Fault Zone Characteristics and Coseismic Surface Ruptures associated with Strike-Slip Faults

Abstract

The Analyses of deformation structures across and/or along large-scale strike-slip faults are carried out to understand how accompanying structures affect fault growth and propagation. First, we describe a cross-fault classification which is performed based on analyses of deformation structures (secondary faults, folds, fractures and veins) across the Yangsan fault (YSF) in Sangcheon-ri, Ulsan, southeastern part of Korean peninsula. The Yangsan fault zone (YSFZ) can be divided into three general categories depending on the analyses of the structures (secondary faults, folds, fractures and veins) and fault rocks; 1) fault core - developing shear zones and accommodating most displacement, 2) mixed zone - developing minor shear zones and fractured bedrocks between fault core and damage zone, 3) fault damage zone - developing secondary faults, folds, and veins in fractured bedrocks. The analyses of accompanying structures in the YSFZ indicate that the YSF has experienced a predominant right-lateral strike-slip and has been reactivated by thrust movements. In the western damage zone, Cretaceous sedimentary bedrocks thrust over young conglomerate deposits along an east-dipping reverse fault. The long axes of the pebbles in the conglomerate deposits show a preferred orientation near the fault plane, which is sub-paralleled to the YSF. This indicates that the reverse movement along the secondary fault in the YSFZ occurred after the deposition of the conglomerates. The detailed cross-analysis of the YSFZ indicates; 1) well-defined cross-fault classification for meso-scale fault zones can be applied to large-scale fault zones, 2) later faulting occurred along the secondary fault in the fault zone as well as the main fault surface. The detailed analyses of rupture patterns and slip changes along the surface rupture associated with the 1957 Gobi-Altay earthquake (Mw=8.1, in Mongolia), which occurred along the nearly E-W trending Bogd left-lateral strike-slip fault. The surface rupture was initiated near the western end and propagated unilaterally eastwards. Geomorphologic offsets show primarily left-lateral slip with a maximum displacement of 7.0 m and average displacement of 3.5-4.0 m. Abrupt changes of rupture patterns and slip distribution indicate that the Bogd surface rupture is composed of three segments; North-Ih, East-Ih and North-Baga Bogd segments from west to east. Although the Bogd rupture is propagated through many geometrical step-overs, abrupt changes of slip occurred only at three step-overs. These step-overs, which acts as barriers of rupture propagation, show underlapping geometries or soft-linked segments types. This supports the opinion that structural maturity of step-overs is one of the main factors influencing rupture propagation. The eastern tip damage zone is characterized by widely developed minor ruptures rather than a western tip zone. Asymmetric tip zones are harmonious with unilateral propagation of the 1957 Bogd surface rupture. Slip distribution indicates that the easternmost step-over acts as tough barrier, so the main rupture is terminated and minor ruptures are developed for releasing of stress at the eastern tip zone. Detailed analyses of rupture patterns and slip distribution indicate; 1) minor ruptures are concentrated at fault linkage and tip zones and their damage patterns strongly resemble the suggested fault damage model, 2) step-over zone and its linked-types are highly affect fault propagation and termination.