Velocity changes in the Haichenghe fault zone in Liaoning, China revealed by S-coda of repeating microearthquakes

Abstract Monitoring temporal seismic velocity changes within active fault zones provides critical insights into crustal stress evolution and earthquake nucleation processes. Leveraging high-density observational data from the Liaoning Seismic Network and the Haicheng Dense Array, we identified 37 repeating sequences (comprising 89 events) and applied coda wave interferometry (CWI) to measure relative seismic velocity changes within the fault zone. Velocity variations show substantial overall dispersion, with their stability jointly governed by sequence duration and epicentral distance. A limited number of continual-type sequences indicate a steady slip process at a depth of approximately 10 km, whereas the predominant burst-type sequences reflect rapid adjustments in the localized fault stress state. The coexistence of these distinct sequence types underscores the pronounced heterogeneity of deep deformation within the fault zone. Notably, although sequence S36 exhibited a pre-seismic velocity increase of ~ 0.1% prior to the M L 3.7 earthquake, this anomaly failed to achieve statistical significance after accounting for background noise and applying multiple-testing corrections. This study demonstrates that monitoring seismic precursors using repeating earthquakes is limited by methodological and observational constraints. Combined, these limitations restrict the detection of long-term repeating sequences and statistically significant pre-seismic velocity changes.