Crustal density structure of the southern segment of the Liaocheng-Lankao fault, China

Abstract

The 1:200,000 middle-large scale Bouguer gravity anomaly data covering the southern segment of the Liaocheng-Lankao fault (SLLF) and its vicinity are analyzed with two methods. First, the Bouguer gravity anomaly data are decomposed by two-dimensional (2D) wavelet to make the family of multi-scale modes correspond with density structure at different depths. Second, a two and half dimension (2.5D) human-computer interaction inversion of the Bouguer gravity anomaly data are conducted with the constraints provided by two deep seismic sounding profiles (DSS1 and DSS2) crossing the study area to get the crustal density profiles.Based on the integrated study, we can draw the following conclusions: 1) SLLF appears to be a deep fault with almost vertical dipping and rooted into the uppermost mantle; 2) In the middle to upper crust SLLF shows an clear turning patterns and segmentation features; 3) In the study area the epicentral distributions of the precisely re-located small earthquakes and the historical large earthquakes have a good correspondence with the turning patterns and segmentation features of SLLF; and 4) The results of the horizontal slices from 2D wavelet decomposition show that there are significant differences in the density structure on the two sides of the fault. A well-defined concave structure with low density exists in the upper crust of the Dongming Depression on the west side of the fault, with the concave center being estimated at a depth of about 8 km. In contrast, the upper crust on the east side presents a relative thinner pattern in depth with a bit higher density. Meanwhile, the low-density structure in the middle crust underneath the fault is presumably caused by the uplift of the upper mantle materials and their intrusion along the deep rupture system.This paper clarified the inconsistency of fault system and epicenters of small earthquakes from upper to lower crust. The results indicated that the fault system plays an important governing role to the seismicity in this area. Keywords: Southern segment of the Liaocheng-Lankao fault (SLLF), Bouguer gravity anomalies, Density structure, Dongming depression, Multi-scale wavelet decomposition, Epicenters of small earthquakes