Calculation Of Co-seismic Horizontal Shortening Amount

It is important to obtain co-seismic slips and their distribution for understanding the focal mechanism of rupture process, forecasting the likely size of the future earthquakes on the active faults, and delimiting the width of set-back zone in engineering construction.Co-seismic slip vector consists of three components: strike slip, vertical offset and horizontal shortening (or extension). A reasonable estimation of all three components is essential to understanding the co-seismic surface deformation. Compared with research of vertical and strike slip, there are few studies on horizontal shortening, because it is very difficult to estimate the co-seismic horizontal shortening amount and its result has big uncertainties.The May 12th, 2008 Wenchuan earthquake (Ms7.9) in Sichuan province led to severe casualties and a huge economic loss. Field investigations show the Wenchuan earthquake ruptured two NW-dipping imbricate reverse faults along the LonmengShan fault zone at the eastern margin of the Tibetan Plateau. This earthquake generated a 250km long surface rupture along the Beichuan-Yingxiu fault characterized by right-lateral oblique motion and a 90km long surface rupture along the Guanxian-Jiangyou fault characterized by dip-slip reverse faulting. This co-seismic surface rupture pattern, involving multiple structures, is among the most complicated events of recent great earthquakes. Its surface length is the longest among the co-seismic surface ruptures zones for reverse faulting events ever reported. After the Wenchuan earthquake, through field investigations the surface rupture data of co-seismic displacement have been obtained, most of which however are of strike slip and vertical offsets, with a little about horizontal shortening. The horizontal shortening amounts were determined by measuring and recovering deformed natural and man-made references near the surface ruptures. Though the regional co-seismic displacement field has been derived from GPS and InSAR data, the interferential data of near surface ruptures region from InSAR are not perfect because of the steeply terrain of the LongmenShan. And because the GPS observational stations are sparse, only far field displacement can be obtained. Many researchers obtained the regional co-seismic displacement field by inversion, which is based on the InSAR and GPS date or seismic waves. But this method cannot reveal displacements nicety, because there are many limiting factors. Otherwise, the subpixel correlation method by pre- and post- earthquake high resolutions images was a new tool for research on the regional co-seismic displacement field. And in many earthquake researches, the method is employed, but which is difficult to compare with field investigation.This work selects the surface rupture of Wenchuan earthquake as an example. Baishahe rupture section, located at the southern end of the rupture caused by the Wenchuan earthquake, extends 14km along the Baishahe River, 11km north of Dujiangyan City. This rupture section presents complicated geometric features, and is composed of several segments. Almost each segment has different strikes (from 0°to 90°), and overall the section has an average strike of 50°. Co-seismic slip along the rupture section also shows complexity and multiformity. Up-thrust of the northwest wall is the main vertical co-seismic slip, and the maximum height of the fault flexure scarp is 6.5m, but local backward flexure scarps exist there; the right-slip is the main co-seismic strike slip, and the maximum right-slip offset is 4.5m, but local left-slip offset also appears. Although there are a lot of phenomena about horizontal shortening, only few horizontal shortening amounts are obtained with some values were conjecture from local persons’words. In this thesis, by the research of surface rupture in the Baishahe segment, I discuss the co-seismic horizontal shortening problems. Based on the analysis of surface rupture of the Wenchuan earthquake, by the research of displacement on the Baishahe segment, I suggest two methods and restrictive conditions for horizontal shortening calculation. One method is measuring and recovering deformed natural and man-made references near the surface ruptures. The other is based on the high resolution satellite images. With determining the geometry of characteristic objects from the high resolution satellite images and field investigations, by comparing these geometries of pre- and post- earthquake, I calculate co-seismic horizontal shortenings. On the either side of the Baishae segment surface ruptures, there are two roads which serve as be perfect characteristic objects. I extract the geometry of the two roads from pre- and post- earthquake Quick Bird satellite images. And in combination with the field investigations, by comparing the spaces of the two roads of pre- and post- earthquake, I calculate co-seismic horizontal shortenings of this area.According to this thesis, I obtain conclusions as follows:(1) By measuring and recovering deformed man-made reference surfaces, this work obtains four co-seismic horizontal shortenings: 3.37m in Shenxigou, 1.44m and 0.36m in Gaoyuancun, and 0.15m in Zhongping. By measuring and recovering deformed natural reference surfaces, I obtain four co-seismic horizontal shortenings: 2.4m and 2.49m in Shenxigou, 1.2m in Tongmakan, and 1.49m in Gaoyuancun.(2) By comparing the spaces between two roads, which are extracted from pre- and post- earthquake satellite images, I obtain a series of co-seismic horizontal shortening amounts, which normal to the average strike. The average, maximum, minimum and errors are 6.6m, 11m, 1.1m and 1.84m, respectively. By comparing the spaces between two roads, which are extracted from pre-earthquake satellite images and field measurements, I obtain a series of co-seismic horizontal shortening amounts, which are perpendicular to the average strike. The average, maximum, minimum and errors are 7.1m, 9.85m, 3.37m and 1.3m, respectively. In the compared regions, the contrast value of the averages of the two methods is 0.3m. It implies that the error analysis of this work is reasonable in another view.(2) The northeastward downtrend of horizontal shortening along the Banshee segment is consistent with the strike slip and vertical offset measured by other researchers.(3) High co-seismic horizontal shortening amount is also found to be related with low vertical offset, and vice versa, which might be explained by the change of dip angle of the rupture plane: higher angle at two ends of the Banshee segment, and lower angle in the middle.(4) The co-seismic horizontal shortening amount in 300-900m is larger than that in 10-100m, suggesting that the co-seismic horizontal shortening should be distributed in a wider area, including the immeasurable permeated deformation far from the rupture and the measurable near the rupture.