Study On The Crustal Stress Field And The Fault Plane Parameter Of Circum-Ordos Block Region

In this paper, we present a detailed study for the stress and fault, the two "Key issues" of earthquake preparation and prediction, in the Circum-Ordos block region. In the past, there were frequently strong earthquake around the Circum-Ordos area, and the normal fault system over there is quite distinctive characteristic. The main researches include three following parts: first, calculating the stress field in the Circum-Ordos by applying composite focal mechanism method on the data of P wave first motion; second, relocating the earthquake cluster in the Circum-Ordos region using the Double Difference Algorithm; third, analyzing the fault plane parameter of 1989 Datong-Yanggao earthquake and the 1303 Hongtong earthquake of Shanxi Province, by applying the sieismic fault plane with small earthquakes based on the relocation results. The details are presented respectively in the below.We calculate the precise and detailed stress field in the Circum-Ordos block region by using the composite focal mechanism method, using the polarity data of 49844 P wave first motion from 8499 earthquakes, which occurred between August 1,2007 to July 21,2013. The results show that the stress field has five distinctive characteristics:first, in the rift zone of Yinchuan-Jilantai, Hetao, Daihai, Shanxi, and Weihe around the Ordos block, the dominant types of composite focal mechanism are normal types, and the strike of the nodal planes is consistent with the strike of the main fault which controls the boundary of the rift zone, that is, in accord with the extension regime in the rift zone around Ordos block; second, the dominant types of the composite focal mechanism are thrust, thrust-strike-slip, and strike-slip types in the southwest boundary of Ordos block, which reflects that the Ordos block is subjected to the northeastward squeezing of the Qinghai-Tibetan plateau. The principal compressive stress direction is eastward in the distance from the southwest boundary of Ordos block, that is originated from the northeastward squeezing of the Qinghai-Tibetan plateau, and the principal compressive stress direction turns to NE-SW when near the Ordos block; third, the P-axis azimuth shows generally regular changes except in some local areas. From south to north, the P-axis azimuth turns more to the north direction in the west boundary of the Ordos block. From west to east, the P-axis turns more to the east direction in the north boundary of the Ordos block. In the south and east boundary of the Ordos block, the P-axis azimuth has small change and is parallel to the main fault’s strike which controls the boundary of the rift zone. The dip angle of the P-axis is nearly horizontal near the southwest boundary of Ordos block, and nearly vertical in the basins around the Ordos block; fourth, the T-axis azimuth is NW-SE overall. The T-axis is generally perpendicular to the strike of the main fault, which controls the boundary of the rift zone and the strike of the basin in the basins around Ordos block; fifth, under the NE pushing force from Qinghai-Tibetan plateau and the NW-SE extension caused by the uplifting of the deep substance, the regions around the Ordos block are shear zones with extensional components, except for the compressional region in the southwest of Ordos-block; these results are consistent with the previous studies which suggest that the Circum-Ordos block is in extensional stress field, and better explain the fault basins surrounding the Ordos block, as well as being consistent with the formation of right-lateral shear zones with extension components in its western and eastern boundary and left-lateral shear zones with extensional components in its southern and northern boundary.We split the margin of Ordos into five areas to analyze, the northern margin of Ordos, the eastern margin of Orods, the southern margin of Ordos, the southwestern margin of Ordos and the western margin of Ordos. Then we use the Double Difference Algorithm to study the relocation of the earthquake cluster in each area. From the study, we find the result of eastern margin of Orods’s relocation only can cover the whole analyzing area, the results from the rest of the areas just have the relocation result for local area and can not reflect the characteristics of seismic activity throughout the whole area. Therefore, we only give the summary and conclusion on the eastern margin of the earthquake relocation result of Ordos block. From the relocation result of Shanxi fault subsidence zone, we have four following conclusions:first, the average root mean square residual is greatly decreased after relocation, reflecting that the Double Difference Algorithm improves the accuracy of the location results; second, the spatial distribution of epicenter is more concentrated to fault nearby and its banded distribution becomes more apparent after relocation; third, the focal depth of the south is relatively deeper than the north, and the focal depth are within 25 km and generally showing a normal distribution, which suggests that the seismogenic layer lies in the upper crust in Shanxi fault subsidence zone; fourth, the focal depth in the basin is relatively shallow, and gradually deepened to the ends of the basin, the focal depth changes with the structural features of the fault basin zone.Finally, based on the assumptions that earthquakes often occur on the fault plane and its vicinitiy area as well as the "Long-term Activity" of large earthquake’s seismic area, we determined the fault plane parameters of 1989 Datong-Yanggao earthquake and the 1303 Hongtong earthquake of Shanxi Province, by using the results of the relocated earthquake clusters and the tectonic stress field obtained in the precious section about the Circum-Ordos block region. The fault plane parameters of 1989 Datong-Yanggao earthquake are as follows:the strike is 199°, dips to the northwest, the dip angle is 64°, and the slip angle is -128°, these show the seismic fault is a right-lateral strike slip normal fault. The fault plane parameters of the 1303 Hongtong earthquake are as follows:the strike is 17°, dips to the southeast, the dip angle is 84°, and the slip angle is -176°, these show the seismic fault is mainly right-lateral strike slip with few normal component.