Joint inversion of observed gravity and GPS baseline changes for the detection of the active fault segment at the Red River fault zone

Abstract

SUMMARY Methods for the determination of active fault segments by inversion of temporal gravity and GPS (global positioning system) baseline changes are presented in this paper. A numerical test is performed for estimating four types of fault movement models from simulated gravity changes with random noise. Results of the numerical test show clearly the possibility of detecting fault movement from observed gravity changes. Based on this work, active segments of the Red River fault zone (RRFZ) are investigated by inversion of observed gravity changes (from 1985 November to 1988 November) and GPS baseline changes (from 1988 August to 1991 October), both separately and jointly. The results from the joint inversion indicate an active segment of the Red River fault, characterized by right-lateral strike slip (1.0 ± 0.2 m), with a length of 232 ± 3 km, a width of 6 ± 3 km, a dip angle of 100°± 5° (80°± 5° NE), a depth of 10 ± 1 km, and a strike direction of 169°± 2° (SE). These characteristics of the detected active segment at the RRFZ agree with those of geological investigations and seismic activity in the area. Estimated horizontal displacements at the RRFZ indicate that drastic movements are concentrated on the northern segment, whereas the southern segment of the fault is relatively stable. The estimated fault slip rate along strike at the Red River fault is 8–11 mm yr−1 larger than the geological estimate of ∼6 mm yr−1, which implies that the RRFZ is presently active. The results obtained in this research support previous interpretations that the fault movements at the RRFZ are mainly a result of the collision of the Indian and the Eurasian plates.