P-wave Velocity Structure of the Lower Crust and the Uppermost Mantle Beneath Sichuan and Yunnan Region

Abstract:

Sichuan and Yunnan region located at the North-South Seismic Zone of China has been going through continuing drastic crustal deformation due to the collision between the Indian and Eurasian plates. The characteristics of its geological structures are so complex that we still need more information to explore the tectonic evolution. Hundreds of broadband seismic stations deployed across this region allow us to make use of a large number of regional earthquakes with good distribution to develop a new P-wave velocity model with high resolution for the lower crust and the uppermost mantle.

We create the new velocity model basing on an approach called Fast Marching Tomography (Rawlinson and Urvoy, 2006). A deep learning model called PickNet (Wang et al., 2019) is applied to automatically pick P-wave onsets from a huge number of seismic waves. We still make some revise to the final auto-pick results by manual to ensure high accuracy of the picks. There are 43540 P-wave traveltimes finally used in the tomography after the 3-D source relocations (Sambridge, 1999), which are conducted basing on the P-wave traveltimes. In addition, we use statistical resolution matrix (An, 2012) to make resolution tests for the inverted model and find the best resolution can reach up to 0.10.1 in some regions.

The lower crust has slower P-wave velocities as compared with that of the other places, while the velocities below 60 km depth in the uppermost mantle are generally comparable with the global average value. The extremely low velocities in the lower crust may indicate the existence of the lower crustal flow. Strong lateral heterogeneities are found across the whole region in the lower crust with close relation to the crustal geological features. The uppermost mantle beneath Sichuan region is also characterized by velocity patterns with high lateral heterogeneities. Some rather high velocity anomalies in the uppermost mantle, which are up to 8.3 km/s, are found in and around the Sichuan-Yunnan Rhombic Block. These anomalies may be of great importance in preventing the soft materials in the lower crust moving south-eastwards and downwards. The rather low velocity zone extending from the lower crust to the uppermost mantle beneath the Sichuan-Yunnan Rhombic Block may also indicate that the uncoupled depth of this block extends into the uppermost mantle.