Qibin Xiao

Crustal structure and rheology of the Longmenshan and Wenchuan Mw 7.9 earthquake epicentral area from magnetotelluric data

The Longmenshan forms the eastern margin of the Tibetan Plateau adjacent to the Sichuan Basin. This range is anomalous because it formed despite low convergence and slip rates and without the development of a foreland basin. The devastating A.D. 2008 Wenchuan earthquake (Mw = 7.9) has renewed debate about the tectonics of the Longmenshan. A magnetotelluric (MT) study was undertaken subsequent to the earthquake to investigate the crustal structure of the Longmenshan, and inversion of the data reveals a low-resistivity (high-conductivity) layer at a depth of ∼20 km beneath the eastern Tibetan Plateau that terminates ∼25 km west of the Wenchuan-Maoxian fault. Its electrical properties are consistent with it being fluid-rich and mechanically weak. Beneath the Longmenshan and Sichuan Basin, a high-resistivity zone extends through the entire crust, but with a zone of low resistivity in the vicinity of the Wenchuan hypocenter. We show that the MT data, combined with other geological and geophysical observations, support geodynamic models for the uplift of eastern Tibet being caused by southeast-directed crustal flow that is blocked by stable lithosphere beneath the Sichuan Basin and Longmenshan, leading to inflation of the Songpan-Ganzi terrane. This rigid high-resistivity backstop not only provided a block to flow, but also may have accumulated stress prior to the earthquake. The MT observations provide new insights into the generation of the Wenchuan earthquake, which occurred in a region with low convergence rates prior to the earthquake.

Eastern termination of the Altyn Tagh Fault, western China: Constraints from a magnetotelluric survey

The left-lateral Altyn Tagh Fault forms the northern boundary of the Tibetan Plateau. The strike-slip rate of the active Altyn Tagh Fault decreases northeastward and reduces close to zero as it passes north of the Qilian Shan. This geometry raises controversies on whether and how the fault terminates or extends further east. To address these controversies, wide-band magnetotelluric data were collected along four profiles across the Altyn Tagh Fault ranging from 135 to 261 km in length. All four profiles are located in the foreland of the Qilian Shan Ranges and are oriented perpendicular to the inferred fault zone that could be the continuation of Altyn Tagh Fault. Both the two-dimensional and three-dimensional electrical resistivity models derived from our magnetotelluric data show that the Hexi Corridor crust is generally of low resistivity, whereas the crust of the Huahai–Jinta basin is, in general, of high resistivity with a local and isolated low-resistivity anomaly within the mid-lower crust. The generally high-resistivity crust of the Huahai–Jinta basin may be rheologically unfavorable for the Altyn Tagh Fault passing through the basin toward the northeast. The entirely different electrical structure between the Hexi Corridor and its northern neighbors indicates the existence of a tectonic boundary that coincides with the Altyn Tagh Fault in the west and reverse faults in the east. The two-dimensional electrical conductivity models suggest that the Altyn Tagh Fault transfers from a single fault in the west to a branching set of mainly dip-slip faults in the east.

Structure and geometry of the Aksay restraining double bend along the Altyn Tagh Fault, Northern Tibet, imaged using magnetotelluric method

Large restraining bends along active strike-slip faults locally enhance the accumulation of clamping tectonic normal stresses that may limit the size of major earthquakes. In such settings, uncertain fault geometry at depth limits understanding of how effectively a bend arrests earthquake ruptures. Here we demonstrate fault imaging within a major restraining bend along the Altyn Tagh Fault of western China using the magnetotelluric (MT) method. The new MT data were collected along two profiles across the Aksay restraining double bend, which is bounded by two subparallel strands of the Altyn Tagh Fault: Northern (NATF) and Southern (SATF). Both two-dimensional (2D) and three-dimensional (3D) inversion models show that the Aksay bend may be the center of a positive flower structure, imaged as a high-resistivity body extending to a ~ 40 km depth, and bounded by subvertical resistivity discontinuities corresponding to the NATF and SATF. In the western section of the Aksay bend, both the NATF and SATF show similar low-resistivity structure, whereas in the eastern part of the bend, the low-resistivity anomaly below the SATF is wider and more prominent than that below the NATF. This observation indicates that the SATF shear zone may be wider and host more fluid than the NATF, lending structural support to the contention that fault slip at depth is asymmetrically focused on the SATF, even though surface slip is focused on the NATF. A south-dipping, low-resistivity interface branching upward from the SATF towards the NATF indicates a fault link between these strands at depth.

Advances in alternating electromagnetic field data processing for earthquake monitoring in China

The alternating electromagnetic (EM) field is one of the most sensitive physical fields related to earthquakes. There have been a number of publications reporting EM anomalies associated with earthquakes. With increasing applications and research of artificial-source extremely low frequency EM and satellite EM technologies in earthquake studies, the amount of observed data from the alternating EM method increases rapidly and exponentially, so it is imperative to develop suitable and effective methods for processing and analyzing the influx of big data. This paper presents research on the self-adaptive filter and wavelet techniques and their applications to analyzing EM data obtained from ground measurements and satellite observations, respectively. Analysis results show that the self-adaptive filter method can identify both natural- and artificial-source EM signals, and enhance the ratio between signal and noise of EM field spectra, apparent resistivity, and others. The wavelet analysis is capable of detecting possible correlation between EM anomalies and seismic events. These techniques are effective in processing and analyzing massive data obtained from EM observations.

Three-dimensional magnetotelluric responses for arbitrary electrically anisotropic media and a practical application: Anisotropic media and a practical application

Electrical anisotropy in earth media increases the complexity of magnetotelluric responses. Magnetotelluric models based on anisotropic media must be developed to fully understand observed data. This paper presents a three-dimensional algorithm for calculating magnetotelluric responses of arbitrary anisotropic media in the frequency domain. Using a staggered-grid finite difference method, the model space is discretized into rectangular blocks with electric fields on the edges of each block and the magnetic fields normal to the faces of each block. The electric field Helmholtz vector equation that considers a full 3 × 3 conductivity tensor is calculated numerically under two orthogonal polarizations. In calculating the boundary values on the four sides of the three-dimensional anisotropic model, we adopt different procedures for calculating the two-dimensional responses on the sides in the x and y directions. The responses for a layered anisotropic model and a three-dimensional isotropic model calculated with this algorithm are compared with the corresponding analytical and numerical solutions, respectively. The comparisons show that the algorithm’s approximations are highly precise for a wide frequency band. A typical two-dimensional anisotropic model and a general three-dimensional anisotropic model were also constructed, and their responses were calculated. These anisotropic models have ordinary structures but can produce phase rolling out of quadrant magnetotelluric responses, which indicates that considering electrical anisotropy may improve our interpretation of observed data. Using this algorithm, we can model the observed data from the northern Qaidam Basin in northern Tibet, where ultrahighpressure metamorphic rocks are exposed along an old suture, and seismic anisotropy was indicated in neighbouring areas. The phase tensors of the magnetotelluric sites at this location show large skew angles, and the corresponding phase splits are distinct in the off-diagonal impedance elements. Although the isotropic three-dimensional electrical structure can model the profile data well, the structure shows a sequence of conductive and resistive bodies in the mid-lower crust of the north Qaidam Basin, which is very spatially inhomogeneous, and a simple intrinsic anisotropic body can also produce similar surficial responses. Using the three-dimensional anisotropic algorithm, we found an equivalent anisotropic replacement for this area. The results of the three-dimensional anisotropy modelling of the magnetotelluric data from the northern Tibetan Plateau show the valuable applicability of the threedimensional anisotropic algorithm in testing the qualitative presumption of electrical anisotropy.