Direct inversion of surface wave dispersion data with multiple-grid parameterizations and its application to a dense array in Chao Lake, eastern China

Summary The direct surface wave tomography has become an efficient tool in imaging three-dimensional (3-D) shallow Earth structure. However, some fundamental problems still exist in selecting the grids to parameterize the model space. This study proposes to implement a model parameterization approach with multiple grids to the direct surface wave tomography. These multiple grids represent several overlapping collocated grids with the same or different grid spacings, such as staggered grids, multiscale grids, and multiscale-staggered grids. At each iteration, direct inversion is applied to each individual set of collocated grids to invert for the shear-wave velocity (Vs) model; the models are then projected onto a set of predefined base grids (usually the finest grids) using 3-D B-spline interpolation. At the end of each iteration, we average the Vs models of all sets of collocated grids to obtain the average 3-D Vs model, which is then used as the initial model for the next iteration. The properties of this approach are explored by applying it to a newly deployed dense array in Chao Lake (CL), eastern China. Synthetic and field data tests demonstrate that the method using multiple grids recovers anomaly patterns better than that using the individual set of collocated grids, though it does not necessarily achieve the smallest traveltime residual. We then obtain a high-resolution 3-D shallow crustal Vs model beneath the CL. The 3-D Vs model reveals two prominent features: (1) a stripe-like structural pattern of velocity variations, where the Hefei basin and eastern CL display low-velocity anomalies while the Tanlu fault zone (TFZ), Zhangbaling uplift, and Yinping mountain present high-velocity anomalies; (2) north-shifted low-velocity anomalies beneath the eastern CL as depths go shallow. The shallow Vs features are consistent well with the local geological units and topography. We suggest that the two main features could be associated with the multistage tectonic activities of the Tanlu fault. The multiple-grid scheme proposed in this study could be conveniently extended to other 3-D direct inversion approaches in the near future.

Characteristics and Implications of Nanocoatings on Gouges in a Seismically Active Fault Zone

Using a ZEISS Axio Scope A1 ordinary polarizing microscope, a ZEISS Sigma 300 scanning electron microscope and a HITACHI S4800 scanning electron microscope, we observe micro/nanocharacteristics of slip planes in gouges sampled from the Tanlu fault zone, the Haiyuan fault zone and several other Late Pleistocene active faults, such as the Haiyang fault, the Shuangshan-Lijiazhuang fault and the Xintai-Mengyin fault, in Shandong Province. Based on microscopic observation of gouges, a straight slip zone is a sign of seismic stick slipping. According to scanning electron microscopy results, the surface of gouges is commonly covered by nanocoatings. Such coatings feature nanoparticles, aggregations, scratches, grooves, cracks and “silver lines.” According to the characteristics of nanomaterials, we believe that nanocoatings on gouges could help rapidly unload tectonic stress in the process of energy accumulation and weaken the strength of the active fault, which is beneficial to creep slipping and has a weakening effect on seismogenesis.