Holocene Slip Rate And Seismogenic Capacity Of The Lenglongling Fault, Northeastern Margin Of The Tibetan Plateau

The mechanisms of large-scale tectonic deformation within and around the Tibetan Plateau are debated. The relative importance of crustal thickening and strike-slip motion in absorbing plate convergence, as well as the proportion of strain accommodated by faults and localized shear zones are still controversial. Slip rates of major active strike-slip faults within and around the plateau provide kinemetic constraints for understanding continent deformation dynamics. The rate accurately determinated provides a basis for correcting dynamic model of the Tibetan Plateau. It represents long-term and average activity level and reflect the accumulating rate of strain energy of the fault. So determination of slip rate contributes to a better understanding of faulting behavior and seismic risk. The Lenglongling fault(LLLF) is a major active left-lateral strike-slip fault along the northeastern margin of the Tibetan Plateau, constituting Qilian-Haiyuan fault zone(QHFZ) together with Tuolaishan fault, Jinqianghe fault, Maomaoshan fault, Laohushan fault and Haiyuan fault. Little is known about the activity of the LLLF. Slip rate remains considerable controversy, which is limited within ~3-24mm/a, a relatively wide range. One earthquake occured on the north side of the LLLF in January 21, 2016. It remains unclear that the tectonic relation between 2016 MS6.4 Menyuan earthquake and the LLLF.Through interpretation of high-resolution remote sensing satellite image(RSSI), we carry on field geologic and geomorphic mapping and outline geometric distribution of the fault. The faulted features of the LLLF are revealed by measurements of field or RSSI and trench excavation. Niutougou site(37.4402 ° N, 102.0940 ° E) and Chailong site(37.4473°N,102.0630°E) are selected as research objects, where faulted landform is typical. Both of them are located at the upstream of Talihua gully in Menyuan county, Qinghai province. Holocene slip rate of the LLLF is redefined by measuring offset and abandment age of faulted landform surface. We assesss seismogenic capacity of the LLLF by utilizing the quantitative parameters of the activity. Combined with the activity and seismogenic capacity of the faults near the epicenter, as well as seismic metarials, we comprehensively analyze tectonic environment of the 2016 MS6.4 Menyuan earthquake. Through the above studies, we mainly draw the following conclusions:(1) The LLLF is active obviously since Holocene, whose faulted landforms is in rich. The fault shows as clear seismic rupture zones in Laohugou section, Taolagou section, Chailong section, Niutougou section and Chumayuan section. It is inferred that the length of the most latest surface rupture zone is at least 80 km or more from the synchronicity when gully bed is offset. Coseismic displacement of the most latest event is 2.9±0.3m, 7.1±0.5m, 7.9±1.1m, 5.7±0.5m, 2.5±0.4m in above sections along the fault from west to east. Cumulative displacement of the small gullies is 5.7±0.5m and 14.1±0.9m in laohugou section and taolagou section. It is better that the multiple relationship between cumulative and coseismic displacement. So the fault may behave as characteristic slip behavior, whose earthquake magnitude is MW7.3-7.5. Trench profile reveales dip angle of the fault is relatively steep, dip trends to NNE, behaving as flower structure in styles. Except for the current soil layer, the layer near the surface dated to 1815-1065 a BP is offset.(2) Left-lateral slip rate of the LLLF is 6.4±0.7mm/a since Holocene, which is between previons results from geological method, also within slip rate range 4.2-8mm/a from In SAR, but slightly larger than that from GPS(4.0±1.0mm/a). Late Quaternary slip rate of QHFZ, displaying arc-shape distribution, turns to be the largest in LLLF region. The most forceful uplift in the LLF region of the NE Tibetan Plateau confirms the important role of the LLLF playing in accommodating the eastward component of movement of Tibetan Plateau relative to the Gobi-Ala Shan block from one side.(3) The January 21, 2016 MS6.4 Menyuan earthquake occures on north side of the LLLF. The distance is relatively far between epicenter and Minle-Damaying fault or Huangchen-Shuangta fault. The strike of seismic rupture surface is NW-trending, which intersects with that of two faults at some angle. However, focal mechanisms solution of the main shock suggests that the earthquake is produced by reverse movement, which is largely different with the kinematic property, Holocene active characteristics and seismogenic capacity of the LLLF. The North lenglongling fault(NLLLF) is an associated fault in the northwest end of LLLF, which is subjected to tectonic pushing effect produed by sinistral shearing of the LLLF in the end. So the NLLLF bends correspondingly, resulting in tectonic rotation of the outward orientation, deviating from dominant rupture direction. So the activity of the NLLLF is correspondingly weakened and the fault evolves into inactive or not active obviously reverse fault, which is coordinated with NE-trending slightly protruding acr-shape and no geologic and geomorphic signs since late Qurternary. But the fault can create moderate-strong earthquakes. The geometric features and kinematic property of the NLLLF is relatively consistent with fault nodal plane 2, kinematic property revealed by focal mechanisms solution and distribution characteristics of aftershocks and seicmic intensity. So it is more appropriate that the NLLLF acts as the causative structure of MS6.4 Menyuan earthquake. The reverse movement of the NLLLF makes room for the next movement of the LLLF. The LLLF and NLLLF generate tectonic uplift effect on local topography in historical evolution process of the NLLLF.