| The North-South Seismic Belt is located in the eastern part of the Tibetan Plateau,serving as the boundary zone between the terrain,geological structure,and geophysical features of the eastern and western regions in China.Complex tectonic activities have resulted in the complex distribution of faults and strong seismic activity in the NorthSouth Seismic Belt,with the occurrence of severe destructive earthquakes.Compared to large earthquakes,there is a higher frequency and wider distribution of moderate strike-slip earthquakes in the North-South Seismic Belt,posing great threats to human life,property,and social stability.However,the rupturing faults of some strike-slip earthquakes are unknown.Identifying the distribution and kinematic characteristics of active faults is crucial for evaluating and preventing seismic disasters,establishing regional kinematic models,and analyzing dynamic mechanisms.Therefore,studying the source parameters and causative faults of moderate earthquakes are not only important for understanding the seismogenic mechanism,mitigating earthquake disasters,and evaluating the future risk of earthquakes but also for understanding the characteristics of the present-day fault activity in these areas and exploring the tectonic deformation mechanism.In this study,the 2014 Yingjiang doublet earthquake,the 2019 Changning earthquake,and the 2022 Delingha earthquake sequence,which occurred in the southern section,middle section,and northern section of the North-South Seismic Belt,are selected as examples to reveal the source characteristics and seismogenic mechanisms and expect to deepen the understanding of the present-day tectonic deformation mechanisms in these areas.The May 2014 M5.6 and M6.1 Yingjiang earthquakes occurred in the northern part of the Yunnan-Myanmar active block,located in the southern section of the North-South Seismic Belt.A series of NE-SW trending and NW-SE trending strike-slip faults are widely developed within the Yunnan-Myanmar block.Various dynamic mechanisms have been proposed to explain the fault deformation in this region,but there still exist many debates.The 2014 Yingjiang doublet earthquake provides an opportunity to further understand the tectonic activity of this region.We determined the source parameters and rupture directivity using seismic waveform data.The result reveals that the centroid depth of M5.6 and M6.0 earthquakes are about 9 km and 6 km.The M5.6 event mainly ruptured along an NW-SE trending right-lateral strike-slip unmapped fault,and the rupture length is about 2～3 km;the M6.1 earthquake mainly ruptured northward along a nearly N-S trending right-lateral strike-slip buried fault,with the rupture length of about 5 km.Comparing the ruptured faults of this doublet earthquake with the existing dynamic models,we think that the shear zone model is more suitable to explain the deformation of the seismogenic faults of this earthquake.The June 2019 M6.0 Changning earthquake occurred in the southeastern part of the middle section of the North-South Seismic Belt.The earthquake close to the shale gas and salt mining region,and its seismogenesis have attracted extensive attention.We analyzed the hypocentral depth,point source parameters,and rupture directivity via modeling local seismic waveforms.The result reveals that its hypocentral depth is about 4 km,much shallower than the most tectonic earthquake in surrounding regions.The moment tensor inversion results show that the mainshock is a dominant strike-slip event with a significant thrust component.Its magnitude is Mw5.8,and the source duration is 6s.The focal plane solutions are 125°/46°/28°(strike/dip/rake)(NP1)and 14°/70°/132°(NP2)for an optimal depth of 4 km.The rupture directivity analysis results indicate that the causative fault of the mainshock is NP1,and the horizontal rupture along the fault to the northwest,with a co-seismic rupture length of about 14 km.Based on its shallow hypocentral depth,and proximity to nearby mining regions,we hypothesize that the Changning earthquake might be an induced event due to long-term fluid injection for salt mining.However,interdisciplinary studies are needed to test the hypothesis,including simulation of fluid migration and investigation of background stress as well as resolving mechanical parameters of the rocks in the epicentral regions.The 2022 Delingha earthquake sequence(M5.8,M6.0,and M5.4)occurred in the Qilian Mountain thrust-fold belt,located in the northern section of the North-South Seismic Belt.The epicenter is located in a low seismicity region at the westernmost end of the Qilian-Haiyuan fault system.These earthquakes provide a precious opportunity to analyze seismogenic settings,fault activity,and seismic hazards within the Qilian Shan fold-thrust belt.We use the local seismic waveforms to analyze its source parameters,aftershock distribution,and rupture directivity.The results show that these mainshocks are shallow strike-slip events with a centroid depth range from 6～8 km.The focal mechanism solutions show that the orientation of the two nodal planes of those earthquakes is nearly N-S and E-W directions.The relocated aftershocks are mainly distributed along the N-S direction.Both the M5.8 and M6.0 events ruptured along the N-S nodal plane and expanded toward the south for～4 km and～3 km.Furthermore,we extracted the co-seismic deformation data of M5.8 and M6.0 events using InSAR observation data.The maximum surface deformation caused by the two earthquakes is about 2 cm and 3 cm,respectively.Therefore,we further constrained the source depth of the earthquakes.Based on those results,we thought that this earthquake sequence ruptured along an unmapped,nearly vertical dextral fault.The accumulated Coulomb stress changes caused by the mainshocks show that the northwest has a higher risk of future earthquakes.Finally,we discuss the possible deformation mechanism for this right-lateral strike-slip faulting and thought that the deformation of this fault was associated with the Qilian-Haiyuan fault,and controlled by long-term protracted nearly N-S trending right-lateral simple shear in the Qilian Shan fold-thrust belt.To further improve the understanding of the characteristics of fault activity in the North-South Seismic Belt,we also studied the focal mechanism parameters of six moderate strike-slip earthquakes that occurred in the southern and middle sections of the North-South Seismic Belt from 2009 to 2020.The analysis results of all earthquakes mentioned above show that:there is right-lateral strike-slip shear inside the YunnanMyanmar block in the southern section of the North-South Seismic Belt,which contrasts with the left-lateral strike-slip shear in this region,further supporting the possibility of the shear zone model.In the Changning area in the southeastern part of the middle section,the influence of industrial activity on seismic activity cannot be ignored.In the western region of the northern section,there are not only thrust and leftlateral strike-slip moderate earthquakes but also right-lateral strike-slip moderate earthquakes,indicating that right-lateral strike-slip activity is also accommodating the deformation in this region.In addition,the rupture directivity results determined in this study show that moderate earthquakes may occur on unmapped faults,emphasizing that studying the distribution and kinematic characteristics of active faults is an important task in seismic risk assessment.Each section of the North-South Seismic Belt is tectonically complex region,and detailed geological,seismological,and geodetic studies can help to further understand the kinematic patterns and dynamical mechanisms in these regions. |
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