Ambient Noise Imaging (ANI) of subsurface structures relies on seismic interferometry of diffuse seismic wavefields. However, the lack of effective methods to quantify and identify highly diffuse waves hampers applications of ANI, particularly in evaluating seismic attenuation and monitoring structural changes with high temporal resolution. Conventional ANI approaches require data normalization, which effectively suppresses the non-diffuse component with large amplitude but also results in significant loss of amplitude and phase information in the continuous seismic records. In this study, we propose a frequency domain method to quantitatively evaluate the degree of diffuseness of seismic wavefields by analyzing their statistical characteristics of modal amplitudes for stationarity and randomness. Tests on synthetic waveform and field nodal records show that the proposed method can effectively distinguish between diffuse and non-diffuse waveforms for either single- or three-component data. As an application, we identify a 60-second-long diffuse coda of a local M 2.2 earthquake recorded by a dense nodal array on the San Jacinto Fault Zone, and successfully extract high-quality dispersion curve and Q-value without performing data normalization. These results are consistent with those obtained by conventional methods that assess the correlation between coherency and the Green's function, and by modeling ballistic waves generated by road traffic. Our proposed method can advance the imaging of subsurface velocity and attenuation structures as well as monitoring temporal changes for scientific studies and engineering applications.

The location and origin of Neoproterozoic-Cambrian sutures provide keys to understand the formation and evolution of the supercontinent Gondwana. The Larsemann Hills is located near a major Neoproterozoic-Cambrian suture zone in the Prydz Belt, but has not been examined locally by comprehensive geophysical studies. In this study, we analyzed data collected from a 1-D joint seismic-MT array deployed during the 36th Chinese National Antarctic Research Expedition. We found that a sharp Moho discontinuity offset of 6-8 km shows up in the stacked image of teleseismic P-wave receiver function analysis; coinciding with the abrupt Moho offset, a near-vertical channel with (1) low resistivity extending to the uppermost mantle depths, and (2) a high crustal Poisson’s ratio in the crust is identified. These findings provide evidence for the determination of the location and collisional nature of the Prydz belt or a portion of it.

Classic surface wave imaging relies mainly on the dispersion of Rayleigh waves or Love waves. In addition to these types of waves, waveforms that arrive prior to the S wave also exhibit dispersion. These early dispersed waves are controlled mainly by the leaking modes and are rarely used for imaging. We applied the frequency-Bessel transform method to waveforms that arrive before the S wave and successfully extracted multiple leaking mode dispersion curves. The extracted “0th” mode shows continuous sensitivity to S-wave velocity (Vs) and mass density structure whereas the “higher” modes behave more like surface waves that are sensitive to P-wave velocity (Vp) structure in the crust. Thus, the “higher” modes can be used to invert the Vp structure, which can compensate for the fact that conventional surface wave imaging methods are mainly sensitive to Vs structure.

With the advent of the array-based methods, the observations of leaky modes from earthquake records and ambient noise are frequently reported. It is urgently needed to have an effective guide to the selection of certain leaky modes to put into practical inversion combined with other imaging methods. To this end the excitation of leaky modes for multi-layered models is theoretically investigated. By computing theoretical seismograms for different source mechanisms and different source depths with the discrete wavenumber method and the normal-mode summation method separately, the contribution of leaky modes to the resultant seismograms is qualitatively evaluated and the effect of the source depth for the same source mechanism is in detail examined. Further, we perform accurate computation of leaky modes for two models and categorize the leaky modes into PL and OP (organ-pipe) modes, whose distinctive properties are characterized both from the attenuation and from the eigen-displacements of these two types of modes. Also, these results could help explain very well the occurrence of certain leaky modes on the dispersion spectrum and be indicative of the reasonable choice of certain leaky modes to put them into practical inversion for P-velocity structures.

By cross-correlating ambient seismic noise, researchers have made significant progress in retrieving Empirical Green’s Functions (EGFs) in the past two decades. EGFs emerging with high Signal-to-Noise Ratios (SNRs), however, requires stacking plenty of cross-correlations. Here, we retrieve clear Rayleigh waves by cross-correlating noise released by different hurricanes moving along the southeastern coast of the United States. Extracted Rayleigh EGFs almost only emerge in the negative lag time windows due to the dominated sources. We show significant similarities of Rayleigh EGFs between the hurricane and one-year noise. Stable dispersion curves obtained from hurricane noise with the F-J transform agree well with that from one-year noise. Our results, indeed, suggest that we can extract reliable dispersion curves from hurricane noise in several days. Therefore, we enlarge the data sets for the future work of illuminating the Earth’s crust and upper mantle.

The inversion of Scholte wave dispersion curves is one of the most important methods employed in the detection of marine sedimentary structures. The measurement methods of Scholte wave dispersion are primarily developed from terrestrial geophysical exploration technologies. Unlike terrestrial seismic source excitation methods, marine exploration typically uses air guns to generate seismic waves in seawater, which will result in strongly dispersed Guided-P waves. We found that the Guided-P waves will truncate the higher-mode dispersive Scholte waves at their lower cutoff frequency. However, a simple individual preprocess of suppressing Guided-P procedure (SGP) before extracting dispersion curves can significantly increase the frequency band and image the full dispersive modes of Scholte waves. The wider frequency band and higher-mode Scholte wave dispersion curves can effectively improve the constraints on the submarine shear wave velocity structure. Based on this, we highly recommend adding this simple step to Scholte wave dispersion imaging.

A semi-analytical spectral element method (SASEM) is proposed to solve for the normal and leaky modes of elastic waves propagating in a planar waveguide with a half-space substrate. For the SH-wave modes, the transparent boundary condition is used to model the SH wavefields in the half-space substrate. To solve for the PSV-wave normal modes on the (+, +) Riemann sheet and leaky modes on the (+, −) Riemann sheet, the elastic wavefields in the finite-thickness layers are modeled using the displacements, whereas the wavefields in the half-space are modeled using the P- and S-wave potentials. In the substrate, the transparent boundary condition is used for the shear wavefields, whereas semi-infinite elements are introduced to treat the radiative boundary condition of the P wavefields. Then, a polynomial eigenvalue problem is derived, which can be transformed into a standard linear eigenvalue problem. Solving the eigenvalue problem, we can obtain the solutions of the normal and leaky modes. Several numerical tests were performed to verify the effectiveness of SASEM, as well as to demonstrate its high accuracy. Modal analyses of the oscillations of the solved modes demonstrate that the leaky modes differ from the normal modes because of the increasing wavefields in the half-space. Moreover, the guided-P modes are confirmed to be more dependent on the P-waves, whereas the normal and organ-pipe modes are primarily determined by the S-waves. Besides the crustal model composed of several homogeneous layers, SASEM is applied to a vertically inhomogeneous offshore model to demonstrate its applicability. The good agreement between the theoretical guided-P modes and the dispersion spectra not only shows the correctness of SASEM when analyzing waveguides composed of gradient layers but also indicates the potential for constraining the P-wave velocity using the guided-P modes.

Traditional finite difference method for electromagnetic simulation is based on staggered grid, whilst researches on collocated grid are few. We present a high-order collocated-grid finite-difference method for modelling electromagnetic waves with a topographic ground surface by solving 2D time-domain Maxwell equations in curvilinear coordinates. The proposed method, incorporating curvilinear coordinates, collocated grids and MacCormack finite-difference scheme techniques, can describe the geometry of the irregular interface better and avoid the numerical scattering caused by the staircase approximation in the conventional finite-difference method for electromagnetic wavefield modelling. The first-order 2D Maxwell equations on curvilinear grids are solved by an optimized MacCormack finite-difference scheme, first presented by Hixon (1997). As the collocated grids are implemented, in which the electric and magnetic fields are discretized at the same grids, the interfacial boundary conditions need to be considered. Therefore, a novel effective interface method is presented to handle the conditions. The proposed method is verified by a series of ground penetrating radar application models, such as homogeneous space, multilayered media and buried cavity models, by comparing synthetic waveforms with independent reference solutions, such as the analytical solution, the generalized reflection/transmission method and an open-source program gprMax. Comparisons show that the proposed method does well in handling multiple reflections and curved interfaces.

In the past two decades, surface wave imaging based on seismic ambient noise cross-correlation (CC) has been one of the most important technologies in the field of seismology. With the development of this technology, high-mode surface waves have received increasing attention, especially after the proposition of the frequency-Bessel transform (F-J) method, which can effectively extract multimode dispersion curves from ambient noise data. In the past few years, our research group has made many attempts to improve this method. We summarized these experiences and the corresponding algorithm for fast CC, and packaged them into a Python package called CC-FJpy. It is commonly understood that CC takes a good deal of time. However, we found that a simple reorganization of the CC logic can achieve computational acceleration by a multiple of tens or even hundreds in comparison with classical CC open-source programs for N stations. For the F-J method, we use Nvidia’s graphics processing unit (GPU) to speed up computation, and this approach achieves a hundreds-fold computational acceleration. We have encapsulated our experiences and technologies into CC-FJpy and submitted it to various types of data tests to ensure its speed and ease of use. We hope that providing the open source of CC-FJpy can benefit the development of surface wave studies and make it easier to start with high-mode surface waves. We look forward to your use and valuable suggestions.

Numerical simulation of rupture dynamics provides critical insights for understanding earthquake physics, while the complex geometry of natural faults makes numerical method development challenging. The discontinuous Galerkin (DG) method is suitable for handling complex fault geometries. In the DG method, the fault boundary conditions can be conveniently imposed through the upwind flux by solving a Riemann problem based on a velocity-strain elastodynamic equation. However, the universal adoption of upwind flux can cause spatial oscillations in cases where elements on adjacent sides of the fault surface are not nearly symmetric. Here we propose a nodal DG method with an upwind/central mixed-flux scheme to solve the spatial oscillation problem, and thus to reduce the dependence on mesh quality. We verify the new method by comparing our results with those from other methods on a series of published benchmark problems with complex fault geometries, heterogeneous materials, off-fault plasticity, roughness, thermal pressurization, and various versions of fault friction laws. Finally, we demonstrate that our method can be applied to simulate the dynamic rupture process of the 2008 Mw 7.9 Wenchuan earthquake along/across multiple fault segments. Our method can achieve high scalability in parallel computing under different orders of accuracy, showing high potential for adaptation to earthquake rupture simulation on natural tectonic faults.