The morphology of thrust faulting in the 21 September 1999, Chichi, Taiwan earthquake

Abstract

The 80-km-long surface rupture of the Chelungpu fault in the 21 September 1999, Chichi, Taiwan Mw=7.6 earthquake resulted in a surface scarp with vertical throws of 2–9 m, and horizontal heaves of 4–8 m. Few major thrust faults have broken the surface in the past century, and the Chelungpu surface rupture is of interest in that it provides a morphological template for the identification of paleo-surface thrusts in similar neotectonic environments such as the Himalaya. The toe of the thrust is found emplaced gently over underlying hanging-wall materials, partly by prograde hanging-wall rotation and partly by simple shear, leaving few overt clues as to the total amount or sense of slip. Despite the large surface slip near the toe of the Chelungpu thrust its emplacement appears to have been relatively slow. MSK (the Medvedev–Sponheuer–Karnik 1981 revision of the Seismic Intensity Scale MSK81 supersedes the Mercalli Intensity scale for the description of acceleration-induced damage to modern buildings) Intensity VIII accelerations were imposed on buildings on the hanging-wall, and Intensity VII on the footwall, decaying in both directions by perhaps one intensity unit a few hundred meters from the rupture. The somewhat moderate amplitude of these accelerations, for a rupture with several meters of slip, is attributed to non-linear dissipative deformation near the toe of the rupture. The partitioning of thrusting into basal slip and hillside steepening in some locations on the Chelungpu fault suggests that the estimation of paleoseismic slip from the offset of piercing points crossing historic thrust faults elsewhere may result in underestimates of fault slip.