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

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