Shear-Wave Tomography and Cause of the 2021 Mw 7.4 Qinghai Maduo Earthquake

Abstract To better understand the cause of the 2021 Mw 7.4 Maduo earthquake in western China, we collect continuous waveforms recorded at 34 portable broadband MaduoArray seismic stations newly deployed from May 2021 to June 2023, as well as those from five permanent stations near the Maduo source area. Applying the ambient noise surface-wave direct tomography method to the data set, we determine detailed 3D crustal shear-wave velocity (VS) tomography of the Maduo source area. Our results show that the shallow VS structure closely correlates with surface geological features. Secondary fault zones in the study area are located at boundaries between high- and low-VS anomalies. The Maduo mainshock took place in a high-VS anomaly, whereas most aftershocks occurred near the transition zone of high- and low-VS anomalies in the upper crust. However, a large low-VS anomaly exists beneath the Maduo source area. Integrating with previous results, we deem that this low-VS anomaly may reflect fluids in the lower crustal flow, which entered a weak section of the seismogenic fault, reduced the effective normal stress, and so triggered the 2021 Maduo earthquake. Our tomographic results reveal detailed seismic structures around two seismic gaps near the Maduo mainshock hypocenter. The eastern gap is located in a stronger high-VS anomaly, whereas the western gap is in a weaker high-VS anomaly. Our two-year earthquake sequence data show that the western gap still exists, whereas the eastern gap eventually disappeared with time.