QUANTITATIVE ANALYSIS OF SURFACE COSEISMIC FAULTING AND POSTSEISMIC CREEP ACCOMPANYING THE 2003, MW = 6.5, CHENGKUNG EARTHQUAKE IN EASTERN TAIWAN

Abstract

On the basis of field observations and geodetic measurements we analyzed coseismic and postseismic deformation of the 2003, Mw = 6.5, Chengkung earthquake across the rupture trace of the Chihshang fault, which lies along the suture zone between the Philippine Sea plate and the Eurasian plate in eastern Taiwan. At three of our investigation sites along the Chihshang fault the earthquake deformation was exhibited by fresh fractures within the 150- to 200-m-wide surface fault zone, reactivating preexisting fractures in most cases. In addition to daily recorded creep meter data, geodetic measurements, including leveling, distance-and-angle electronic distance meters, and GPS measurements, were carried out during surveys 20-25 days before, 20-25 days after, and 120-125 days after the earthquake. The near-fault surface deformation is mainly characterized by anticlinal folding in the hanging wall and minor gentle synclinal folding in the footwall. The geodetic data show that within our 150- to 250-m-wide networks the coseismic deformation of the main shock produced only about 1-2 cm of horizontal shortening and vertical offset across the fault zone. A larger additional displacement of about 7-9 cm for both the horizontal shortening and the vertical offset occurred as postseismic creep during the 120-125 days following the main shock. We interpret the predominant folding and the rather large postseismic creep as a result of strong velocity strengthening along the fault plane near the surface, which caused locking or coupling effect during the coseismic rupturing. The depth of locked segment, along which the coseismic and postseismic slip decreased dramatically upward, is estimated to be 25-100 m, depending on the site. We interpreted the velocity strengthening and coupling effect at the shallow level as viscoelastic behavior of unconsolidated deposits in the footwall and mélange mudstone in the hanging wall. By incorporating far-fault continuous GPS data, it was found that minor but significant deformation occurred outside of the surface ruptures zone of the Chihshang fault. In addition, a possible back thrust or back fold might have occurred in the hanging wall. We note that the Chengkung earthquake occurred during the dry season. This is consistent with the fact that the mechanical coupling of the fault at shallow depth is higher during drier periods, inferred from the previous seasonal creep data. At outcrop scale, regarding damage of construction such as a concrete retaining wall, most of the coseismic and postseismic horizontal shortening was essentially absorbed by multiple distributed brittle ruptures, which deserves attention in terms of earthquake hazard mitigation. Copyright 2006 by the American Geophysical Union.