【摘要】 利用2007年8月1日至2013年7月21日发生在鄂尔多斯块体周缘的8499个地震的49844个P波初动符号资料,应用综合震源机制解法获得了鄂尔多斯块体周缘0.25°×0.25°的精细地壳应力场,所得应力场结果基本上覆盖了整个鄂尔多斯周缘地区.研究结果表明鄂尔多斯周缘地壳应力场具有以下特征:(1)在环绕鄂尔多斯周缘的银川—吉兰泰断陷带、河套断陷带、岱海断陷带、山西断陷带和渭河断陷带内,综合震源机制解结果以正断层型为主,且综合震源机制解节面走向大体与控制断陷带边界的主要断裂走向相一致,与鄂尔多斯周缘断陷带现今的拉张状态相一致.(2)在鄂尔多斯西南缘,综合震源机制解类型主要为逆冲、逆冲走滑和走滑型,反映了鄂尔多斯块体在西南缘受到青藏高原北东向挤压作用.鄂尔多斯西南缘的应力场的主压应力方向在远处为东向,源自于青藏高原向东北挤压作用,靠近鄂尔多斯块体表现为北东—南西向.(3)P轴方位在局部地区变化较大,但总体呈现规律性变化.P轴方位在鄂尔多斯块体西缘,从南向北,主压应力轴方位更加偏北;在其北缘,由西向东,主压应力轴方位更加偏东.在其南缘和东缘,主压应力轴方位变化不大,大体上平行于控制各断陷带主要断裂走向.P轴倾角在西南缘为近水平,在其周缘各盆地内P轴倾角近直立.(4)T轴方位总体表现为北西—南东向;在鄂尔多斯周缘各断陷带内,T轴走向大体与控制断陷带主要断裂走向以及断陷盆地走向相垂直.(5)鄂尔多斯块体在其西南角受到来自青藏高原的北东向挤压和其东北角深部物质上涌形成的北西—南东向拉张力联合作用,上述作用使得鄂尔多斯块体周缘地区除西南区为挤压区外,其余区域均为剪切拉张区,与先前研究认为鄂尔多斯周缘地区处于引张应力场作用相符合,较好地解释了环鄂尔多斯周缘的断陷盆地构造,亦符合鄂尔多斯块体东西两侧的右旋剪切拉张带以及南北两侧的左旋剪切拉张带的认识.更多还原

【Abstract】 With more and more densely seismic stations deployed,a large number of P wave firstmotion data are observed,which provides us an opportunity to further study the tectonic stress field.In this paper,by using the 49844 Pwave first motion polarity data from 8499 earthquakes,which occurred in and around the Ordos block from August 1,2007 to July 21,2013,we obtained the precise and detailed stress field in the study area.The composite focal mechanism method was used in this study.By fitting the focal mechanism radiation pattern with P wave polarity data from a large number of earthquakes,this method can obtain the composite focal mechanism in specific grid,and P,B,Taxes can be regarded as the principal stress axes of the tectonic stress field.The grid search method is used to fit the focal mechanism radiation pattern,the focal mechanism which has the minimum contradictory ratio was regarded as the optimal solution.The inconsistent ratio is the ratio of the number of P wave first motion polarity which is inconsistent with the focal mechanism and the total number of P wave first motion polarity data,which is weighted by the distance of the earthquake epicenter to the grid point.The study results show that the stress field has the following characteristics:First,in the rift zone of the Yinchuan-Jilantai,Hetao,Daihai,Shanxi,and Weihe around the Ordos block,the main 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 the Ordos block.Second,the main types of the composite focal mechanism are thrust,thrust-strike-slip and strike-slip types in the southwest boundary of the 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 may derived from the northeastward squeezing of the Qinghai-Tibetan plateau,the principal compressive stress direction turns to NE-SW direction when near the Ordos block.Third,the Paxis azimuth shows generally regular changes except in some local areas.From south to north,the P-axis azimuth becomes 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 in the southwest boundary of the 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 the Ordos block.Fifth,under the NE pushing force from the 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,also better explain the fault basins surrounding the Ordos block,also accord 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.Overall,we obtained the stress field around the Ordos block,and its general characteristics are consistent with the results of previous study,our results are more consistent with the basin structure around the Ordos block.In this study we focus on the general characteristics of our results.In the further study,we will pay more attention on the inconsistency of our study with the previous study,try to find out the reason and give a reasonable explanation;and pay more attention on the phenomenon of sudden changes in the direction of the stress axis,try to find whether it caused by the inhomogeneity of stress field,or the complex geological structure,or other reasons.更多还原