Middle segment of the Red River fault: Numerical simulation of slip rates with implications for the absence of major historical earthquakes

• Slip rates are simulated for middle Red River fault (RRF) and its parallel faults to north. • Simulated slip rates of this RRF are 1.0–1.5 mm/a and comparable with geodetic ones • The plastic yield strength of modeled fault zones affects simulated slip rates seriously. • Low slip rates make large earthquakes on the RRF recur at intervals of 1.2–3.5 millenniums. The middle segment of the Red River fault (RRF) in southwestern China has historically lacked large earthquakes, unlike its parallel northern fault zone. To explain the reason for the absence of historical seismicity on the middle RRF, we developed a 3D viscoelastic–plastic model simulating slip partitioning between these structures, using GNSS and short-baseline geodetic constraints. The simulated horizontal velocities match observations well. Based on data fitting, the optimal mechanical parameters are determined as follows: a fault viscosity of 10 19 Pa·s, and plastic yield strengths of 10 MPa and 20 MPa for the straight and Big Bend sections of the middle segment of the RRF, respectively; the corresponding dextral slip rates are: 1.5 mm/a for the straight RRF section, 1.0–1.5 mm/a for its Big Bend section, 1.0–1.5 mm/a for the Chuxiong fault, 2.5–3.0 mm/a for the Qujiang fault, and 0.5 mm/a for the Shiping fault. Slip rates are more sensitive to plastic yield strength than viscosity, as simulations exceed viscoelastic relaxation time. Estimated recurrence intervals with slip rates we obtain for M 7–8 earthquakes on the middle segment of the RRF range from 1200 to 3500 years. This long recurrence interval—compared to local earthquake written history about four to five centuries in the region—may explain the absence of large historical earthquakes on the middle segment of the RRF. Another possible reason is that the middle RRF may currently be in a seismically quiescent period if its parallel strike-slip faults alternate in activity.