Research on Double Differential Velocity Ratio Method Based on Normal Distribution——A Case Study of Changdao Earthquake Swarm in Shandong Province
For the earthquake swarm activity with relatively concentrated source locations,the method of double differential wave ve-locity ratio is used to make two differences on the arrival time data of the station.By doing this,the method makes full use of the ar-rival time data of the P and S waves of different stations,in which it takes advantage of that it does not need the information of the time and location of the earthquake.The range of the calculated wave velocity ratio is limited to the vicinity of the source area,which elimin-ates the influence of the propagation path from the source area to the station.Furthermore,the combination of normal distribution is used to remove the influence of the error distribution,and the two-dimensional normal distribution of the time of the P and S waves after the error distribution is obtained.Then,the correlation coefficient of two-dimensional normal distribution is determined by formula deriva-tion.Finally,the slope of the major axis of the confidence ellipse is calculated to obtain the overall wave velocity ratio of Changdao earthquake swarm.Using the method above in this paper,we obtained that the ratio of the whole wave velocity of Changdao earth-quake swarm is 1.7275,and the ratio interval of wave velocity calculated by equal number of earthquakes sliding(60)is[1.4887,3.0541].We concluded that the change of wave velocity ratio of Changdao earthquake swarm from 2017 to 2018 is closely related to the process of swarm activity,and reflects the changes of fluid saturation,fracture density and state of the medium in the source area.
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