Characteristics Of Icequakes By Triggering And Locating In Yulong Snow Mountain

As global warming,glaciers on the Tibetan Plateau in China are melting at an accelerating rate.Correspondingly,glacier surging occur frequently,and the flood,debris flow and other related natural disasters is greatly increased.Therefore,it is important and urgent to understand glacier change and its mechanism.In recent years,satellite remote sensing,unmanned aerial vehicle are the main means to study the changes of glacier flow,area and ice volume,but these methods are still insufficient to study the glacier structure changes and the glacial flow mechanism.Since 2003,the seismological method gradually applied successfully in study the glacier crevassing,basal stick-slip motion,subglacial lake changes and so on.Compared with radar,airborne gravity satellite and remote sensing,seismic data with high spatial and temporal resolution in glacier changes is playing its significant advantages.The combination of seismology and glaciology research established the glacial seismology.In glacial seismology,icequake is the seismicity generated when glacial ice crevassing,stick-slipping or calving,which is closely related to the changes of glacial structure and extremely common in the process of glacier flow,and is an important information for learning the crevasse,basal motion and calving process.As a typical Marine glacier in the world,Baishui River Glacier No.1 of Yulong Snow Mountain,Yunnan Province,China,is in a state of accelerating loss.Its accumulation window in winter is shrinking year by year,and it shows strong melting characteristics in summer.Complex crevasse zones are distributed on the glacier surface.Because it is easy to reach,has excellent observation conditions and observation basis,and is supported by many research background in other fields,it is one of the suitable glaciers to make seismic observation and icequake study in China.We selected the Baishui River Glacier No.1 of Yulong Snow Mountain to study icequakes,sited ten short-period seismonitors from June 12,2021 to July 15,getting 34 days' continuous seismic data.We study the temporal and spatial distribution characteristics of short-duration icequakes by triggering and locating methods.The specific research work and conclusions of this paper are as follows:1)Based on the classical STA/LTA algorithm,we use 0.1 s STA window length and 1 s LTA window length to detect 41,124 icequakes by 34 days' Z channel data of the joint stations,and more than 270,000 icequakes by 34 days' three channel data of all the single station.Furthermore,based on the detection results of the single station,a hierarchical clustering algorithm was used to cluster the icequakes in the first 24 hours of the observation period,and a total of 13 icequake clusters were obtained.Based on the detection results of the joint stations,the automatic and manual picking methods were used to pick up the first arrive of the P-wave,and 8228 icequakes with high signal-noise ratio during the whole observation period were located according to the first arrive,travel-time and grid-search method.2)The duration of the icequakes in Baishui River Glacier No.1 was less than 2 s,and most of them were shorter than 1 s,showing the high frequency of 5-150 Hz.The icequake clusters show that most of the icequakes are composed of small P-wave and strong surface wave,and S wave is almost invisible.In the spectrum,part of the icquakes show obvious single-frequency characteristics(50 Hz or 80 Hz),and part of the icquakes are in widefrequency band and have dispersion,with the main frequency ranging from 30 Hz to 60 Hz.The seismic phase,wavelength and coda wave length of one icequake are different between stations,which shows the heterogeneous of the galcier.Part of the events show the special phenomenon of the successive occurrence of multiple subicequakes in the 1-2 s window.These multiple subicequakes show consistent waveform and phase in a few seconds window,indicating that the stress release during the development of crevassing is characterized by impulse burst.3)In terms of temporal distribution,the seicmic actvity of Baishui River Glacier No.1 is higher in daytime than at night.In a single day,the number of icequakes increased significantly during the period of temperature rise,with the highest between 12:00 and 14:00.The results indicate that the activity of icequakes in summer of marine glacier is closely related to the temperature rise at noon.4)The source location of the 8228 icequakes show that more than 80 % of the shortduration icequakes are concentrated on the glacier surface,and they mainly comes from the dense crevasse zone of the glacier.The average source depth is 18 m.Icequakes located at the bottom of glacier are most like the crevasse icequakes caused by water flowing deep into the basal of glacier,but not the basal icequakes.Based on glacier field investigation,radar detection,velocity of glacier flow and meteorological studies,as a Marine glacier,the basal of Baishui River Glacier No.1 was formed by a soft medium layer due to the ice-rock debris and large amount of meltwater in summer.The glacier basal flow was in a stable sliding state,and the basal icequakes were few or covered under the environmental noise.This indicates that the crevasses in this region are in a continuous development state during the rapid glacier flow in summer.