Milin M6.9 Earthquake Sequences Monitored And Namche Barwa Seismicity Analysis

Namche Barwa region is located in the eastern Himalaya,which is the core area of the eastern Himalayan syntaxis as a result of collision between the northward Indian and Eurasian plates.The complex geological structure of this region makes it a hotspot for geological study.However,due to the complex geographical conditions of high altitude,difficult traffic,high mountain fluctuation and intricate canyon distribution in the eastern Himalaya syntaxis,it is difficult to carry out seismological observation in this hotspot region.The real-time seismology reserch group of Institute of geophysics,China Earthquake Administration set up several temporary seismic stations in the Namche Barwa region at December 2016 for building the Namche Barwa Temporary Seismic Network(NBTSN).During the operation,the Mainling M6.9 earthquake occurred,just within the area monitored by the NBTSN.The seismic network recorded lots of aftershocks,provideing an opportunity to explore the tectonic processes of the Namche Barwa region.In this paper we firstly analyzed the noise of the temporary stations and evaluated the monitoring capacity of the NBTSN through investigating the construction,data transmission and operation of this network.Then we established the regional crustal velocity model,detected earthquakes and picked seismic phases,located earthquakes,and computed the standard magnitude.Based on the double difference location method,we relocated the Mainling seismic sequences.The obtained earthquake catalogs and relocated results are used for spatial and temporal scanning the b-value of the Mainling seismic source region.Combined with the regional geological background of this area,we analyzed the b-value distribution.In addition,we discussed the seismicity and tectonic characteristics of the study region with the historical seismic resources.The paper strives to be objective and rigorous in the process of event monitoring,phase picking,velocity model selection and calculation.Based on that,the following main conclusions and understandings are obtained:1.The addition of the temporary seismic stations of the NBTSN has greatly improved the seismic monitoring capacity for the Namche Barwa region,and provided valuable local earthquake data for the studying the Mainling earthquake.3.The relocation of the Mainling mainshock is 29.85?N and 95.06?E,at a focal depth around 14 km beneath the surface.The seismogenic fault is located in the northeastern region of the link between the Namche Barwa Peak and Gyala Peri Peak.The aftershocks distribution trend almost parallel to the Palong-Pangxin Fault,reaches at Mainling Fault in the northwest part while ends at the NE truncation fault of Palong-Pangxin Fault in the southeastern part.The focal depth of aftershocks is concentrated in the range of 3~20 km.3.The Mainling seismic sequence presents normal dacay.The h-value of one-month Mainling sequence is 1.26 and the b-value is 0.84,showing no more stronger earthquakes after short term.4.The b-value before the Mainling earthquake is lower than the background b-value of the study area,indicating that the stress accumulation in the study area is strong before the Mainling earthquake.After the mainshock,the b-value has a sharp rise,representing the release of stress.5.The variation of b-values in the Mainling earthquake source region is between 0.52 and 1.29.The b-value distribution of the aftershocks with focal depth < 15 km is NW trend,while the b-value distribution of those with focal depth > 15 km is NE-SW trend.The b-values variation in the shallow layer is related to the degree of rupture,and the characteristic of b-values in the deep layer of source area reflects the difference between the Bilu tectonic rock and the Yarlung Tsangpo suture zone.6.According to the analysis of earthquake distribution in the study area in last 40 years,the period with the highest seismicity is between 1977 and 1993.Thereafter it presented a seismicity cycle of 12 years for ML 6.0 earthquakes.In the spatial distribution,the Mainling earthquake source region and its vicinity are the areas with the strongest seismicity in nearly 40 years in the study area,and earthquakes with ML ? 4.0 occurred mostly in the eastern part of the Namche Barwa region.7.The seismic distribution features in the study area can reflect the movement characteristics of the Namche Barwa syntaxis.The Mainling earthquake source region has the strongest seismicity in the study area and is the convergence zone.From the view of mechanics,the convergence zone is the key area for stress adjustment and can explain the phenomenon of a cyclical seismic activity in the study area.