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The feasibility of using a dense seismic array with an airgun source to study the quality factors of shallow media is verified. Data were obtained from 37 stations in the dense seismic array located in Binchuan, Yunnan Province, China, and the amplitude–distance attenuation method and the coda normalization method were applied to calculate the S-wave quality factors in the area. The amplitude–distance attenuation method yielded Q s - 1 =0.0260 ± 0.0103, and the frequency-dependent Q s - 1 calculated by the coda normalization method can be expressed by the power law Q s - 1 (f) ≅ 0.0554 f - 0.5643 . The consistency between the results of these two methods shows that a dense seismic array with an airgun source can be used to study the attenuation characteristics of shallow media. The amplitudes at some points deviate substantially from the fitted curve and thus have a certain influence on the fitting results; hence, we must select high-precision data for the calculation. Given the topography, we speculate that the anomalous stations located on the edge of the Binchuan Basin and in the western hilly area are due to the edge effect of the basin and the weak attenuation of the hilly area and that the anomalous station located in the northern Binchuan depocenter is attributable to local site factors. Compared with the Q s - 1 estimated by previous studies, the Q s - 1 in the Binchuan area is found to lie between those of the hard soil and sedimentary rock and is similar to the Q s - 1 in the North China Basin, corresponding to the shallow velocity structure in this area. [ABSTRACT FROM AUTHOR]

Low‐velocity zones (LVZ) in the shallow crust such as basins infilled with unconsolidated sediments could significantly amplify seismic waves and cause intense ground motions. In this study, we estimate the seismic site response in the Binchuan basin across the active Chenghai fault zone in northwest Yunnan Province, Southwest China using data from a large‐aperture (∼8 km) dense linear array of 125 three‐component seismometers deployed in 2018 for ∼1 month. We observe that local earthquakes with larger epicentral distances tend to produce stronger spatial variability in peak ground velocity (PGV) across the array. The PGV values are overall larger in the middle of the array than the two sides, especially in the horizontal directions, which is consistent with both tomographic and traveltime results. Using both nearby and distant earthquakes, we find that horizontal ground motions from 0.2 to 2 Hz at stations inside the LVZ are up to 20 times as large as that outside, and a more localized zone (∼500 m wide) enclosing the fault surface trace is characterized by broader frequency bands and larger factors of amplification. As the distance from the fault increases, amplification factors become smaller while the lower limit of the amplification frequency band gradually increases. Variations in predicted ground motion from one‐dimensional SH‐wave modeling reveal amplification and attenuation effects of the LVZ, suggesting that strong lateral heterogeneity should be considered. Our results demonstrate that based on dense array data, analyzing seismic site response is an alternative method to effectively constrain the shallow subsurface fault structure. Plain Language Summary: Seismic site response refers to the effects of local site conditions on arriving seismic waves and is an important factor for precisely predicting ground motions. It could vary considerably around major crustal faults, where numerous ruptures and many other processes have together generated complex subsurface structures. Here we investigate seismic site response with its spatial variability across the Chenghai fault zone in northwest Yunnan Province, Southwest China using a ∼8‐km‐long dense array of 125 seismometers. We find clear variations in peak ground velocity (PGV) produced from local earthquakes across the array and higher PGV values around the fault. Moreover, the spatial variability in PGV increases with the epicentral distance. From local, regional, and teleseismic earthquake recordings in horizontal directions, we observe that ground motions within the previously identified low‐velocity zone are amplified by factors up to 20 between 0.2 and 2 Hz, and a ∼500‐m‐wide zone near the fault with the strongest amplification effects is newly identified. Predictions using one‐dimensional SH‐wave modeling and the shear‐wave velocity model indicate complicated effects from near‐fault subsurface structures on ground motions. Our results suggest seismic site response analysis based on dense array data can help effectively constrain low‐velocity structures around active fault zones. Key Points: We investigate the variations of ground motions in the Binchuan basin in Northwest Yunnan with a temporary dense arrayThe amplitude observed in the low velocity zone across the Chenghai fault is up to 20 times as large as outside stationsAnalyzing seismic site response based on dense array data could be an effective method to constrain fault zone structure [ABSTRACT FROM AUTHOR]

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 ~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 ~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. Key Points: We deployed a linear dense array of 8 km in length across the Chenghai fault that is critical to constrain the fault zone depthBased on teleseismic travel times and ambient noise tomography, we found a low‐velocity zone of 3.4 km in width associated with the faultAmbient noise tomorgraphy showed that the low‐velocity zone extends to 1.5 km in depth, consistent with teleseismic travel times [ABSTRACT FROM AUTHOR]