Automatic Identification Of Major Faults And Their Stress States In The Tangshan Area

To investigate the primary fault structures and their associated stress states within the Tangshan area,this study is divided into two parts.Firstly,the main faults in the region are automatically identified based on the spatial distribution of earthquakes,and their fault parameters are determined.Subsequently,using focal mechanism solution data specifically acquired from the identified faults,the stress field at each fault and the overall regional stress field are inferred.These investigations furnish fundamental data for regional dynamic analysis,seismogenic environment characterization,and studies related to seismic hazard within the Tangshan area.To automatically discern the principal faults and acquire fault parameters in the Tangshan area,this paper introduces an automated fault identification technique based on an enhanced DBSCAN algorithm.The proposed method enhances the unsupervised clustering technique,DBSCAN,by adapting it to the dataset characteristics,thereby enabling the automatic selection of optimal clustering parameters and the identification of fault segments.Additionally,fault parameters are determined by employing a combination of the Simulated Annealed(SA)global search method and the Gaussian Newton(GN)local search method.Subsequently,adjacent fault segments exhibiting similarities are merged,ultimately providing a comprehensive set of faults and their corresponding parameters.This paper rigorously validates the reliability of the proposed method using synthetic data and successfully applies it to the Tangshan area in China.Building upon the acquisition of the primary fracture structures in the Tangshan area,the stress field at each fracture is calculated using the grid search method.To facilitate comparison with previous research outcomes,this paper also computes the tectonic stress field within the Tangshan area.Moreover,the impact of the 2011 Mw9.0 Tohoku-Oki earthquake in Japan on the tectonic stress field in North China is taken into consideration to further analyze the stress field alterations within the Tangshan area and at the major active ruptures before and after the Tohoku-Oki earthquake in Japan.Based on the aforementioned study,this paper yields the following results: I.The improved DBSCAN algorithm effectively identifies fault segments,a validation accomplished through its application to synthetic data and the double-difference location data of the Tangshan area.II.Utilizing the improved DBSCAN algorithm with the double-difference location data of the Tangshan area,eight fault segments are identified.The specific names of these faults are as follows: Douhe Fault Segment,Weishan-Fengnan Fault Segment,Luanxian-Laoting Fault Segment,Lulong Fault Segment,Xujialou-Wangxizhuang Fault Segment,Luanxian Fault North Segment,Leizhuang Fault Segment,and Chenguantun Fault Segment.The fault parameters for the first five faults align with previous research findings,while the last three faults represent novel identifications derived from the earthquake catalog in this study.III.The current regional stress state within the Tangshan area corresponds to a strike-slip stress field,characterized by a P-axis NEE-SWW direction and a T-axis near NS direction or NNW-SSE direction,featuring R-values ranging from 0.0 to 0.1.These findings align with previous studies,suggesting that the Tangshan area has adjusted to the stress field prior to the Mw7.8Tangshan earthquake,exhibiting a tensile stress state.IV.The stress state of the Douhe Fault Segment and the Weishan-Fengnan Fault Segment signifies a strike-slip stress field.Specifically,the stress field at the Douhe Fault segment exhibits a P-axis NEE-SWW direction,a T-axis NNW-SSE direction,and an R-value of 0.1,while the stress field at the Weishan-Fengnan Fault Segment demonstrates a P-axis near EW direction,a T-axis near NS direction,and an R-value of 0.1.The main directions of compressive stress mentioned above are consistent with the findings of related studies,such as the primary strain directions in the respective regions and the fast wave polarization directions of shear wave splitting.V.The occurrence of the 2011 Mw9.0 Tohoku-Oki earthquake in Japan resulted in a minor clockwise rotation of the overall tectonic stress field within the Tangshan area.