Xian‐Kang Zhang

Velocity structure of the crust and upper mantle in Xingtai earthquake region and its adjacent area

Two seismic refraction profiles which are perpendicular to each other, running through Xingtai earthquake region, reveal the anomalous variations of crust-mantle velocity structure and deep tectonics. Pg wave attenuates rapidly with distance in the earthquake region. A group of strong reflections from a depth of 21.0 km can be identified along the section from Longyao to the piedmont of Taihang Mountain, but Pm waves characterized generally by strong amplitude are not obvious. Under the earthquake region and its western neighboring region, the crustal velocity structure features high and low velocities changed alternatively. From North China plain to Shanxi plateau, the velocity at the top of the upper mantle decreases progressively, while crustal thickness increases by 11 km. Moho uplifts locally in the earthquake region. The crustal fault stretching deeply to Moho and the discontinuous sections of Moho in the earthquake region are supposed to be the channels and zones for magmatic intrusion. The uplifting of upper mantle and magmatic intrusion are responsible for the formation of anomalous crust-mantle structures and extending basins, and for the occurrence of Xingtai earthquake as well.

Three-dimensional crustal velocity structure model of the middle-eastern north China Craton (HBCrust1.0)

Lithosphere thinning and destruction in the middle-eastern North China Craton (NCC), a region susceptible to strong earthquakes, is one of the research hotspots in solid earth science. All 42 seismic wide-angle reflection/refraction profiles have been completed in the middle-eastern NCC. We collect all the 2-D profiling results and perform gridding of the velocity and interface depth data, building a 3-D crustal velocity structure model for the middle-eastern NCC, named HBCrust1.0, by using the Kriging interpolation method. Our result shows that the first-arrival times calculated by HBCust1.0 fit well with the observations. The result demonstrates that the upper crust is the main seismogenic layer, and the brittle-ductile transition occurs at depths near interface C (the interface between upper and lower crust). The depth of interface Moho varies beneath the source area of the Tangshan earthquake, and a low-velocity structure is found to extend from the source area to the lower crust. Based on these observations, it can be inferred that stress accumulation responsible for the Tangshan earthquake may have been closely related to the migration and deformation of the mantle materials. Comparisons of the average velocities of the whole crust, the upper and the lower crust show that the average velocity of the lower crust under the central part of the North China Basin (NCB) in the east of the craton is obviously higher than the regional average. This high-velocity probably results from long-term underplating of the mantle magma.