Joint Inversion for Crustal and Pn Velocities and Moho Depth for Red River fault, northern Vietnam

The more-then-900-km-long Red River fault (RRF) crossing through southwestern China and northern Vietnam is perhaps the most significant structural discontinuity in south-east Asia, which is mechanically responsible for the collision and extrusion between Indian and Eurasian Plate. It accommodated 700 ± 200 km of left-lateral offset prior to the end of Miocene (5Ma) and had changed the sense to right-lateral slip up to recent. Present seismicity is active in northwest of RRF (southwestern China) but relatively quiet in southeast (northern Vietnam) and the volcano-related low velocity zone was detected in seismic imaging beneath the northwestern RRF. Although the geodetic and field surveys are abundant, there are still few studies focusing on the seismic imaging of southeastern RRF in northern Vietnam. For the reason, we combined P wave phase picks from the Vietnam local seismic network with 52 stations from 1996 to 2006 and 30 seismic stations we deployed since 2006. We use a total of 267 events and 2268 arrivals. Both first and secondary crustal arrivals are picked whenever possible for our own stations. Using a joint inversion method recently developed, we invert crustal and Pn velocities as well as Moho depth simultaneously to help determine rapid variation of crustal structure and Moho without flat-discontinuity assumption. Our preliminary results show that Moho on average is around 31 km and relatively shallower beneath the Red River delta with a RRF-parallel trend. In crustal velocity image, the low velocities also correlate with the delta and offshore area. The Pn velocity is consistently low near the RRF, which seems to indicate RRF is a crustal scale structure as a continental block boundary.