Crustal velocity-resistivity structure of Shizhu earthquake zone, Southwest China: Revealing interaction with the Yangtze river using dense seismic and magnetotelluric data

The occurrence of the isolated 2013 Shizhu M_S4.3 earthquake in a historically seismically quiet region makes it important to understand its underlying mechanisms, including the role of geological and anthropogenic factors, especially for seismic risk assessment in future mineral resource extraction. This study investigates the upper-crustal geological structure across the Shizhu seismogenic zone within the Eastern Sichuan Fold-and-Thrust Belt (ESFB), as revealed by a co-located seismic and magnetotelluric array. The high-resolution velocity-electrical models demonstrate a strong spatial correlation between low-velocity and low-resistivity anomalies above 5 km along an NE-SW orientation, which aligns with the distribution of the Yangtze River and its tributaries. The widely distributed detachment layers further strengthen the hydrological pathways, potentially acting as preferential conduits for fluid infiltration, while deeper anomalies, extending to depths of approximately 15 km, are potentially linked to metamorphic processes. The M_S4.3 mainshock occurred near the intersection of high-and low-resistivity zones, at the intersection of distinct velocity gradient zones, suggesting that fluid dynamics and fault reactivation play a significant role in earthquake triggering. We also observe a prominent noise source in the low-frequency range, closely aligned with the path of the Yangtze River, further supporting that the dynamics of river flow and its interactions with riverbanks contribute to local seismic noise. The pronounced diurnal variations in noise levels, along with a noticeable reduction in seismicity during the holidays, highlight the anthropogenic origin of the high-frequency noise sources from the nearby quarries. The results can also enhance our understanding of the seismicity within the ESFB.