Guoze Zhao

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.

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.

Transformation of XML Data Sources for Sequential Path Mining

In recent years XML has become one of the most promising ways to define semi-structured data. Data mining techniques devised for detecting interesting patterns from semi-structure data have also grown in popularity, but carrying out such techniques on XML data can be problematic due to its hierarchical structure. Therefore, it has become necessary to transform XML into flattened, path data, so as to enable data mining to be carried out efficiently. However, problems may arise when the XML tree needs to be reconstructed from the traversal path. There are currently many transformation techniques for XML data, many of which take advantage of its tree-like hierarchical structure; but most of these approaches do not allow the XML tree to be reconstructed from the traversal path. In this paper we propose a new approach to the transformation of XML data into path data. The new approach employs a 5 step transformation process along with a new ‘Postorder Sequencing’ method of traversing the XML tree. The proposed method, on the one hand, can be seen an efficient and effective way of transforming XML data into collections of paths, and on the other hand enables XML trees to be generated from the traversal paths.

Digitalizing Seismograms Using a Neighborhood Backtracking Method

In this paper, we present a new algorithm for tracing waves on seismograms and digitalising the waves into single vectors in the form of a time series data. The algorithm consists of two main components that will be used to handle the smooth and complicate cases, respectively. The underlying feature of the algorithm lies in a novel searching process based on examining the context of pixels to ascertain how the tracing moves forward. The algorithm has been evaluated on a limited number of samples and the result demonstrates its competence. The work presented can be regarded as an effort of developing a uniform earthquake archive covering both the historical and the digital device periods for future reassessment of the seismic hazard cross the world. The archive developed will serve as an effective means for discovering precursor of earthquakes by characterising the spectral seismograms and the source parameters of less active sources, whereby permitting comparative studies on earthquakes and development of prediction models.

A Fuzzy Inspired Approach to Seismic Anomaly Detection

In this work we investigate the use of a fuzzy inspired approach for anomaly detection in different electromagnetic sequential time series datasets. The method proposed consists of simple component methods that are aggregated in a serialized way to achieve anomaly detection. Each of the component methods adds an element towards anomaly detection, i.e. a smoothing filter removes any unwanted noise, an automated peak finding with Fast Fourier Transformation and correlation, reduces the dimensionality of the signal, a fuzzy inference system encodes the signal before the final comparison and its respective output. This method is evaluated on 6 benchmark datasets with promising results in terms of F-measure accuracy. The method is also evaluated over real datasets gathered from the SWARM satellites for the detection of possible anomalies in relation to seismic events. The preliminary experimental results also prove to be promising for the proposed method for the detection of anomalies in electromagnetic sequential time series datasets.