Apparent Low-Velocity Belt in the Shallow Anninghe Fault Zone in SW China and Its Implications for Seismotectonics and Earthquake Hazard Assessment

The Anninghe fault forms the eastern boundary of the Sichuan-Yunnan block in Southwest China and has been identified as an earthquake gap zone. This study intends to construct the upper crustal shear wave velocity (Vs) structure beneath the Anninghe fault to understand its seismotectonics and potential large earthquake hazards. We deployed a dense seismic array along the southern central Anninghe fault valley. From the 3-month continuous records, we calculated vertical-component cross-correlation functions (CCFs). However, the surface wave signals in the CCFs are intensely interfered by near zero-time-lag noise. We proposed a mode separation method based on the high-resolution linear Radon transform, which suppressed the interfered noise and greatly enhanced the surface wave signals for ambient noise tomography of the Vs structure. The fine upper crustal structure reveals a distinct narrow low-velocity belt within a depth of 3 km beneath the Anninghe fault zone. At deeper depths (4.5-8 km), the narrow low-velocity belt shifts to the east and correlates with the distribution of local earthquakes. Combining previous results with our new findings, we presented a seismotectonic model of the southern central Anninghe fault, which interprets the narrow low-velocity belt as a water-contained fracture zone that forms a seismogenic zone at deeper depths under transpression. In addition, we demonstrated through scenario earthquake simulations that fine structures play a significant role in the assessment of earthquake hazards along the Anninghe fault. As such, this study provides a typical window into seismotectonics and large earthquake hazards in the active southeastern Tibetan Plateau.