Host-Rock Rheology Controls Seismicity Segmentation along the Red River Fault

The Red River Fault (RRF) presents a significant seismological paradox: its northern segment hosts frequent large earthquakes, whereas its southern segment remains largely quiescent despite similar tectonic loading. To investigate how fault-zone structure and frictional properties govern this seismicity contrast, we sampled outcrops where mylonitic shear zones host multiple layers of cataclasite within the fault core. In the northern segment, the mylonites are dominated by quartz and feldspar, by contrast, the mylonite is hornblende rich in the southern segment. Hydrothermal friction experiments are then conducted on the sampled mylonite and cataclasite fault rocks at 100–500 °C, fluid pressures of 50–100 MPa, and confining pressures of 150–200 MPa, approximating upper-midcrustal earthquake nucleation conditions.

Our experimental results reveal a critical rheological contrast between the two segments. In the aseismic southern segment, the fault core cataclasites exhibit a transition to velocity weakening at intermediate temperatures; however, the surrounding mylonitic host rocks display stable velocity strengthening behavior across nearly the entire temperature range. Conversely, mylonitic host rocks from the seismically active northern segment exhibit unstable velocity weakening behavior over a wide temperature range of 150–500°C. Based on the architecture of the fault and numerical modeling, we propose that the frictional stability of the surrounding mylonitic rock acts as a rheological gate for earthquake propagation. In the south, although nucleation may initiate within the relatively weak and velocity-weakening cataclasite (μ=0.53-0.62), the contrasting stable response of the surrounding mylonite acts as a damper, arresting rupture and suppressing large events. In the north, the unstable velocity-weakening nature of the host rock promotes a "runaway" rupture process, amplifying nucleation events into large earthquakes. These results challenge models focused only on single fault rock properties, highlighting how host-rock rheology modulates seismic hazard along major continental faults.