Abstract:

The Longmenshan fault zone, situated along the eastern margin of the Tibetan plateau, represents a significant thrust tectonic belt characterized by pronounced segmentation. It is delineated into northern and central-southern segments at Beichuan, and along its depth, it features three sub-parallel fault belts: the Houshan fault, the Central fault, and the Qianshan fault, extending from the northwest to the southeast. Geological research indicates that since the Quaternary, the central-southern segments of the Longmenshan fault zone have exhibited considerable seismic activity, whereas the northern segment has shown minimal signs of movement. However, paleo-earthquake studies have identified substantial historical seismic events in the Qingchuan fault, a component of the northern segment, dating back to the Holocene. The devastating 2008 Wenchuan earthquake(M_S8.0), which occurred in the middle section of the Longmenshan fault zone, resulted in a 240-km-long surface rupture along the Central fault. A multitude of aftershocks radiated northward from the epicenter, with no discernible surface ruptures observed in the northern segment. This study aims to provide a comprehensive analysis of the kinematic features of the northern segment by re-evaluating the Wenchuan earthquake's aftershocks and employing focal mechanisms derived from previous studies.
Seismic activity is intrinsically linked to active tectonics, and the precise localization of minor earthquakes can offer critical insights into the underlying seismogenic processes and mechanisms. In this paper, we have compiled early aftershock relocation data and further refined the relocation of small earthquakes using an integrated seismic location technique. Seismic phase data were obtained from the networks in Sichuan, Gansu, and Shaanxi over the past decade, spanning from 2010 to 2020. To mitigate the impact of crustal velocity variations, an optimal one-dimensional velocity model for the study area was initially inverted using the VELEST program. The Hypo2000 program was then utilized to adjust the initial seismic source positions, followed by the application of the double-difference method for the relocation of minor earthquakes. The reliability of the localization outcomes, determined using the LSQR method, was verified by the SVD method. Consequently, 10 653 minor earthquakes were relocated with an average travel time residual of 0.053s, a horizontal location error of 281m, and a vertical location error of 260m.
In the southern extremity of the study area, the relocated earthquakes are predominantly aligned along the parallel faults flanking the primary rupture zone. In the south-central region, the relocated earthquakes exhibit deviations from the rupture zone, revealing multiple seismic clusters. Towards the northern end, the relocated earthquakes demonstrate a migration from the main rupture towards the Qingchuan fault. The depth profiling of seismic sources reveals that the relocated earthquakes are concentrated between 8-15km deep, all situated above the 500℃ isothermal surface. The depth profile in the southern region continues the characteristics of the main rupture surface of the Wenchuan earthquake, while the dip angle becomes increasingly steep as it progresses northward. The northern end's depth profile suggests an interaction between the rupture surface and the Qingchuan fault. Additionally, the analysis of 32 focal mechanisms exceeding M_L4.0 within the study area corroborates the geometrical structures of the fault zone, as revealed by the spatial distribution of the relocated earthquakes, further validating the reliability of relocation.
A comprehensive analysis suggests that the current seismicity in the northern section of the Longmenshan fault zone is multifaceted, with ongoing activity on the main rupture surface(afterslip)and slip on secondary new rupture surfaces triggered by the mainshock. It is hypothesized that the spatial distribution of the relocated earthquakes retains segmented characteristics. In the southern region of the study area, thrust slip induced by the main rupture continues; in the middle region, new ruptures are concurrently active with the main rupture; and in the northern region, influenced by the high velocity of the upper crust around Ningqiang-Mianxian, the rupture zone vanishes at the surface, with the deep triggering of the Qingchuan fault by stress transfer being evident. In conclusion, the complex spatial characteristics of the current seismic activity in the northern section of the Longmenshan are attributed to the interplay of pre-existing faults, new ruptures, and the main rupture, reflecting the spatially heterogeneous process of stress transfer and adjustment following the Wenchuan earthquake, potentially linked to the complex geological structure of the region.

Key words: Northeastern segment of Longmenshan fault zone, earthquake relocation, seismicity characteristic, spatial distribution, faults interaction

摘要：

文中基于中国测震台网统一地震编目网2010年1月—2020年6月的正式地震观测报告反演了龙门山断裂带北东段的最优一维速度结构, 并矫正了初始震源位置, 在此基础上开展了小地震精定位工作, 结合汶川 M_S8.0 地震早期余震精定位结果和震源机制解资料综合分析了龙门山断裂带北东段现今的地震活动特征。结果显示: 南坝地区(S区)小地震平行于主破裂带分布; 中段(M区)小地震偏离主破裂带分布, 在两侧分别形成了丛集区M2和M3; 北段青川附近(N区)地震沿主破裂带和青川断裂密集分布(N1区), 地震活动强度较大, 且沿主破裂带与青川断裂震源深度剖面特征有所不同。推测研究区南端S区的小地震空间分布延续了汶川地震主破裂的特征; 北端N区汶川地震破裂受到宁强—勉县一带上地壳高速体的阻挡, 地表破裂带消失, 应力向深部传导, 对青川断裂运动的触发作用明显。综上分析认为, 先存断层、 新生破裂与主破裂共同作用导致了龙门山北东段现今地震活动复杂的空间特征, 暗示了汶川地震后应力传递和调整过程的空间不均匀性, 这可能与北段复杂的地质结构有关。

关键词: 龙门山断裂带北东段, 重定位, 地震空间分布, 断层相互作用