关键词: 喀什凹陷, 短周期密集地震台阵, 背景噪声成像, 低速异常

Abstract:

High-density short-period seismometers are increasingly employed in urban environments and local geological structures to explore the crustal structure, their high-resolution images facilitate the precise identification of subsurface faults, the spatial distribution of mineral resources, and conduct building response analysis. In this study, 101 short-period seismometers were deployed across Kashgar for continuous seismic monitoring. Integrating with ambient noise tomography, high-resolution seismic velocity imaging of the shallow crust within the Kashgar was conducted. This study aims to delineate potential subsurface faults, elucidate their tectonic genesis, and provide critical insights for regional seismic risk assessment.
The empirical Green’s functions extracted from the cross-correlation of the Z component yielded a total of 1 752 Rayleigh wave phase velocity and group velocity dispersion curves. By applying one-step ambient noise tomography, the three-dimensional S-wave velocity structure of the study area was resolved down to a depth of 5km, achieving a lateral resolution of approximately 0.04°. The horizontal and vertical cross-sections of the S-wave velocity model reveal that the S-wave velocities within the upper 5km of the crust in the study area are generally lower than the global average velocity model. The Kashgar Depression is characterized predominantly by Cenozoic sediments, with continuous Quaternary alluvial deposits reaching thicknesses of up to 10~12km. The relatively weak Cenozoic sedimentary basin likely contributes to the overall low S-wave velocities observed in the region.
The velocity structures exhibit remarkably consistent patterns in varying depths. Below the depth of 1.2km, three notable low-velocity anomalies(LVAs), labeled L1, L2, and L3, are identified beneath the Kashgar. Among these, L1 and L2 form an approximately 16km long, east-west trending bowtie-shaped LVAs that align with the structural trend of the Kashgar anticline. These anomalies cover much of central Kashgar and extend nearly vertically to depths shallower than 5km, showing variation in shape and size at different depths. L3, located at the central southern edge of Kashgar, appears as a semicircle with a diameter of about 10km. Its extent diminishes gradually with increasing depth, which may indicate lithological variations at different depths.
Based on the integration of seismic reflection profiles, we infer that the frontal zone of the Southwestern Tianshan fold-and-thrust belt has developed multiple north-dipping thrust structures and south-dipping secondary thrust faults propagating toward the basin. These deformations penetrate the entire sedimentary cover, forming multi-level detachments at varying depths. Notably, the slippage of the mud layer(approximately 4km deep)at the base of the sedimentary cover in the Kashgar Depression represents the shallowest detachment layer identified in previous studies. This suggests that the multi-layered weak slippage zones within the sedimentary sequence of the Kashgar Depression may be responsible for the formation of the bowtie-shaped LVAs. Mechanically weak detachment layers likely play a key role in shaping these anomalies.
Furthermore, the Kashgar-Atushi fold-and-thrust system has experienced both lateral propagation and along-strike shortening during ongoing tectonic activity, resulting in the progressive advancement of the fold-and-thrust system towards the Kashgar Depression, which lies adjacent to the collapse-reverse fault system, may also have been subjected to intense tectonic action to form similar faults. Consequently, the east-west trending bowtie-shaped LVAs may indicate the presence of a secondary blind fault parallel to the Kashgar anticline. This inferred fault crosses the tectonic boundary between the southwestern Tianshan and Pamir regions, exhibiting a significant east-west structural discontinuity. Geological and geomorphological evidence reveals that the Kizilsu River, the largest river in the region, and its tributaries intersect the LVAs beneath Kashgar. We hypothesize that these LVAs may also reflect high-porosity fluvial sediments and folded scarps associated with paleo-river deformation.
In summary, high-density short-period seismic array imaging enables the precise detection of shallow subsurface structures in urban environments. This approach provides robust datasets for urban active fault detection, seismic amplification effect evaluation, and subsurface resources and energy exploration and development.

Key words: Kashgar depression, short-period dense seismic array, ambient noise tomography, low-velocity anomaly