The Alishan area of Taiwan spans the transition from the platform with full thickness of the Eurasian continental margin in the north to the thinning crust of its slope in the south. This part of the foreland thrust and fold belt includes important along-strike changes in structure, stratigraphy, and seismic velocities. In this paper we present the results of new geological mapping from which we build geological cross sections both across and along the regional structural trend. Fault contour, stratigraphic cutoff, and branch line maps provide 3-D consistency between the cross sections. Minimum shortening is estimated to be ~15 km with displacement overall to the northwest. A P wave velocity model helps constrain the structure at depth by providing insight into the possible rock units that are present there. P wave velocities of ≥ 5.2 km/s point toward the presence of basement rocks in the shallow subsurface throughout much of the southeastern part of the area, forming a basement culmination. The changes in strike of thrusts and fold axial traces, the changing elevation of thrusts and stratigraphic contacts, and the growing importance of Middle Miocene sediments that take place from north to south are interpreted to be associated with a roughly northeast striking lateral structure coincident with the northern flank of this basement culmination. These transverse structures appear to be associated with the inversion of Eocene- and Miocene-age extensional faults along what was the shelf-slope transition in the Early Oligocene, uplifting the margin sediments and their higher P wave velocity basement during Pliocene-Pleistocene thrusting.
Abstract We propose a workflow for the recognition of the hierarchical segmentation of faults through earthquake hypocenter clustering without prior information. Our approach combines density‐based clustering algorithms (DBSCAN and OPTICS), and principal component analysis (PCA). Given a spatial distribution of earthquake hypocenters, DBSCAN identifies first‐order clusters, representing regions with the highest density of connected seismic events. Within each first‐order cluster, OPTICS further identifies nested higher‐order clusters, providing information on their number and size. PCA analysis is applied to first‐ and higher‐order clusters to evaluate eigenvalues, allowing discrimination between seismicity associated with planar features and distributed seismicity that remains uncategorized. The identified planes are then geometrically characterized in terms of their location and orientation in the space, length, and height. This automated procedure operates within two spatial scales: the largest scale corresponds to the longest pattern of approximately equally dense earthquake clouds, while the smallest scale relates to earthquake location errors. By applying PCA analysis, a planar feature outputted from a first‐order cluster can be interpreted as a fault surface while planes outputted after OPTICS can be interpreted as fault segments comprised within the fault surface. The evenness between the orientation of illuminated fault surfaces and fault segments, and that of the nodal planes of earthquake focal mechanisms calculated along the same faults, corroborates this interpretation. Our workflow has been successfully applied to earthquake hypocenter distributions from various seismically active areas (Italy, Taiwan, and California) associated with faults exhibiting diverse kinematics.
Ripartizione della deformazione in Taiwan Centrale: nuove prospettive da dati di geologia di superficie, meccanismi focali dei terremoti, rilascio di energia sismica e misure GPS lungo il sistema di faglie transpressivo Shuilikeng.Il sistema di faglie transpressivo Shuilikeng e una zona di faglia ad alto angolo che separa le Western Foothills dall'Hsuehshan Range, le due principali unita tettoniche del settore centro-occidentale della catena di Taiwan costituite dai sedimenti del margine continentale passivo Asiatico. Si presenta una nuova cartografia geologica integrata con dati geofisici per porre nuovi vincoli e meglio definire la ripartizione della deformazione lungo questo sistema di faglie.
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