to‐noise ratio of the first 〈span〉P〈/span〉 arrivals generally decreases from a few hundred down to a single number with increasing source‐to‐receiver distance, from deep (∼3000  m) to shallow water (∼100  m), with increasing sediment thickness and decreasing air‐gun shot spacing and time interval. On average, the ratio appears 5–10 times lower at inland stations than at offshore stations. The sediment thickness estimated in this study demonstrates significant along‐strike variations, which is up to 5 km offshore Oregon and 3.5 km offshore Washington. The coincidence of thick sediments with seismicity suggests the importance of sediments on the coupling of the plate interface.〈/span〉
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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  • 10
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    A Comprehensive Quality Analysis of Empirical Green’s Functions at Ocean‐Bottom Seismometers in Cascadia (2019)
    Seismological Society of America (SSA)
    Details
    Publication Date: 2019
    Description: 〈span〉〈div〉ABSTRACT〈/div〉The deployment of oceanic seismic arrays facilitated unique data sets for the science community in imaging the seismic structures and understanding the lithosphere and mantle dynamics at subduction zone systems and other tectonic settings. The data quality is fundamental to ensure reliable seismic results using records from ocean‐bottom seismometers. In this study, we conduct a comprehensive analysis of factors that may affect the signal‐to‐noise ratio (SNR) of the fundamental‐mode Rayleigh waves, as a proxy for the waveform quality, within the Cascadia subduction zone. We use stations from Cascadia Initiative, Gorda deformation zone experiment, Blanco transform fault experiment, and Neptune Canada array. The empirical Green’s functions (EGFs) of Rayleigh waves are extracted from ambient‐noise seismic waveforms and filtered at 10‐ to 35‐s periods. In general, the SNR of the EGFs decreases with increasing interstation distance and increasing sediment thickness. A portion of stations, mainly located within the Gorda plate and along the trench, demonstrates temporal variations of the data quality, with the highest SNR observed during the fall and winter seasons. The SNR demonstrates a complicated pattern in terms of the length of the time series used to extract EGFs. Most stations within the Juan de Fuca (JDF) plate show improvement of data quality with increasing length. However, for many stations located within the accretionary wedge and the Gorda plate, the ratio does not increase much by stacking more data. The distinctly different patterns of the SNR between the Gorda and JDF plates indicate possible impacts of lithosphere properties on data quality.〈/span〉
    Print ISSN: 0895-0695
    Electronic ISSN: 1938-2057
    Topics: Geosciences
    Published by Seismological Society of America (SSA)
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