Abstract:

The 2023 Jishishan M_S6.2 earthquake struck within the Linxia Basin along the eastern front of the Jishishan Mountains. This region is characterized by the Jishishan Fault thrusting over the Linxia Basin. However, the dip direction of the seismogenic fault remains debated, with arguments for both west- and east-dipping geometries. Faults near the epicenter include the East Margin Fault of the Jishishan Mountains(EJSF); in addition, the South Margin Fault(SLJF) and North Margin Fault(NLJF) of the Lajishan Mountains may extend southward toward the epicentral area. Active anticlines are also present. Consequently, determining whether the earthquake originated on a single fault(and which one)or involved rupture of multiple faults is an urgent and critical question. To address this, we investigated faults, mountain-basin geological sections, and earthquake-induced fissures near the epicenter. Integrating these observations with a more complete relocated earthquake catalog and five shallow-seismic profiles, we conducted a comprehensive analysis of the seismogenic fault and rupture mechanism of the 2023 Jishishan M_S6.2 earthquake. The results are as follows.
First, the faults near the epicenter include the EJSF, SLJF, and NLJF. The EJSF, situated along the eastern margin of the Jishishan Mountains, comprises multiple west-dipping reverse faults, with its most recent activity in the late Pleistocene to Holocene. The SLJF is an east-dipping reverse fault primarily north of the Yellow River and shows no discernible activity since the late Quaternary. The NLJF is a west-dipping reverse fault mainly developed north of the Yellow River; south of the river it is concealed beneath younger deposits. Its latest activity occurred primarily in the late Pleistocene.
Second, the Jishishan-Linxia Basin section shows early Paleozoic magmatic rocks thrusting over the Linxia Group. The Linxia Group dips overall SW at 10°~20°, locally up to 29°. Near the epicenter, an asymmetric anticline deforms the Linxia Group, with a steeper eastern limb-indicative of EJSF propagation into the basin. Overlying early-Middle Pleistocene deposits display minor folding, but deformation amplitudes are markedly weaker than within the Linxia Group.
Third, the meizoseismal zone exhibits diverse earthquake-induced fissures, including gravity, tectonic, and landslide-related fissures. Most tectonic fissures are narrow(<1cm), with maximum widths of ~5cm. They are concentrated at NWW(21%), NNW(30%), and NE(16.5%) within the EJSF's left-stepping zone, with predominant orientations matching the fault strikes. Over 50% of fissures exploit pre-existing bedrock weaknesses(faults, bedding, joints), while most others follow artificial discontinuities(e.g., road-embankment joints). Their preferential development along weak zones indicates these features result from ground shaking rather than primary fault rupture, further evidenced by mixed sinistral/dextral offsets lacking uniform sense.
By integrating fault mapping, mountain-basin sections, and shallow-seismic profiles, we infer that both the SLJF and NLJF terminate abruptly south of the Yellow River and do not extend to the epicentral area. Only the EJSF and its associated thrust system-including blind faults and folds within the Linxia Basin-are developed near the epicenter. Analysis of relocated aftershocks from the Gansu digital seismic network, early-warning stations, and temporary arrays indicates the Jishishan earthquake likely nucleated on a blind thrust or fold branching from the EJSF. The >10km hypocentral depth greater than 10km further argues against an east-dipping back-thrust as the seismogenic source. Fault geometry and slip-rate results suggest that left-lateral slip along the West Qinling Fault transfers strain via vertical uplift along the EJSF and western Jishishan margin faults, together with crustal shortening in basins flanking the Jishishan Mountains. This strain partitioning constitutes the primary driving mechanism for the 2023 M_S6.2 event.

Key words: Jishishan M_S6.2 earthquake, seismogenic structures, east margin fault of Jishishan Mountains, earthquake-induced fissures, west Qinling Fault

摘要：

2023年12月18日积石山 M_S6.2 地震发生在积石山东缘, 不同研究人员基于不同学科的研究数据, 得到的对发震构造的认识截然不同。文中通过系统考察震中附近的活动构造、 地震裂缝、 地质剖面等, 结合精定位地震序列和浅层地震勘探剖面, 综合分析此次地震的发震构造, 建立其构造模型。研究结果如下: 1)震中以西为积石山东缘断裂的左阶拐弯部位, 断裂西支发生左阶拐弯, 衔接积石山山前断裂, 断裂东支可能沿NNW的线性山梁和河谷向N延伸, 拉脊山南缘和北缘断裂终止在大河家以南, 没有延伸到震中附近; 2)极震区发育了大量不同类型的地表裂缝, 如边坡重力裂缝、 构造裂缝、 滑坡后缘张裂等, 除边坡重力裂缝和滑坡后缘张裂外, 其他裂缝宽度大多<1cm, 最宽5cm, 且在震中以西的积石山东缘断裂左阶拐弯部位尤为密集, 其走向集中于NWW(21%)、 NNW(30%)和NE(16.5%), 与积石山东缘断裂的走向一致, 先存断裂控制了过半的地表裂缝, 地表建筑弱连接部位(如公路护坡接缝)控制了绝大多数裂缝。综合多学科资料, 我们推测发震断层为临夏盆地内部的一条倾向W的隐伏断层, 该隐伏断层属于积石山向临夏盆地的逆冲扩展体系。在晚新生代青藏高原向NE扩展、 顺时针旋转和挤出的构造背景下, 西秦岭北缘断裂的左旋走滑转换为积石山断裂的逆冲活动, 促使积石山发生构造抬升并向两侧盆地扩展, 在临夏盆地内部形成逆断层-褶皱带和显著的挤压缩短变形, 2023年积石山 M_S6.2 地震便是积石山向临夏盆地的扩展事件。

关键词: 积石山M_S6.2地震, 发震构造, 积石山东缘断裂, 地震裂缝, 西秦岭北缘断裂