Geometry and tectonic deformation of the seismogenic structure for the 8 August 2017 M_S 7.0 Jiuzhaigou earthquake sequence,northern Sichuan, China

To reveal the geometry of the seismogenic structure of the Aug. 8, 2017 M_S 7.0 Jiuzhaigou earthquake in northern Sichuan,data from the regional seismic network from the time of the main event to Oct. 31, 2017 were used to relocate the earthquake sequence by the tomoDD program, and the focal mechanism solutions and centroid depths of the M_L ≥ 3.5 events in the sequence were determined using the CAP waveform inversion method. Further, the segmental tectonic deformation characteristics of the seismogenic faults were analyzed preliminarily by using strain rosettes and areal strains(As). The results indicate:(1) The relocated M_S 7.0 Jiuzhaigou earthquake sequence displays a narrow ～ 38 km long NNW-SSE-trending zone between the NW-striking Tazang Fault and the nearly NSstriking Minjiang Fault, two branches of the East Kunlun Fault Zone. The spatial distribution of the sequence is narrow and deep for the southern segment, and relatively wide and shallow for the northern segment. The initial rupture depth of the mainshock is 12.5 km, the dominant depth range of the aftershock sequence is between 0 and 10 km with an average depth of 6.7 km. The mainshock epicenter is located in the middle of the aftershock region, showing a bilateral rupture behavior. The centroid depths of 32 M_L ≥ 3.5 events range from 3 to 12 km with a mean of about 7.3 km, consistent with the predominant focal depth of the whole sequence.(2) The geometric structure of the seismogenic fault on the southern section of the aftershock area(south of the mainshock) is relatively simple, with overall strike of ～150° and dip angle ～75°, but the dip angle and dip-orientation exhibit some variation along the segment. The seismogenic structure on the northern segment is more complicated; several faults, including the Minjiang Fault, may be responsible for the aftershock activities. The overall strike of this section is ～159° and dip angle is ～59°, illustrating a certain clockwise rotation and a smaller dip angle than the southern segment. The differences between the two segments demonstrate variation of the geometric structure along the seismogenic faults.(3) The focal mechanism solutions of 32 M_L ≥ 3.5 events in the earthquake sequence have obvious segmental characteristics. Strike-slip earthquakes are dominant on the southern segment, while 50% of events on the northern segment are thrusting and oblique thrusting earthquakes, revealing significant differences in the kinematic features of the seismogenic faults between the two segments.(4) The strain rosettes for the mainshock and the entire sequence of 31 M_L ≥ 3.5 aftershocks correspond to strike-slip type with NWW-SEE compressional white lobes and NNE-SSW extensional black lobes of nearly similar size. The strain rosette and As value of the entire sequence of 22 M_L ≥ 3.5 events on the southern segment are the same as those of the M_S 7.0 mainshock,indicating that the tectonic deformation here is strike-slip. However, the strain rosette of the entire sequence of 10 M_L ≥ 3.5 events on the northern segment show prominent white compressional lobes and small black extensional lobes, and the related As value is up to 0.52,indicating that the tectonic deformation of this segment is oblique thrusting with a certain strike-slip component. Differences between the two segments all reveal distinctly obvious segmental characteristics of the tectonic deformation of the seismogenic faults for the Jiuzhaigou earthquake sequence.

Characteristics of Late-Quaternary Activity and Seismic Risk of the Northeastern Section of the Longmenshan Fault Zone

Following the 2008 Wenchuan M8 earthquake,the seismic risk of the northeastern section of the Longmenshan fault zone and the adjacent Hanzhong basin has become an issue that receives much concern.It is facing,however,the problem of a lack of sufficient data because of little previous work in these regions.The northeastern section of the Longmenshan fault zone includes three major faults：the Qingchuan fault,Chaba-Lin＇ansi fault,and Liangshan south margin fault,with the Hanzhong basin at the northern end.This paper presents investigations of the geometry,motion nature,and activity ages of these three faults,and reveals that they are strike slip with normal faulting,with latest activity in the Late Pleistocene.It implies that this section of the Longmenshan fault zone has been in an extensional setting,probably associated with the influence of the Hanzhong basin.Through analysis of the tectonic relationship between the Longmenshan fault zone and the Hanzhong basin,this work verifies that the Qingchuan fault played an important role in the evolution of the Hanzhong basin,and further studies the evolution model of this basin.Finally,with consideration of the tectonic setting of the Longmenshan fault zone and the Hanzhong basin as well as seismicity of surrounding areas,this work suggests that this region has no tectonic conditions for great earthquakes and only potential strong events in the future.