Preliminary Study On The Tectonic Stress Field In Henan And Its Adjacent Areas

Henan Province tectonically joins the North China plate and the South China plate.It is controlled tectonically by the faults with NNE and NW direction,and a series of depression and uplift zones has been evolved with active seismicity.It is of great significance to study the tectonic stress field and hence to promote the studies of geological activity and earthquake prediction in this area.In this study,by using composite focal mechanism technique and grid search stress determination method,stress fields in several sub-regions in Henan province are estimated and analyzed.By using the 5241 P wave first motion polarity data from 2008 to March2017 in Henan province and its adjacent areas(31°-37°,110°-117°),the composite focal mechanisms are determined in sub-regions of Yu-Lu-Ji,Jin-Ji-Yu,Jin-Shan-Yu and E-Yu-Wan from P first motion polarity of the earthquakes occurred in each subregion.The gross stress field in Henan Province and its adjacent areas shows NE compression and NW extension,which consistent with the overall stress field pattern in North China.Except for the consistence with the features of the overall tectonic field,the stress fields insub-regions have their distinct characteristics.Firstly,the compressive axes of the local stress field near the Anyangnan fault in Jin-Ji-Yu region rotates from NE to NEE or EW direction,with large plunge angle.The extensive stress axes gradually rotate form NW direction to NWW direction.The stress field is consistent with the sinistral and normal slip feature of the Anyangnan fault,which verifying our study strategy.Secondly,The stress field shows local tectonic disturbance near the Jiaxiang and Juye faults.This region is cut by series of faults with azimuths of NE and NW direction,which forming difference subsidence.The stress field disturbance may be caused by second-order division of the tectonic blocks.Thirdly,in the Linfen graben zone,which enclosed by the Luoyunshan frontal fault and Zhongtiaoshan southern piedmont fault,the stress field shows NS or NW compression and nearly vertical extension,which is significant difference with the background gross stress field in a larger region.Lastly,in the intersectant area of the Northern Qinling uplift and the Tongbaishan uplift,the stress field shows dominantly NW or nearly NS compression and extensional axes clockwise rotation from east part to west part,which is discordant with the background stress field.By using grid search stress determination technique from qualitative and quantitative data of active faults.The stress fields are estimated in the north,west and south of Henan.The stress fields are estimated in northern part of Henan shows NE compression and NW extension with their compressive and extensional stress axes being horizontal,which is consistent with the previous studies.The stress field in western part of Henan shows vertical compression and NNW extension.The difference between azimuth of local fault and primary fault nearby shows tectonically complex in this region,and hence the distinctive stress field from the overall stress field.The stress field in southern part of Henan shows NNE compression and NWW extension.By using two methods to study stress fields in Henan province and its adjacent areas,we can see our results are consistent with previous research,except for the control of the overall background tectonics,the stress fields insub-regions have their distinct characteristics.