Abstract We present the first in a series of dataset and model assessment products for investigating Africa’s lithosphere (ADAMA). This is a comprehensive catalog of short-period interstation surface-wave dispersion measurements and uncertainties. It is derived from processing continuous recordings of all publicly available three-component seismograms, spanning four decades, from ∼1372 stations, across 62 seismic networks deployed in and around the African continent. It includes Love- and Rayleigh-wave dispersion derived from frequency-domain ambient noise cross-correlation functions (NCFs). Phase and group dispersion, as well as their uncertainties, are then obtained with an iterative nonlinear waveform fitting of the NCFs, using a spectral element representation of a path-average a priori Earth model. Our catalog represents the following advances: (1) a large distribution of short period dispersion measurements: ∼114,000 interstation pairs at periods between 5 s and 40 s, (2) inclusion of uncertainties useful for regularization in continent-wide model building, (3) preliminary model assessments for different tectonic domains on the continent, and (4) an exemplary Love-wave phase velocity map obtained by Bayesian inversion revealing detailed features not previously detected. ADAMA will be used to prepare short-period, high-resolution dispersion maps, and for assessment and updates of widely used seismic velocity models of the crust across a diversity of terranes on the continent.
more »
« less
Numerical comparison of Rayleigh and Love full waveform inversion in characterizing soil spatial variability for near‐surface engineering applications
The use of surface wave testing for near‐surface engineering applications has increased in recent years. Typical surface wave analysis is based on the dispersion of surface waves in one‐dimensional layered models. One‐dimensional models are inappropriate for measurements at sites with appreciable lateral variability, a likely scenario in many engineering applications. Use of such models can subsequently undermine the reliability and accuracy of the surface wave results. Full waveform inversion (FWI) is a high‐resolution imaging technique that is proven to outperform the conventional dispersion‐based analysis of surface waves. Much of the near‐surface literature has focused on full waveform inversion of Rayleigh waves developed by the interaction of primary‐ and vertically polarized shear waves (P‐SV), and the capabilities of surface waves generated by horizontally polarized shear waves (Love waves) in mapping near‐surface spatial variabilities have not been fully explored. In this numerical study, full waveform inversion of Rayleigh and Love waves was performed on two different spatially correlated Gaussian random fields (mean
- PAR ID:
- 10376594
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Near Surface Geophysics
- Volume:
- 18
- Issue:
- 5
- ISSN:
- 1569-4445
- Format(s):
- Medium: X Size: p. 497-513
- Size(s):
- p. 497-513
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)Two types of surface wave anisotropy are observed regularly by seismologists but are only rarely interpreted jointly: apparent radial anisotropy, which is the difference in propagation speed between horizontally and vertically polarized waves inferred from Love and Rayleigh waves, and apparent azimuthal anisotropy, which is the directional dependence of surface wave speeds (usually Rayleigh waves). We show that a new data set of Love and Rayleigh wave isotropic phase speeds and Rayleigh wave azimuthal anisotropy observed within and surrounding eastern Tibet can be explained simultaneously by modeling the crust as a depth-dependent tilted hexagonally symmetric (THS) medium. We specify the THS medium with depth-dependent hexagonally symmetric elastic tensors tilted and rotated through dip and strike angles and estimate these quantities using a Bayesian Monte Carlo inversion to produce a 3-D model of the crust and uppermost mantle on a 0.5° × 0.5° spatial grid. In the interior of eastern Tibet and in the Yunnan-Guizhou plateau, we infer a steeply dipping THS upper crustal medium overlying a shallowly dipping THS medium in the middle-to-lower crust. Such vertical stratification of anisotropy may reflect a brittle to ductile transition in which shallow fractures and faults control upper crustal anisotropy and the crystal-preferred orientation of anisotropic (perhaps micaceous) minerals governs the anisotropy of the deeper crust. In contrast, near the periphery of the Tibetan Plateau the anisotropic medium is steeply dipping throughout the entire crust, which may be caused by the reorientation of the symmetry axes of deeper crustal anisotropic minerals as crustal flows are rotated near the borders of Tibet.more » « less
-
more » « less
Abstract We develop an automatic workflow for enhancing surface wave signals in ambient noise cross correlations (ANCs) calculated for a one‐dimensional (1‐D) linear array. The proposed array‐based method is applied to a 1.6 km‐long dense linear nodal array crossing surface traces of the San Jacinto fault near Anza, California. Fundamental and higher modes of surface waves are observed in ANCs of the nodal array. After attenuating the surface wave overtones by applying a frequency‐dependent tapering window to the ANCs, signals dominated by the fundamental mode surface wave are then enhanced through a denoising process based on three‐station interferometry of direct waves. The signal‐to‐noise ratio is significantly increased at high frequencies (>2 Hz) after denoising. Phase travel times are extracted reliably in the frequency domain for the period ranges of 0.3–1.2 s and 0.3–1.6 s for Rayleigh and Love waves, respectively. The corresponding period‐dependent phase velocity profiles derived from the eikonal equation reveal high‐resolution details of fault zone internal structures beneath the array. A broad (500–1,000 m) low‐velocity zone that narrows with increasing period is observed, illuminating a flower‐shaped structure of the San Jacinto fault damage zone.
-
more » « less
SUMMARY Fundamental mode surface wave data have often been used to construct global shear velocity models of the upper mantle under the so-called ‘path average approximation’, an efficient approach from the computational point of view. With the advent of full-waveform inversion and numerical wavefield computations, such as afforded by the spectral element method, accounting for the effects of the crust accurately becomes challenging. Here, we assess the merits of accounting for crustal and uppermost mantle effects on surface and body waveforms using fundamental mode dispersion data and a smooth representation of the shallow structure. For this we take as reference a model obtained by full-waveform inversion and wavefield computations using the spectral element method, model SEMUCB-WM1 and compare the waveform fits of synthetics to different parts of three component observed teleseismic records, in the period band 32–300 s for body waves and 40–300 s for surface waves and their overtones for three different models. The latter are: a dispersion-only based model (model Disp_20s_iter5), and two models modified from SEMUCB-WM1 by successively replacing the top 200 km (model Merged _200 km) and top 80 km (model Merged _80 km), respectively, by a model constrained solely by fundamental mode surface wave dispersion data between periods of 20 and 150 s. The crustal part of these three models (resp. SEMUCB-WM1) is constrained from dispersion data in the period range 20–60 s (resp. 25–60 s), using the concept of homogenization which is tailored to simplify complex geological features, enhancing the computational efficiency of our seismic modelling. We evaluate the fits to observed waveforms provided by these three models compared to those of SEMUCB-WM1 by computing three component synthetics using the spectral element method for five globally distributed events recorded at 200+stations, using several measures of misfit. While fits to waveforms for model 3 are similar to those for SEMUCB-WM1, the other two models provide increasingly poorer fits as the distance travelled by the corresponding seismic wave increases and/or as it samples deeper in the mantle. In particular, models 1 and 2 are biased towards fast shear velocities, on average. Our results suggest that, given a comparable frequency band, models constructed using fundamental mode surface wave data alone and the path average approximation, fail to provide acceptable fits to the corresponding waveforms. However, the shallow part of such a 3-D radially anisotropic model can be a good starting model for further full waveform inversion using numerical wavefield computations. Moreover, the shallow part of such a model, including its smooth crustal model, and down to a maximum depth that depends on the frequency band considered, can be fixed in full-waveform inversion iterations for deeper structure. This can save significant computational time when higher resolution is sought in the deeper mantle. In the future, additional constraints for the construction of the homogenized model of the crust can be implemented from independent short period studies, either globally or regionally.
-
more » « less
SUMMARY We use broad-band stations of the ‘Los Angeles Syncline Seismic Interferometry Experiment’ (LASSIE) to perform a joint inversion of the Horizontal to Vertical spectral ratios (H/V) and multimode dispersion curves (phase and group velocity) for both Rayleigh and Love waves at each station of a dense line of sensors. The H/V of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the Green’s function. The presence of low-frequency peaks (∼0.2 Hz) in H/V allows us to constrain the structure of the basin with high confidence to a depth of 6 km. The velocity models we obtain are broadly consistent with the SCEC CVM-H community model and agree well with known geological features. Because our approach differs substantially from previous modelling of crustal velocities in southern California, this research validates both the utility of the diffuse field H/V measurements for deep structural characterization and the predictive value of the CVM-H community velocity model in the Los Angeles region. We also analyse a lower frequency peak (∼0.03 Hz) in H/V and suggest it could be the signature of the Moho. Finally, we show that the independent comparison of the H and V components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field.
