The recently developed frequency‐Bessel transformation (F‐J) method is effective to extract multimode surface wave dispersion curves from ambient noise cross‐correlation functions (CCFs). However, ...this method is currently limited to the vertical‐vertical component CCFs, and only Rayleigh wave dispersion curves can be obtained. In this study, we first relate the F‐J spectrogram to the spatial autocorrelation coefficients; we then extend the F‐J method to the full multicomponent CCFs tensor, including the radial‐radial, transverse‐transverse, and the mixed‐component CCFs. Using the newly derived formulation, not only the signal of higher‐mode Rayleigh wave phase velocity dispersion can be enhanced, but also the multimode Love wave phase velocity dispersion curves and the higher‐mode Rayleigh wave ellipticity can be extracted. The formulation is tested in several numerical examples and is applied to field data in North America. Our derivation and formulation provide an extension to the current F‐J method and help to take usage of multicomponent CCFs. The resulting higher‐mode surface wave dispersions and Rayleigh wave ellipticity provide complementary constraint on the Earth structures.
Key Points
Frequency‐Bessel transform method for ZZ component CCFs is extended to full noise cross‐correlation tensor
The formulation helps to extract multimode surface wave, phase velocities, and multimode Rayleigh wave ellipticity
Synthetic tests and an application to field data in North America validate the formulation
The Xiaojiang fault zone system (XJFS) is located in the southeastern Tibet with high seismicity. In this study, we invert Rayleigh and Love wave dispersion curves obtained from three dense seismic ...arrays jointly for high-resolution 3-D crustal average shear wave velocity and radial anisotropy models simultaneously in XJFS. Our model reveals that the upper crust and mid-lower crust generally exhibit negative and positive radial anisotropy, respectively, implying that the deformation pattern is depth-dependent. To the east of the Lvzhijiang Fault, most of the low velocity zones in the mid-crust correspond to the positive radial anisotropy (Vsh > Vsv); the channelized weak zone seems to be continuous across the Red River Fault at depth of 20 km, probably within a thin layer. West of the Lvzhijiang Fault, where it is inferred to be the inner zone of the Permian Emeishan Large Igneous Province, the high velocity zone and positive radial anisotropy in the mid-lower crust can be attributed to the underplating and intruded magmatic rocks. In the upper crust, the high and low velocity zones are mostly belt-shape and follow the north-south direction parallel to the regional major faults; the lateral variation of the radial anisotropy spatially corresponds to the activity of different segments of the Xiaojiang fault zone. Our models depict detailed geometry of the channelized weak zone in the mid-lower crust and provide new insight into the role of major faults in regional tectonics.
•High-precision crustal Vs and radial anisotropy models in XJFS (SE Tibet) are derived.•Our models depict detailed geometry of the channelized weak zone in mid-lower crust.•Radial anisotropy is spatially associated with activity of different segments of XJF.
SUMMARY
A recent study analysed the Rayleigh wave ellipticity obtained by ambient noise cross-correlation in periods of 8–20 s, and observed the Rayleigh wave ellipticity is backazimuth-dependent ...with a 180° periodicity in the contiguous United States. However, the azimuthal anisotropic parameters have not been inverted to depths, and the comparison with other seismic results has not been possible so far, partially due to the lack of related theoretical investigations. Here, we first derive explicit formulation to relate the period-dependent backazimuthal Rayleigh wave ellipticity with the depth-dependent azimuthal wave speed variation in a slightly anisotropic medium based on the variational principle; by carefully examining relations among different parametrizations of a horizontally transverse isotropic medium, we then express the final formulation in terms of Crampin’s notation. The formulation is verified by comparison with the results of anisotropic propagator matrix technique. Tests show the backazimuth-dependent Rayleigh wave ellipticity provides complementary information on anisotropic parameters in addition to the widely used phase velocity. A simple application of the derived formulation to real data in North America is also provided. Our formulation can be regarded as an extension of the classic work on azimuthal-dependent phase velocity, and helps to quantitatively explain the backazimuth-dependent Rayleigh wave ellipticity.
SUMMARY
Teleseismic receiver functions (RFs) are frequently used to determine depths of seismic discontinuities in the crust and upper mantle. We developed an efficient reverse-time migration (RTM) ...method that is applied to teleseismic receiver functions directly. Both the primary P-to-S converted phases and their crustal multiples in RFs can be used for imaging seismic discontinuities. The method uses the phase-shift-plus-interpolation algorithm to extrapolate both the source and receiver wavefields in a 3-D velocity model, which greatly reduces the computation costs compared with those using a full wave-equation numerical solver. Tests using synthetic data in various crustal models demonstrate the effectiveness of the method and its superiority over the common-conversion-point stacking method. In particular, the method handles diffraction caused by strong lateral structural variations correctly and there is no limitation on the maximum dip of the interface. We applied the method to real data of a linear array in the Wabash Valley Seismic Zone in the central USA and obtained a crustal structural image across a failed continental rift. We suggest that future passive-source seismic recording experiments for crustal scale imaging use station spacing less than 5 km, and a 2-D array with even smaller station spacing is desired for regions with strong lateral structural variations. With increasing numbers of sensors used in passive-source recording experiments nowadays, our RF-RTM method can be a useful tool for structural imaging on scales ranging from sedimentary basins, crust to lithosphere.
A novel method is implemented to invert Rayleigh and Love wave dispersion curves of all paths jointly for 3‐D shear wave velocity and radial anisotropy simultaneously without intermediate steps. We ...use the method to derive high‐precision crustal shear wave velocity and radial anisotropy models around the eastern Himalayan syntaxis using ambient noise dispersion data (5–40 s). Results show that the crust can be divided into several subregions with different rigidity and preferred mineral alignment orientation depth‐dependently. In the middle crust, combined with other geophysical observations, 3‐D geometry of two continuous branches of eastward channelized weak zones is outlined in detail. Both branches of the weak zone are blocked by the high velocity zone with radial anisotropy of Vsh>Vsv at around 96–97°E. In the upper crust, the radial anisotropy model depicts a complex pattern, which is associated with the ongoing surface uplifting and shear strain rate distribution.
Key Points
A novel method of 3‐D direct surface wave radial anisotropy tomography is implemented
High‐resolution crustal VS and radial anisotropy model of the eastern Himalayan syntaxis is derived using ambient noise data
The models reveal depth‐dependent rigidity and outline 3‐D geometry of the mid‐crustal channelized weak zone
Azimuthal anisotropy retrieved from surface waves is important for constraining depth‐varying deformation patterns in the crust and upper mantle. We present a direct inversion technique for the ...three‐dimensional shear wave speed azimuthal anisotropy based on mixed‐path surface wave traveltime data. This new method includes two steps: (1) inversion for the 3‐D isotropic Vsv model directly from Rayleigh wave traveltimes and (2) joint inversion for both 3‐D Vsv azimuthal anisotropy and additional 3‐D isotropic Vsv perturbation. The joint inversion can significantly mitigate the trade‐off between strong heterogeneity and anisotropy. With frequency‐dependent ray tracing based on 2‐D isotropic phase speed maps, the new method takes into account the ray‐bending effect on surface wave propagation. We apply the new method to a regional array in Yunnan, southwestern China. Using Rayleigh wave phase velocity dispersion data in the period band of 5–40 s extracted from ambient noise interferometry, we obtain a 3‐D model of shear wave speed and azimuthal anisotropy in the crust and uppermost mantle in Yunnan. This model reveals that two midcrust low‐velocity zones are possible weak channels, and the azimuthal anisotropy at a depth of 5 to 30 km is mainly controlled by nearby strike‐slip faults, some of which also approximately coincide with the lateral boundaries of the crustal low‐velocity zones. Approximately south of 26°N, the upper crustal azimuthal anisotropy from our model is significantly different from the upper mantle anisotropy inferred by shear wave splitting, indicating different deformation styles between the crust and upper mantle in southern Yunnan.
Key Points
We developed a new method for direct inversion of 3‐D Vs azimuthal anisotropy from surface wave traveltime data
The new method considers period‐dependent surface wave ray tracing based on 2‐D isotropic phase velocity maps
The obtained azimuthal anisotropy in Yunnan reveals apparent differences in upper crustal and upper mantle deformation styles
A magnitude (Ms) 6.9 earthquake occurred on November 18, 2017, in Mainling, southeastern Tibet, which is the largest earthquake to occur in or around the eastern Himalayan syntaxis within the last ...50 years. To further understand the nucleation mechanism of this earthquake, the local seismicity distribution, and the regional tectonics, the detailed crustal velocity structure is obtained by direct ambient noise tomography using the Rayleigh wave phase velocity data (T=5 ∼ 40 s) in this area. The spatial correlation of the velocity model with the distribution of the seismicity reveals that the nucleation mechanism of the 2017 Ms 6.9 Mainling earthquake is closely related to the local stress condition and velocity structure and to the aqueous fluid and geothermal states. Our velocity model is extensively compared with previous magnetotelluric inversion results and reveals the distribution and connectivity of the crustal low velocity zones (LVZs). The general patterns of the velocity model in the middle crust show LVZs in the northwest and high velocity zones in the southeast that are truncated by major faults. We also find a relatively low-velocity anomaly in the upper crust near the Namche Barwa massif; this can be explained by the conceptional ‘tectonic aneurysms' model, which emphasizes the coupling effects of climate, erosion, and tectonics. To reconcile our velocity model with previous observations, we propose a tentatively integrated geodynamic model to explain the regional seismicity and stress condition.
•A high-resolution crustal Vs model around the eastern Himalayan syntaxis is obtained and compared with MT imaging results.•Nucleation mechanism of the 2017 Ms 6.9 Mainling earthquake is revealed, and spatial distribution of seismicity is discussed.•We propose an integrated regional geodynamic model to explain the regional seismicity and stress condition.
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