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Fault activity and structure are important factors for the assessment of seismic hazards. The Anninghe fault is one of the most active strike‐slip faults in southwestern China but has been experiencing seismic quiescence for M > 4 earthquakes since the 1970s. To understand better the characteristics of its highly locked southern segment, we investigate seismicity and ground motion variability using recently deployed multi‐scale dense arrays. Assisted by machine learning (ML) seismic phase picking and event discrimination models, we first compile a high‐resolution catalog of local seismic events. We find limited earthquakes that occurred on the Anninghe fault, consistent with its generally acknowledged high locking degree. Whereas, most newly detected events appear within off‐fault clusters, among which four are closely related to anthropogenic activities (e.g., mining blasts), and two neighboring faults host the remaining ones. We further apply an ML‐based first‐motion polarity (FMP) classifier and successfully obtain a reliable small earthquake focal mechanism, which agrees well with the geologically inferred north‐south trending and eastward dipping of the Anninghe fault. Analyses of ground motion variations along two across‐fault linear arrays show abrupt changes in FMPs and obvious frequency‐dependent site amplifications near the mapped fault traces. It further suggests that, at finer scales, the damaged Anninghe fault zone may have split into two smaller damaged zones at shallower depths, resulting in a typical "flower‐type" fault structure. The efficient workflow developed in this study can be well applied for the longer‐term monitoring and better characterization of the southern Anninghe fault, or other similar regions. Plain Language Summary: The Anninghe fault in southwestern China has great potential to produce devastating earthquakes. However, its southern segment has experienced seismic quiescence during the past 40 years, resulting in less knowledge about its detailed seismicity and structures which are crucial for seismic hazard assessment. Here, we apply a series of novel machine learning methods to seismic data of multi‐scale temporary dense arrays we deployed near the southern segment of the Anninghe fault. Our event detection and location results show that earthquakes along this segment are very limited, consistent with the well‐acknowledged highly locked fault plane status. The detected seismic events are mostly located off the fault and many are closely related to human activities (e.g., mining explosions). The source parameters of one on‐fault earthquake agree with the geologically inferred fault orientation and slip direction. The dense across‐fault recordings from local and regional sources reveal much larger amplitudes around the mapped fault traces, corresponding to two shallower fault damage zones that can trap seismic energies and promote earthquake ruptures. The efficient methods used here can be extended to future longer‐term seismic experiments over the Anninghe fault, or other similar faults. Key Points: We have compiled a high‐resolution seismic event catalog with anthropogenic and natural sources better separated around the southern Anninghe faultA reliable focal mechanism inverted using dense seismic records agrees well with the geologically inferred fault orientation and slip directionGround motion amplifications are obvious and frequency‐dependent around fault traces, suggesting distributed damage zones at shallower depths [ABSTRACT FROM AUTHOR]

We determine the 3-D shear wave speed variations in the crust and upper mantle in the southeastern borderland of the Tibetan Plateau, SW China, with data from 25 temporary broad-band stations and one permanent station. Interstation Rayleigh wave (phase velocity) dispersion curves were obtained at periods from 10 to 50 s from empirical Greens function (EGF) derived from (ambient noise) interferometry and from 20 to 150 s from traditional two-station (TS) analysis. Here, we use these measurements to construct phase velocity maps (from 10 to 150 s, using the average interstation dispersion from the EGF and TS methods between 20 and 50 s) and estimate from them (with the Neighbourhood Algorithm) the 3-D wave speed variations and their uncertainty. The crust structure, parametrized in three layers, can be well resolved with a horizontal resolution about of 100 km or less. Because of the possible effect of mechanically weak layers on regional deformation, of particular interest is the existence and geometry of low (shear) velocity layers (LVLs). In some regions prominent LVLs occur in the middle crust, in others they may appear in the lower crust. In some cases the lateral transition of shear wave speed coincides with major fault zones. The spatial variation in strength and depth of crustal LVLs suggests that the 3-D geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur. Consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity.

In response to the need for high‐resolution imaging of shallow crustal structure, we present a linear array double‐difference (DD) adjoint tomography method, using DD Rayleigh wave traveltime measurements for enhanced spatial resolution. This method, validated through synthetic experiments, improves velocity anomaly detection with fewer iterations compared to absolute traveltime measurements. Applied to data from four linear seismic arrays in the central Tanlu fault zone (TLFZ) in the eastern China, our approach integrated both DD and absolute difference (AD) of adjoint traveltime measurements. We performed cluster analysis for data quality control, reducing data outliers and increasing reliability, particularly in suppressing cycle skipping for short‐period measurements. The resulting high‐resolution S‐wave velocity profiles in the shallow crust well delineate geological structures, revealing a continuous low‐velocity anomaly beneath the eastern branch of TLFZ. Our comparative analysis with the southern segment of TLFZ further highlights the segmented nature of the fault zone structure. These variations might suggest a dominant influence of deep magmatic processes due to destruction of the North China Craton. Our study links shallow structural features to deeper geodynamic activities, emphasizing the role of TLFZ as a critical tectonic boundary. Plain Language Summary: The Tanlu Fault Zone (TLFZ) in Eastern China, a region crucial for its dense population and economic significance, has historically experienced severe earthquakes. Particularly, the Weifang segment of the central TLFZ, despite its current low seismic activity, raises questions about potential seismic risks. And the weak structures beneath the fault zone can promote earthquake rupture and its associated hazards. To get a clearer picture of the underground structures, we developed an imaging method focusing on the seismic traveltime differences between pairs of stations of linear seismic arrays. We also developed a data screening method for the linear array to reduce the effect of unusual or extreme data points on our analysis. We used this new technique and surface wave data recovered from ambient noise to produce clear images of in the upper crust of the central TLFZ. We found a weak structure beneath the eastern branch of the TLFZ, indicating a higher risk of earthquakes in this region. We compared our results with those from the southern TLFZ and noticed links between the shallow underground features and the deeper dynamic process associated with the destruction of the North China Craton. Key Points: A linear array double‐difference adjoint tomography using ambient noise data is proposed and applied to the central Tanlu fault zone (TLFZ)The refined upper crust Vs models reveal a continuous low‐velocity anomaly beneath the eastern branch of TLFZSeismic velocity variations along the TLFZ indicate the influence of deep magmatic processes resulting from the destruction of the North China Craton [ABSTRACT FROM AUTHOR]

We determine the 3-D shear wave speed variations in the crust and upper mantle in the southeastern borderland of the Tibetan Plateau, SW China, with data from 25 temporary broad-band stations and one permanent station. Interstation Rayleigh wave (phase velocity) dispersion curves were obtained at periods from 10 to 50 s from empirical Green's function (EGF) derived from (ambient noise) interferometry and from 20 to 150 s from traditional two-station (TS) analysis. Here, we use these measurements to construct phase velocity maps (from 10 to 150 s, using the average interstation dispersion from the EGF and TS methods between 20 and 50 s) and estimate from them (with the Neighbourhood Algorithm) the 3-D wave speed variations and their uncertainty. The crust structure, parametrized in three layers, can be well resolved with a horizontal resolution about of 100 km or less. Because of the possible effect of mechanically weak layers on regional deformation, of particular interest is the existence and geometry of low (shear) velocity layers (LVLs). In some regions prominent LVLs occur in the middle crust, in others they may appear in the lower crust. In some cases the lateral transition of shear wave speed coincides with major fault zones. The spatial variation in strength and depth of crustal LVLs suggests that the 3-D geometry of weak layers is complex and that unhindered crustal flow over large regions may not occur. Consideration of such complexity may be the key to a better understanding of relative block motion and patterns of seismicity. © Journal compilation © 2008 RAS.