Determination of velocity structure variations within the Zhaotong-Ludian Fault Zone based on Repeating Earthquakes

The repeating earthquake method constrains repeated rupture processes on the same or adjacent source patches of a fault by identifying small-magnitude earthquake events with highly similar source locations, focal mechanisms, and waveforms. This approach effectively captures information on seismic velocity variations and fault stress evolution and has been widely applied to studies of fault slip rates, strain accumulation, and earthquake preparation processes. The NE-striking Zhaotong-Ludian Fault Zone, located in the eastern segment of the Sichuan-Yunnan border region, has exhibited pronounced thrusting combined with right-lateral strike-slip motion since the Late Quaternary, shows a high degree of fault locking and significant strain accumulation potential, and hosted the 2014 Ludian M_S 6.5 earthquake, making it an ideal case for investigating fault-zone velocity variations and earthquake preparation mechanisms using repeating earthquakes. In this study, we collected near-field three-component continuous seismic waveform data from January 2018 to March 2021 in the Zhaotong-Ludian Fault Zone, Yunnan Province, and identified a group of repeating earthquake events located on the eastern side of the fault zone through waveform cross-correlation analysis to investigate velocity structure variations within the fault zone. Analysis of repeating earthquake waveforms recorded at different stations shows that the three-component waveforms recorded at station YIL, which is located on the eastern side of the fault zone, exhibit a high degree of similarity, whereas waveforms recorded at station YUD on the western side of the fault zone remain generally consistent but display subtle differences within approximately 2-5s after the P-wave arrival. We further constrained seismic velocity variations within the fault zone by comparing observed waveforms of the repeating earthquakes with synthetic waveforms generated using a two-dimensional finite-difference method. The results indicate that when the P-wave velocity decreases by approximately 4% at depths of 6-12 km in the central portion of the fault zone, the synthetic waveforms successfully reproduce the phase delays observed in the recorded waveforms, with good agreement in both time shifts and waveform morphology. These findings not only quantitatively constrain the magnitude and spatial distribution of velocity variations within the fault zone, but also demonstrate the feasibility of identifying localized changes in velocity structure using waveform differences of repeating earthquakes, and they provide important insights into fault-zone fluid processes and their role in earthquake preparation.