Frequency-Dependent Velocity Changes of the 2018 Ms 4.5 Shimian Earthquake Revealed by Repeating Earthquakes

Abstract

The wide application of seismic dense arrays has facilitated the monitoring of the coseismic velocity disturbance of small and medium earthquakes. In this study, a repeating earthquake cluster near the 2018 Ms 4.5 Shimian earthquake was relocated based on 10 groups of repeating earthquakes that occurred from 2013 to 2019, which were recorded by the Xichang seismic array. A repeating pair was obtained by estimating the overlap of the rupture region. Coda-wave interferometry was carried out in different frequency bands using the moving window cross spectrum and wavelet-domain trace stretching (WTS) methods. Our results show that velocity change at each frequency point can be obtained with the WTS method, and thus its frequency resolution is higher. In addition, the velocity changes of coda waves strongly depend on the frequency in the Shimian area, varying from +0.10% in the high-frequency band (5–10 Hz) to −0.23% in the low-frequency band (0.5–2 Hz). In particular, XC04, which is the station that closest to the epicenter, shows the largest velocity change in the low-frequency band, but the velocity change gradually decreases as the distance from the epicenter increases. It has been suggested that the low-frequency components of the coda waves of repeating earthquakes are more sensitive to medium variation. Combined with the earthquake relocation in the Shimian area, it was found that the normalized depth sensitivity calculated based on scattered waves can retain >10% sensitivity in the source area of the Shimian earthquake. Furthermore, the negative velocity changes calculated from low-frequency coda waves are likely attributed to the Shimian earthquake. It is recommended that the average interstation spacing of seismic dense array should be <30 km to facilitate the monitoring of the coseismic changes of small and medium earthquakes.