High-Resolution Shear-Wave Velocity Model of the Source Region of the 1970 Ms 7.7 Tonghai Earthquake, Yunnan, China

Abstract The Ms 7.7 Tonghai earthquake, which struck the southeastern margin of the Tibetan Plateau on 5 January 1970, along the Qujiang fault (QJF), led to a high number of casualties and severe economic losses. However, research about the high-resolution velocity structure and seismotectonics around this seismogenic fault remains limited. To fill this gap, we deployed 110 short-period seismometers around the QJF from March to September 2023 to construct an S-wave velocity (VS) model of the source region, adopting ambient noise tomography. The VS structure is mainly characterized by relatively high velocities in the western part of the study area and relatively low velocities in the eastern part. The significant low-velocity anomalies in the shallow eastern portion of the fault zone, which align with the broader isoseismic lines in the southeastern section of the main rupture zone, suggest a strong-motion amplification effect due to shallow structures. The correlation between the velocity structure and surface rupture displacement indicates that the inhomogeneous characteristics of the deep medium significantly influenced the rupture process of the Tonghai earthquake. The initial rupture nucleated at the intersection between the QJF and the high- and low-velocity anomaly boundary, a location likely prone to stress accumulation. At depths of 7.5–9.0 km, a prominent low-velocity anomaly is imaged in the eastern study area. We attribute this anomaly to geothermal fluids within the upper-crustal fault system and high temperatures within the crust caused by mantle-derived thermal activity. Along the southern side of the QJF within the same depth range, a narrow velocity anomaly correlates with the granitic intrusion. This research improves our understanding of the seismotectonics of the strong Tonghai earthquake.