Temporal Variations Of Seismic Velocity Related To Earthquakes And Environmental Factors From Single-station Ambient Noise

In the past ten years,the studies on the application of seismic ambient noise to detect temporal variations of seismic velocity have developed rapidly.Among them,most works utilize the cross-correlation of ambient noise between station pairs.In fact,the single-station autocorrelation or the cross-component cross-correlation from a single station can also be used to study the changes of velocity.In this paper,a single-station ambient noise method is used to carry out a study related to the strong earthquakes and environmental factors.We studied the coseismic velocity drop and postseismic recovery characteristics caused by the L' Aquila,Italy Mw 6.3 earthquake on April 6,2009.We collected about 15 years of three-component continuous waveform data recorded at six stations around the earthquake,in several frequency bands(0.2-0.6 Hz,0.3-0.8 Hz,0.5-1 Hz,0.7-1.2 Hz,0.9-1.4 Hz,1.2-2 Hz,2-5 Hz)to detect changes in the seismic velocity of the underground medium,focusing on the analysis of the results of the AQU station closest to the earthquake,and the following understandings were obtained:1.For all frequency bands we can detect obvious coseismic velocity drop.The results of the three frequency bands of 0.3-0.8 Hz,0.5-1.0 Hz and 0.7-1.2 Hz have smaller error and show that the coseismic velocity drop increases with increasing frequency,and the maximum coseismic velocity drop is 4.5%.The coseismic velocity drop detected in the 1.2-2 Hz frequency band is 4.8%,but the measurement errors are relatively large and there are obvious seasonal changes.Seven years after the L'Aquila earthquake,55% to 85% of the coseismic velocity drops in the above frequency bands were recovered.Generally,the larger the coseismic velocity drop,the higher the postseismic recovery degree;2.According to the results of this study and previous studies,we inferred that the main mechanism causing coseismic velocity drop may be shallow crust destruction and structural damage in the fault zone caused by strong ground motion,and the main mechanism of seismic velocity recovery after earthquake may be the poroelastic rebound;3.The seismic velocity change in the L'Aquila area detected in the 1.2-2 Hz frequency band has obvious seasonal changes.We focused on the analysis of the impact of rainfall on the seismic velocity change.It was found that when the rainfall is large,the medium pore pressure increases,and the seismic velocity decreases,and vice versa.We also used a single-station method to study the seismic velocity changes of the two national stations(BJT and KMI)of the China Digital Seismic Network for more than a decade,and tried to analyze the environmental factors such as precipitation,temperature and atmospheric pressure on seismic velocity changes in areas with weak seismic activity levels.We found that there are obvious seasonal changes in seismic velocity,which may be affected by environmental factors.For example,the change in seismic velocity near BJT station has a good correlation with the change in temperature and atmospheric pressure;the change in seismic velocity near KMI station from 2000 to 2006 has a strong correlation with the temperature change,however,the seismic velocity change has a longer time delay than the temperature change after 2007.