Geodetic measurement of horizontal strain across the Red River fault near Thac Ba, Vietnam, 1963-1994

Geodetic measurements from 1963 through 1994 are used to estimate horizonta l strain rates across the Red River fault near Thac Ba, Vietnam. Whether or not this fault system is currently active is a subject of some debate. By combining: (1) triangulation from 1963, (2) triangulation in 1983, and (3) Global Positioning System (GPS) observations in 1994, horizontal shear stra in rates are estimated without imposing any prior information on fixed stat ions. The estimated rates of shear strain in ten triangular subnetworks sur rounding the fault trace are not significantly different from zero at 95% c onfidence. The maximum rate of dextral shear is less than 0.3 mu rad/year i n all but one of the triangles. The estimates help bound the slip rate in a simple elastic dislocation model for a locked, vertical strike-slip fault. By assuming a locking depth of 5-20 km, the most likely values for the dee p slip rate are between 1 and 5 mm/year of right-lateral motion. These valu es delimit the 23% confidence interval. At 95% confidence, the slip rate es timate falls between 7 mm/year of left-lateral motion and 15 mm/year of rig ht-lateral motion.