Constraints on the crustal structure of northern Vietnam based on analysis of teleseismic converted waves

Abstract Here we report estimates of crustal thickness and Poisson's ratios for northern Vietnam, based on teleseismic receiver function analysis of observations from a dense broadband seismic array. The seismic array comprised of 24 stations distributed evenly over northern Vietnam, from whose records we examined 190 teleseismic events of Mw > 5.5 for the period 2006–2008. Using the radial receiver functions calculated from teleseismic records at individual stations, the optimum crustal thickness and V p /V s (where V p and V s are the velocities of P- and S-waves, respectively) ratio beneath each station were determined using the H–κ (where H is crustal thickness and κ is defined as the V p /V s ratio) stacking algorithm. Determined values of crustal thickness range from 26.5 km to 36.4 km, with an average of 31.0 ± 2.1 km. The simple pattern of variation of crustal thickness in the northeastern region of the study area, with a mean of ~ 31 km, suggests that the sector belongs to the craton of the South China block. A highly variable crustal thickness is found over the northwestern region of northern Vietnam, ranging from ~ 29.5 km to ~ 36.4 km, implying that complex tectonic processes have taken place in this region. The thinnest crust is found in the Red River Delta, where it ranges from 26.5 km to 30.4 km, which is suggestive of a recent rifting process. The determinations of crustal thickness show a good linear correlation with Bouguer gravity anomalies. Lower values of Poisson's ratio in the northeastern and Red River Delta sectors suggest a more felsic crust, and larger values in the northwestern sector suggest lithospheric extension in the Song Da depression. The findings enhance our understanding of the geotectonic architecture of the northern Vietnam region.