Abstract:In this study, we analyzed the characteristics of different types of earthquakes occurring in the north rim of the Tibetan Plateau, Tianshan, and Altai in view of regional geodynamic background, focal mechanisms, and research results of seismic sequences of the Xinjiang area considering the effect of earthquake rupture and sequences.
West Kunlun is located in the eastern wing of Pamir arc, with a tectonic style of extrusion conversion system consisting of both strike-slip and thrust structures, thus indicating upliftment of West Kunlun in the Cenozoic.In the area, earthquakes have a combination of characteristics of strike-slip and major-after earthquakes, with the dominant strike-slip characteristics accounting for 60% of the sequences, and major-after characteristics accounting for 63% of sequences. Earthquakes with magnitudes greater than 6 are primarily major-after and individual multiple-shock type earthquakes. Seismic activity characteristics of major-after earthquakes indicate that the stress state of West Kunlun is strong and the rupture strength is medium.
The Arkin fault has been pushed to the north and west by the Qingzang Block. The escape wedge formed by the Arkin and Karakoram faults causes the thrust of northern West Kunlun and North Qilian faults to gradually transit through the Arkin fault zone. On the other hand, the sinistral displacement of the Arkin fault transforms through both-end thrust extrusion, resulting in an arc bending at the eastern and western end of the West Kunlun and North Qilian. Therefore, the northern border of the whole plateau extends outward. Similarly, earthquake sequence types in the Arkin fault are characterized by strike-slip, major-after, and isolated earthquakes. Strike-slip ruptures are dominant in the area, and major-after and isolated earthquakes each account for 44% of the seismic sequence. Isolated earthquakes typically have a magnitude below 6.5. The 7.3 magnitude Yutian earthquake that occurred in 2014 conformed to the characteristics of this type of earthquake.
Owing to the effect of India-Asia continental collision and its effect on the later continental convergence distance, Tianshan shows complete vertical crust shortening under the horizontal compressive, oblique shear transform deformation, and lateral deformation expanding on both sides of the basin. Active faults include reverse and strike-slip faults with rotating characteristics. Seismic rupture mode of Tianshan is complex, with strike-slip and thrust earthquakes being dominant (46% and 38%, respectively). Contact ways of tectonics in different directions and movement characteristics add to the complexity and diversity of seismic sequence types in the Tianshan region. Major-after earthquake sequences account for 50% of seismic sequences, whereas isolated earthquake sequences account for approximately 40% of sequences. In individual regions such as the western part of south Tianshan, tectonics crisscross with strong movement, and the earthquake sequence shows multiple aftershocks.
The Altai fault exhibits dextral shear dislocation under NNE and SW horizontal extrusion, and some of the deep fractures experienced significant thrust movement with obvious fracture topography. Strike-slip, thrust and major-after, and isolated combined earthquake seismic types are formed under different conditions such as regional tectonic stress and fault movement. Isolated earthquakes are primarily medium earthquakes with a magnitude of approximately 5.