Small earthquakes( Ms > 2.0) have been recorded from 1970 to the present day and reveal a significant difference in seismicity between the stable Ordos block and its active surrounding area.The southern Ordos block...Small earthquakes( Ms > 2.0) have been recorded from 1970 to the present day and reveal a significant difference in seismicity between the stable Ordos block and its active surrounding area.The southern Ordos block is a conspicuous small earthquake belt clustered and isolated along the NNW direction and extends to the inner stable Ordos block;no active fault can match this small earthquake cluster.In this paper,we analyze the dynamic mechanism of this small earthquake cluster based on the GPS velocity field( from 1999 to2007),which are mainly from Crustal Movement Observation Network of China( CMONOC) with respect to the north and south China blocks.The principal direction of strain rate field,the expansion ratefield,the maximum shear strain rate,and the rotation rate were constrained using the GPS velocity field.The results show that the velocity field,which is bounded by the small earthquake cluster from Tongchuan to Weinan,differs from the strain rate field,and the crustal deformation is left-lateral shear.This left-lateral shear belt not only spatially coincides with the Neo-tectonic belt in the Weihe Basin but also with the NNW small earthquake cluster( the Ziwuling small earthquake cluster).Based on these studies,we speculate that the NNW small earthquake cluster is caused by left-lateral shear slip,which is prone to strain accumulation.When the strain releases along the weak zone of structure,small earthquakes diffuse within its upper crust.The maximum principal compression stress direction changed from NE-SW to NEE-SWW,and the former reverse faults in the southwestern margin of the Ordos block became a left-lateral strike slip due to readjustment of the tectonic stress field after the middle Pleistocene.The NNW Neo-tectonic belt in the Weihe Basin,the different movement character of the inner Weihe Basin( which was demonstrated through GPS measurements) and the small earthquake cluster belt reflect the activated southern margin of the Ordos block,which was generated through readjustment of the tectonic stress field after the middle Pleistocene.展开更多
In this article,we relocated the seismic source location of the earthquakes in the Muli area of Sichuan,inverted the focal mechanism of the larger earthquakes and analyzed the relationship between the water level of t...In this article,we relocated the seismic source location of the earthquakes in the Muli area of Sichuan,inverted the focal mechanism of the larger earthquakes and analyzed the relationship between the water level of the Jinping reservoir and the frequency of the earthquake swarm. The results show that:( 1) The epicenters of the relocated small earthquake swarms are distributed in a seismic zone,and the earthquake focal depths were in the range of 0- 12 km.( 2) By analyzing the earthquake swarm spatial distribution,we found that the swarms were generated by one branch fault on the west of Xiaojinhe fault.( 3) The focal mechanism of the three earthquakes with magnitude greater than 4. 0 is significantly different,with the shallow source thrust events affected by vertical stress,and the strike-slip events are related to regional stress tectonic activity.展开更多
基金supported by the Program of the National Natural Science Foundation of China(41174083)the Research Fund of China Earthquake Administration(201208009)
文摘Small earthquakes( Ms > 2.0) have been recorded from 1970 to the present day and reveal a significant difference in seismicity between the stable Ordos block and its active surrounding area.The southern Ordos block is a conspicuous small earthquake belt clustered and isolated along the NNW direction and extends to the inner stable Ordos block;no active fault can match this small earthquake cluster.In this paper,we analyze the dynamic mechanism of this small earthquake cluster based on the GPS velocity field( from 1999 to2007),which are mainly from Crustal Movement Observation Network of China( CMONOC) with respect to the north and south China blocks.The principal direction of strain rate field,the expansion ratefield,the maximum shear strain rate,and the rotation rate were constrained using the GPS velocity field.The results show that the velocity field,which is bounded by the small earthquake cluster from Tongchuan to Weinan,differs from the strain rate field,and the crustal deformation is left-lateral shear.This left-lateral shear belt not only spatially coincides with the Neo-tectonic belt in the Weihe Basin but also with the NNW small earthquake cluster( the Ziwuling small earthquake cluster).Based on these studies,we speculate that the NNW small earthquake cluster is caused by left-lateral shear slip,which is prone to strain accumulation.When the strain releases along the weak zone of structure,small earthquakes diffuse within its upper crust.The maximum principal compression stress direction changed from NE-SW to NEE-SWW,and the former reverse faults in the southwestern margin of the Ordos block became a left-lateral strike slip due to readjustment of the tectonic stress field after the middle Pleistocene.The NNW Neo-tectonic belt in the Weihe Basin,the different movement character of the inner Weihe Basin( which was demonstrated through GPS measurements) and the small earthquake cluster belt reflect the activated southern margin of the Ordos block,which was generated through readjustment of the tectonic stress field after the middle Pleistocene.
基金sponsored by the Regular Project of Earthquake Monitoring and Prediction in 2016(16C23ZX327)
文摘In this article,we relocated the seismic source location of the earthquakes in the Muli area of Sichuan,inverted the focal mechanism of the larger earthquakes and analyzed the relationship between the water level of the Jinping reservoir and the frequency of the earthquake swarm. The results show that:( 1) The epicenters of the relocated small earthquake swarms are distributed in a seismic zone,and the earthquake focal depths were in the range of 0- 12 km.( 2) By analyzing the earthquake swarm spatial distribution,we found that the swarms were generated by one branch fault on the west of Xiaojinhe fault.( 3) The focal mechanism of the three earthquakes with magnitude greater than 4. 0 is significantly different,with the shallow source thrust events affected by vertical stress,and the strike-slip events are related to regional stress tectonic activity.
