| The plate is constantly moving,accompanied by the dynamic change of the earth medium stress.Earthquake and volcanism are physical processes of stress accumulation and release in the underground medium.Monitoring and studying the relevant stress state will help deepen the understanding of earthquake preparation and occurrence and volcanism.In the field of seismology,monitoring seismic velocity changes is an important approach to study the property change of underground medium and analyze the seismic physical process and volcanism.The location and occurrence time of repeated earthquakes are impossible to control,which leads to many restrictions for continuous velocity changes monitoring.The active source has a controllable position and excitation time,and the repeated signals it generates can be used to continuously monitor the velocity changes.It often requires the construction of a special monitoring system for monitoring the fault zone with special attention.However,the study of velocity changes in a larger area needs to be supplemented by other methods.Using ambient noise to monitor velocity changes in underground medium provides a new way for seismology to enter the fourth dimension(time),and has become a low-cost method for monitoring velocity changes that has been widely and successfully used in earthquake and volcanic areas in recent years.In our thesis,based on the method of monitoring velocity changes by ambient noise,using continuous waveform records around strong earthquakes and volcanic areas,we quantitatively studied the coseismic and postseismic velocity changes characteristics and control factors of two typical strong earthquakes in recent years.Also we discussed and analyzed the long-period velocity changes and influence factors in volcanic areas.The purpose of this thesis is to deepen the understanding of seismic physical process and volcanism which may provide support for the exploration of velocity changes laws in earthquake and volcanic regions.The main achievements and innovative understanding of our thesis are as follows:(1)Study on coseismic velocity changes of Yangbi MS 6.4 earthquake in 2021,Yunnan.Using the continuous waveform data(January to December 2021)of 16 stations around the MS 6.4 Yangbi earthquake,we quantitatively studied the time-space distribution characteristics of coseismic and postseismic velocity changes within the 1-4Hz frequency band.And then we discussed and analyzed the physical mechanism of velocity changes from dynamic stress,static stress,fluid effect and other aspects.The results show that the coseismic velocity between pairs of stations in the study area is significantly reduced.The postseismic velocity reached the lowest value within a few days after the mainshock,and gradually recovered to the level before the earthquake around August,indicating the destruction and healing process of shallow medium.The temporal and spatial distribution characteristics of coseismic velocity changes show that the relative wave velocity dv/vpair between stations decreased significantly after the main earthquake,with the amplitude of-0.29%～-0.02%,which decreased with the increase of station spacing.The amplitude of relative velocity changes dv/vstation around the station is-0.16%--0.02%,which is large in the source area and generally decreases with the increase of epicenter distance.Coseismic velocity changes are all from shallow medium within 2km,and are mainly controlled by rock fragmentation and large-scale adjustment of stress caused by strong ground motion,due to the joint influence of static strain and dynamic strain.The northern station far away from the main earthquake has a large velocity change and a high sensitivity to stress disturbance,which may be related to the distribution of thermal fluid near the station.In conclusion,the variation of coseismic velocity of Yangbi earthquake is affected by static strain,dynamic strain caused by strong ground motion and fluid effect.Strong ground motion and fluid effect play an important role in controlling the coseismic reduction of seismic velocity of shallow medium.(2)Study on coseismic and postseismic velocity changes of the 2019 Ridgecrest earthquake in California.We collected the continuous waveform records from January 1,2018 to December 31,2020 at 22 seismic stations around the main earthquake rupture area and Coso geothermal field(CGF).Within the frequency band range of 0.1-0.9Hz,we obtained the temporal velocity changes in our study area,and carried out a quantitative analysis on coseismic and postseismic recovery process,separating the role of dynamic stress and static stress in coseismic velocity changes.Their contributions to the M 6.4 foreshock and the M 7.1 main shock are also quantitatively separated.The research results show that the earthquake sequence has caused a significant coseismic velocity drop.The average velocity changes of the station pair in the fracture area is 0.35%,with the maximum of 0.55%.While the average dv/v is 0.25%in CGF with fluctuations,this may be related to the fluid activity.The velocity between stations began to recover gradually 1.5 months after the earthquake.The shortest recovery time observed was 40 days,and the longest prediction time was 10 years.Coseismic velocity changes in fault area is mainly caused by ground motion and medium breakage caused by earthquake.The dynamic velocity changes range of a single station is-0.15%to-0.02%,while the static velocity changes range is-0.2%to 0%.The static velocity changes of 6 stations is greater than the dynamic velocity changes,and the static dv/v of 3 stations located at both ends of the fracture is significantly greater than the dynamic dv/v,which indicates that the medium fragmentation caused by this earthquake at its ends is relatively serious.Except for individual stations,the dynamic velocity changes of stations caused by the M 6.4 and M 7.1 earthquakes is positively correlated with the peak value of dynamic microstrain.The static microstrain of each station obtained at the depth of 8km has the highest positive correlation with static dv/v,which indicates that the static dv/v changes observed at the station mainly come from the depth of about 8km.As with dynamic dv/v and dynamic microstrain,individual stations that do not conform to the correlation may be related to the nonlinear response,which needs further analysis.(3)Study on velocity changes of crustal medium in Tengchong volcanic area.In this paper,using the historical continuous waveform data of 9 stations in Tengchong Volcanic Area,Yunnan Province from 2010 to 2018,based on the method of monitoring medium velocity changes with seismic ambient noise,the characteristics of long period velocity changes in the volcanic area are studied.Combined with meteorological,fluid,seismic activity and other observation data,through quantitative analysis,the reflection of seismic and fluid activities on the dynamic change of medium velocity is analyzed.The results show that the velocity changes of the medium in Tengchong volcanic area has a strong seasonal variation feature,which is generally shown as a decline in wave velocity from May to September,and a rise in wave velocity from September to May of the next year.The period of velocity changes is about 6 months with a increasing trend year by year.The annual increase rate of TNC_RHT is about 0.012%/year,while TNC_SBT reaches 0.023%/year.Precipitation is the main factor controlling the seasonal velocity changes in the volcanic area.When the rainfall increases,the water content in the rock pores increases,leading to the velocity decrease.After the rainy season,the wave velocity gradually increases with the decrease of rainfall.The phenomenon that the wave velocity gradually increases may be caused by the shrinkage of porous elastic medium due to the gradual loss of underground thermal fluid.However,it is necessary to further collect relevant fluid data and conduct quantitative analysis to further reveal the mechanism of velocity changes in the medium in Tengchong volcanic area. |
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