Fine Structure of the Chenghai Fault Zone, Yunnan, China, Constrained From Teleseismic Travel Time and Ambient Noise Tomography

To derive high-resolution fault zone (FZ) structure of the Chenghai fault in Yunnan, southwestern China, we deployed a linear dense array crossing the fault from January to February 2018. The array consisted of 158 short-period (5 s) three-component instruments and spanned an aperture of similar to 8 km with average station spacing of 40-50 m. During the 1-month deployment, 20 teleseismic earthquakes with moment magnitudes larger than 5.5 and 41 local earthquakes were recorded. We first analyzed the travel times of P and S waves from teleseismic earthquakes to determine the boundaries of the FZ. After correcting the station-event geometry effects, teleseismic travel time differences between the reference station, and all other stations clearly marked a zone of 3.4 km in width with distinct travel time anomaly, suggesting a low-velocity zone (LVZ) surrounding the Chenghai fault. We then conducted ambient noise tomography and found that the S wave velocity of the LVZ was reduced by 60% and 40% compared to the northwest and southeast of the LVZ, respectively. Our ambient noise results suggested the LVZ extending to similar to 1.5 km in depth, consistent with the travel time anomalies of teleseismic earthquakes. Integrating ambient noise tomography with teleseismic travel times in a dense array with such an aperture is an effective approach for resolving FZ structure and depth extent. Plain Language Summary High-resolution fault zone (FZ) structure and evolution are critical to understand earthquake physics. As the FZ width is usually on the order of 100 to 1,000 m, resolving the structure requires dense seismic arrays with small interstation spacing. One of the difficulties is to robustly constrain the depth extent by small-aperture arrays. Here we demonstrate that deployment of a large-aperture (8 km) array across a fault can robustly resolve the FZ structure and depth extent. Using the data acquired from a linear dense array crossing the Chenghai Fault in northwest Yunnan, China, we show that the low-velocity zone of the fault extends to 1.5 km from the results of ambient noise tomography, which are consistent with observations of travel times of body waves from teleseismic earthquakes. Such large-aperture array is an effective tool that enables integration of a variety of methods to robustly constrain high-resolution FZ structure.