The Tan-Lu Fault Zone (TLFZ) is the largest strike-slip active fault zone in eastern China, with different segments exhibiting distinct structural characteristics and seismic activity. To investigate the mid-upper crustal structure of the Zhangbaling Uplift segment of the TLFZ, a research team from the Geophysics of Earth and Planets Program at the University of Science and Technology of China (USTC) utilized 192 SmartSolo three-component nodal seismometers deployed in a dense array. Applying ambient noise tomography (ANT), they discovered several NW-trending, band-like low-velocity anomalies (LVAs) within the upper crust of the Zhangbaling segment. These anomalies suggest the presence of a series of NW-striking sinistral (left-lateral) blind faults. The study further infers that these blind faults modulate the near E-W oriented stress loading on the TLFZ.
The TLFZ is divided into northern, central, and southern segments based on its tectonic evolution history and seismicity (Figure 1a). Seismically, the central segment is highly active, having hosted several major historical earthquakes. This includes the 1668 Tancheng Ms 8.5 earthquake – the largest inland earthquake in eastern China – which produced a 230 km-long surface rupture between Suqian and Zhucheng. In recent years, numerous M3-4 earthquakes have also occurred in the west of the central-southern TLFZ, particularly near the intersection of the near E-W trending Fei Zhong Fault and the western branch of the TLFZ in Liangyuan Town, Feidong County. Notably, the largest event here was the M 4.7 earthquake on September 18, 2024. Consequently, delineating the spatial distribution of NW-trending faults conjugate to the NE-striking TLFZ is crucial for understanding the seismogenic environment of the fault zone and its adjacent regions.
Figure 1: Geological setting and seismic station deployment map of the study area.
Using continuous seismic data recorded by the 192 SmartSolo seismometers deployed in 2020, the team obtained a 3D shear-wave velocity (Vs) model down to 10 km depth beneath the study area via ANT (Figure 2). The results reveal:
A distinct LVA at 0–2 km depth directly beneath the TLFZ surface trace, corresponding to the intra-fault graben structure. This graben extends to 3 km depth beneath the Mingguang Graben in the northern Zhangbaling segment and to 1.5 km depth east of the Hefei Basin in the south.
Several NW-trending band-like LVAs developed at depths of 3–8 km beneath the Bengbu Uplift and east of the Hefei Basin, likely corresponding to the NW-striking sinistral blind faults. These faults do not extend to the surface, meaning they do not cut through or disrupt the shallow structure of the NE-striking TLFZ.
Figure 2: Horizontal slices of shear-wave velocity (Vs) at different depths obtained from ambient noise tomography.
Integrating the 3D ANT velocity structure, surface fault geometry, earthquake focal mechanisms, and GPS motion vectors, the research team proposed a contemporary tectonic model for the Zhangbaling segment (Figure 3). This model suggests that the NW-striking blind faults play a key role in modulating the near E-W oriented stress loading on the TLFZ.