Study On The Entropy Rate Of Crustal Deformation In The Near-source Area Before The Ms8.0Wenchuan Earthquake

At2:28p.m. on May12,2008, a strong devastating Ms8.0earthquake took place in Wenchuan, Sichuan province, China. It generated about330kilometers long ruptures along the central fault and front fault, with vertical offsets of4to6meters and right-lateral horizontal offsets of3to5meters. This earthquake attracts wide attention of global scholars for its low sliding velocity, long recurrence interval, large magnitude, huge damage, inland occurrence, right-lateral and high-angle thrusting pattern.The preseismic deformation feature of Ms8.0Wenchuan earthquake is different from other same-type great earthquakes. According to the study of historical earthquakes, the precursory anomaly, such as variety, intensity, scope et al, will extend with the increase of magnitude, i.e. there supposes to be conspicuous pre-seismic anomalous deformation. There are several deformation observation methods before it occurred around the hypocentral region, and some stations observed some middle-term and long-term anomalous deformation, however, it still did not observe short-term anomalous deformation.This paper will process a mass of observation data in near-field area, and apply entropy theory to analysis its entropy rate, and simulate the tilts change caused by the fault slip propagation. Then integrate with the previous research achievements of geological background, regional tectonic evolution, geodynamical and kinematic characteristic in Longmenshan fault zone to explain the pre-seismic deformation distribution in near-field area.This paper’s main research efforts and conclusions include following parts:1. The interpretation of information entropy is a further development of entropy theory, it solves the problem of quantitative description of information. The nondeterminacy or randomness of nature science and social science can be described by information entropy. It is also widely used in the study of earthquake.This paper introduces deformation entropy to study the mechanism of the pre-seismic deformation.2. Collecting pre-earthquake deformation observation data, such as leveling across fault and tilts. Then studying the similarity and dissimilarity indicated by the observational data, and the of temporal and spatial distribution characteristics.3. Collecting the relevant literatures of the Longmenshan fault zone, and having a good knowledge of the geologic structure, tectonic evolution, geodynamical and kinematic background. The Longmenshan fault zone is consist of a series of roughly paralleled imbricate thrust belt, extending from NE to SW with a "Piggyback Type" spreading type, and located in the North-South seismic belt, the junction of Tibetan Plateau and Sichuan Basin. According to the quaternary activity characteristics of Longmenshan fault zone, it can be divided into NE and SW parts. The elevation variation of SW segment is quite large, while the NE segment’s is tiny. Tibetan Plateau continues to uplift because the Indian Plate pushes northwards and the Siberian rigid block resists the Tibetan Plateau. Meanwhile the inter-crustal temperature rises, and the inter-crustal substance moves eastwards and westwards. At eastern margin of Tibetan Plateau, the substance accumulates and moves southwards and northwards along the western margin of Sichuan basin because of the resistance of the rigid Sichuan basin. The author realizes that the geologic structure, geodynamical and kinematic characteristic of Longmenshan fault zone makes for the continuous accumulation of strain.4. Entropy theory calculation. Based on the seismic observational data, the author uses the MATLAB to write relevant codes. The codes’function includes:1) Calculating the entropy rate of all the observational data, saving the result data and plotting entropy diagram.2) Calculating the entropy rate in different window’s length and different step’s length, and obtaining the characteristic of entropy rate result and entropy rate diagram.3) Data fitting and residual entropy calculation.4) Calculating modulus entropy of EW trending and NS trending with tilt observational data.5) Calculating entropy in different dimension and summarize its varying pattern.6) Drafting the temporal and spatial distribution maps of the tilt entropy rate of each station.Then, this paper comprehensively analyzes the tilt entropy rate change and its spatial distribution feature in different stages of the Wenchuan earthquake.It indicates the wenchuan station located in epicenter shows a entropy rate reduction process four years before the earthquake, and reachs minimum level when the earthquake occured.The Yaan station and Emei station located in the southeast of the longmenshan fault zone also show the same entropy rate reduction process, but those stations located in far filed such as Maoxian, Songpan, Guzan, Kangding, Dingongjing, Malanshan show a entropy rate increase process before the earthquake.Based on the spatial distribution fearture, the entropy rate of Wenchuan stataion located in the epicentre is minimum, Yaan and Emei sataion take second place.5. Based on the previous research of the distribution characteristic of Longmenshan fault zone, this paper will use the two following methods to explain the temporal and spatial distribution of deformation entropy rate:1) Analyzing the variation of observational data’s variance and velocity.The variation of variance indicates the degree of deviating from the mean value. The large the variance is, the more diverging and out-of-order the date is, the smaller the entropy rate is. On the contrary, the smaller the variance is, the more concentrated the data is, the larger the entropy rate is.The variation of observational data’s velocity is difference processing of observational data. Then calculating the variation of velocity. The variation of velocity indicates the variation of tilt change, the smaller the expected value of tilt change is and the smaller the variance is, the smaller the entropy rate is.2) According to the previous studies of Longmenshan fault zone, the horizontal shortening of Longmenshan fault zone is very small, it is "locked".Therefore, this paper builds a reasonable model of fault sliding extension, the hanging wall and foot wall keep motionless, and there si a detachment fault blocks which extends under stress. Then this paper uses numerical simulation method of deformation modified by Frank press to simulate the tilt change during the slip extension.Then this paper synthetically considers the geologic feature of Longmenshan fault to analyze the pre-seismic spatial distribution of entropy rate.