Qinya Liu

Adjoint Tomography of the Southern California Crust

Crustal Details Revealed In seismic tomography, a large collection of data representing paths through Earth are inverted to provide an analysis of variation of density in which errors are minimized. Typically, the inversion starts with a simple layered model of the tomographic region. Tape et al. (p. 988) show how, starting with a three-dimensional model, based on synthetic seismograms, an improved iterative inversion approach can lead to a much more detailed view of a region. Using the rich data for Southern California, the model reveals details of the geologic history of the crust in this region. Analysis of seismic data using a more realistic crustal model reveals detailed variations in density beneath southern California. Using an inversion strategy based on adjoint methods, we developed a three-dimensional seismological model of the southern California crust. The resulting model involved 16 tomographic iterations, which required 6800 wavefield simulations and a total of 0.8 million central processing unit hours. The new crustal model reveals strong heterogeneity, including local changes of ±30% with respect to the initial three-dimensional model provided by the Southern California Earthquake Center. The model illuminates shallow features such as sedimentary basins and compositional contrasts across faults. It also reveals crustal features at depth that aid in the tectonic reconstruction of southern California, such as subduction-captured oceanic crustal fragments. The new model enables more realistic and accurate assessments of seismic hazard.

Simulations of Ground Motion in the Los Angeles Basin Based upon the Spectral-Element Method

We use the spectral-element method to simulate ground motion generated by two recent and well-recorded small earthquakes in the Los Angeles basin. Simulations are performed using a new sedimentary basin model that is constrained by hundreds of petroleum-industry well logs and more than 20,000 km of seismic reflection profiles. The numerical simulations account for 3D variations of seismic-wave speeds and density, topography and bathymetry, and attenuation. Simulations for the 9 September 2001 M_w 4.2 Hollywood earthquake and the 3 September 2002 M_w 4.2 Yorba Linda earthquake demonstrate that the combination of a detailed sedimentary basin model and an accurate numerical technique facilitates the simulation of ground motion at periods of 2 sec and longer inside the basin model and 6 sec and longer in the regional model. Peak ground displacement, velocity, and acceleration maps illustrate that significant amplification occurs in the basin.

Finite-Frequency Kernels Based on Adjoint Methods

We derive the adjoint equations associated with the calculation of Frechet derivatives for tomographic inversions based upon a Lagrange multiplier method. The Frechet derivative of an objective function χ(m), where m denotes the Earth model, may be written in the generic form δχ = ∫ K_m(x) δ ln m(x) d^3x, where δ ln m = δm/m denotes the relative model perturbation and K_m the associated 3D sensitivity or Frechet kernel. Complications due to artificial absorbing boundaries for regional simulations as well as finite sources are accommodated. We construct the 3D finite-frequency “banana-doughnut” kernel K_m by simultaneously computing the so-called “adjoint” wave field forward in time and reconstructing the regular wave field backward in time. The adjoint wave field is produced by using time- reversed signals at the receivers as fictitious, simultaneous sources, while the regular wave field is reconstructed on the fly by propagating the last frame of the wave field, saved by a previous forward simulation, backward in time. The approach is based on the spectral-element method, and only two simulations are needed to produce the 3D finite-frequency sensitivity kernels. The method is applied to 1D and 3D regional models. Various 3D shear- and compressional-wave sensitivity kernels are presented for different regional body- and surface-wave arrivals in the seismograms. These kernels illustrate the sensitivity of the observations to the structural parameters and form the basis of fully 3D tomographic inversions.

Spectral-Element Moment Tensor Inversions for Earthquakes in Southern California

We have developed and implemented an automated moment tensor inversion procedure to determine source parameters for southern California earthquakes. The method is based upon spectral-element simulations of regional seismic wave propagation in an integrated 3D southern California velocity model. Sensitivity to source parameters is determined by numerically calculating the Frechet derivatives required for the moment tensor inversion. We minimize a waveform misfit function, and allow limited time shifts between data and corresponding synthetics to accommodate additional 3D heterogeneity not included in our model. The technique is applied to three recent southern California earthquakes: the 9 September 2001, M_L 4.2 Hollywood event, the 22 February 2003, M_L 5.4 Big Bear event, and the 14 December 2001, M_L 4.0 Diamond Bar event. Using about half of the available three-component data at periods of 6 sec and longer, we obtain focal mechanisms, depths, and moment magnitudes that are generally in good agreement with estimates based upon traditional body-wave and surface-wave inversions.

Three-dimensional simulations of seismic-wave propagation in the Taipei basin with realistic topography based upon the spectral-element method

We use the spectral-element method to simulate strong ground motion throughout the Taipei metropolitan area. Mesh generation for the Taipei basin poses two main challenges: (1) the basin is surrounded by steep mountains, and (2) the city is located on top of a shallow, low-wave-speed sedimentary basin. To accommodate the steep and rapidly varying topography, we introduce a thin high-resolution mesh layer near the surface. The mesh for the shallow sedimentary basin is adjusted to honor its complex geometry and sharp lateral wave-speed contrasts. Variations in Moho thickness beneath Northern Taiwan are also incorporated in the mesh. Spectral-element simulations show that ground motion in the Taipei metropolitan region is strongly affected by the geometry of the basin and the surrounding mountains. The amplification of ground motion is mainly controlled by basin depth and shallow shear-wave speeds, although surface topography also serves to amplify and prolong seismic shaking.

Time reversal location of glacial earthquakes

In 2003, Ekstrom et al. reported the detection and location of a new class of earthquakes occurring in the polar regions of the Earth. The proposed source mechanism involves large and sudden sliding motions of glaciers, which gave the name “glacial earthquakes” to these events. In this study we localize some of these earthquakes with a time reversal mirror (TRM) algorithm, which, contrary to ordinary back projection methods, does not involve testing each possible source location. In TRM localization, an earthquake is located on the basis of only one 3-D spectral element simulation of seismic wave propagation by using the full complexity of recorded data as simultaneous time-reversed sources. We show that on the basis of this approach, even glacial earthquakes with a faint signal can be correctly localized and that the pattern of the time-reversed wavefield is coherent with the motion of glaciers down their valley.

Multiparameter adjoint tomography of the crust and upper mantle beneath East Asia: 1. Model construction and comparisons

We present a 3-D radially anisotropic model of the crust and mantle beneath East Asia down to 900 km depth. Adjoint tomography based on a spectral element method is applied to a phenomenal data set comprising 1.7 million frequency-dependent traveltime measurements from waveforms of 227 earthquakes recorded by 1869 stations. Compressional wave speeds are independently constrained and simultaneously inverted along with shear wave speeds (VSH and VSV) using the same waveform data set with comparable resolution. After 20 iterations, the new model (named EARA2014) exhibits sharp and detailed wave speed anomalies with improved correlations with surface tectonic units compared to previous models. In the upper 100 km, high wave speed (high-V) anomalies correlate very well with the Junggar and Tarim Basins, the Ordos Block, and the Yangtze Platform, while strong low wave speed (low-V) anomalies coincide with the Qiangtang Block, the Songpan Ganzi Fold Belt, the Chuandian Block, the Altay-Sayan Mountain Range, and the back-arc basins along the Pacific and Philippine Sea Platemargins. At greater depths, narrow high-V anomalies correspond to major subduction zones and broad high-V anomalies to cratonic roots in the upper mantle and fragmented slabs in the mantle transition zone. In particular, EARA2014 reveals a strong high-V structure beneath Tibet, appearing below 100 km depth and extending to the bottom of the mantle transition zone, and laterally spanning across the Lhasa and Qiangtang Blocks. In this paper we emphasize technical aspects of the model construction and provide a general discussion through comparisons.

A 3-D spectral-element and frequency-wave number hybrid method for high-resolution seismic array imaging

We present a three-dimensional (3-D) hybrid method that interfaces the spectral-element method (SEM) with the frequency-wave number (FK) technique to model the propagation of teleseismic plane waves beneath seismic arrays. The accuracy of the resulting 3-D SEM-FK hybrid method is benchmarked against semianalytical FK solutions for 1-D models. The accuracy of 2.5-D modeling based on 2-D SEM-FK hybrid method is also investigated through comparisons to this 3-D hybrid method. Synthetic examples for structural models of the Alaska subduction zone and the central Tibet crust show that this method is capable of accurately capturing interactions between incident plane waves and local heterogeneities. This hybrid method presents an essential tool for the receiver function and scattering imaging community to verify and further improve their techniques. These numerical examples also show the promising future of the 3-D SEM-FK hybrid method in high-resolution regional seismic imaging based on waveform inversions of converted/scattered waves recorded by seismic array.

A Structural VP Model of the Salton Trough, California, and Its Implications for Seismic Hazard

We present a high-resolution, three-dimensional P-wave seismic veloc- ity model of the sedimentary basin in the Salton Trough, southern California, and use the model for spectral-element method (SEM) wave propagation and ground- motion simulations to quantitatively assess seismic hazard in the region. The basin geometry is defined by a surface representing the top of crystalline basement, which was constrained by seismic refraction profiles and free-air gravity data. Sonic logs from petroleum wells in the Imperial Valley and isovelocity surfaces defined by seismic refraction studies were used to define P-wave velocity within the sedimentary basin as a function of two variables:(1) absolute depth and (2) depth of the underlying crystalline basement surface (CBS). This velocity function was used to populate cells of a three-dimensional spatial array (voxet) defining the P-wave velocity structure in the basin. The new model was then resampled in a computational mesh used for earthquake wave propagation and strong ground motion simulations based upon the SEM (Komatitsch et al., 2004). Simulation of the 3 November 2002 Mw 4.2 Yorba Linda earthquake demonstrates that the new model provides accurate simulation of strong ground motion amplification effects in the Salton Trough sedimentary basin, offering substantial improvements over previous models. A hypothetical Mw 7.9 earthquake on the southern San Andreas fault was then simulated in an effort to better understand the seismic hazard associated with the basin structure. These sim- ulations indicate that great amplification will occur during large earthquakes in the region due to the low seismic velocity of the sediments and the basin shape and depth.

A simulation of a lake effect snowstorm with a cloud resolving numerical model

[1] Lake-effect snowstorms (LES), linearly organized bands of convective clouds, are a major source of snowfall and severe weather in the North American Great Lakes region. LES develop as cold and dry air flows over the warm lake surfaces triggering convection that is often organized into quasi-linear structures known as band clouds. The small horizontal width of these bands, often less than 5 km, combined with their regional-scale evolution that is impacted by the distribution of open water, lake-ice and land makes the forecasting of LES particularly challenging. Here, we describe the simulation of an observed LES event using a cloud resolving numerical model in a domain that includes much of the Great Lakes region. The model was able to successfully capture many of the characteristics associated with the event. This simulation suggests that it soon may be possible to forecast the development of this class of convective weather systems.