Background Characteristics and Seismic Response of M2 Tidal Parameters in Confined Aquifers of the Beijing Seismic Monitoring Well Network

Abstract

Groundwater level responses to solid tides in confined wells provide a means for investigating aquifer parameter variations and earthquake-related effects. Using hourly water-level observations from eight confined wells of the Beijing seismic monitoring well network (January 2017–December 2025), this study applied the Baytap-G program to extract the M2 tidal constituent contribution, amplitude, tidal factor, and phase lag, and analyzed their temporal variations and responses to earthquakes. The eight wells are classified into five groups: Group 1 (BQ, CP), stable during non-interference intervals but with permanent instrument-induced jumps; Group 2 (MF, XJ), stable background references; Group 3 (LX), reflecting genuine hydrological processes; Group 4 (DHC, XXZ), signals too weak for reliable analysis; and Group 5 (WLY), dominated by persistent instrumental disturbances. Using mean ±3σ thresholds derived from seismically quiet periods, nine persistent ≥3σ anomaly segments were identified; none coincides in time with the 14 Ms ≥ 4.0 earthquakes or the 41 coseismic water-level response events, and all can be attributed to documented instrument events. The hypothesis that earthquakes alter M2 parameters before or after their occurrence is not supported, nor is the hypothesis that coseismic water-level responses produce short-term M2 parameter changes; the hypothesis that instrument operational conditions and long-term hydrological changes dominate the observed variations is supported. The absence of earthquake-triggered M2 parameter responses is attributed to basin attenuation, limited strain conversion efficiency, response mechanism decoupling, and observational resolution. These results provide a background reference for applying well water-level tidal parameters to earthquake precursor monitoring in the Beijing area, showing that independent baselines must be established for each well and that instrument operational records are necessary for anomaly identification.