1999 Chi-Chi Earthquake: A Case Study on the Role of Thrust-Ramp Structures for Generating Earthquakes

Abstract

The 21 September 1999 Chi-Chi earthquake (M w 7.6) occurred on eastdipping shallow thrust faults that produced a high-relief surface rupture. Extraordinary surface breaks appeared that could be clearly traced for about 100 km across many counties. These thrust faults, the Chelungpu and Shihkang, are part of an active fold-and-thrust belt related to ongoing recent arc-continent collision. Measurement of slip vectors along the earthquake rupture indicates that the orientation of the maximum shear stress changed from a westward direction (N70-90ЊW) on the Chelungpu fault to a northwestward direction (N30-40ЊW) on the Shihkang fault. The stress field underwent a clockwise rotation of about 40Њ during the Chi-Chi earthquake. Near-rupture vertical displacements in the hanging wall of the Shihkang fault have more cumulative displacement than on the Chelungpu fault, which is consistent with Global Positioning System (GPS) measurements. Maximum vertical offset on the rupture was found to be about 10 m by the surficial rupture and GPS measurements. In addition, analysis of crustal deformation by GPS measurements on the hanging wall defines a coseismic uplift related to a fault ramp structure. Our synthesis of geological and geodetic analyses shows the importance of ramp structures associated with thrust faults for generating large earthquakes and provides a general framework for understanding earthquake in fold-and-thrust belts. Large surficial coseismic uplift and strong asperities appear to be a function of fault ramp geometry. Our analysis also indicates that, in general, ramp structures in fold-andthrust belts may have a high potential in generating large earthquakes.

Thrust-related river terrace development in relation to the 1999 Chi-Chi earthquake rupture, Western Foothills, central Taiwan

“…8. produces near-surface uplift (Jackson and Bilham, 1994;Chen et al, 2001). Such a ramp is interpreted to be responsible for the recent uplift of the Western Foothills and is consistent with seismic reflection profiles.…”

Paleoseismic evidence for coseismic growth-fold in the 1999 Chichi earthquake and earlier earthquakes, central Taiwan

“…Based on seismic reXection proWles and focal mechanisms of the mainshock, the earthquake occurred on a shallow-dipping (20-30°E) thrust ramp of the Chelungpu fault ( Fig. 1; Chiu, 1971;Kao and Chen, 2000;Chen et al, 2001d;Wang et al, 2002). This shallow fault dip is consistent with the inferred dip of the fault as determinated by paleoseismic and subsurface cored borings (Chen et al, 2001a(Chen et al, ,b, 2004Ota et al, 2001Ota et al, , 2005.…”

Lushan M S7.0 earthquake: A special earthquake occurs on curved fault

“…Furthermore, as shown by coseismic GPS observation, the shortening sub-parallel to east-west direction is a dominant component of the horizontal deformation in near-field of the Tohoku-Oki earthquake; meanwhile, significant southward component occurred on the north flank of hanging wall and significant northward component occurred on the south flank [39]. The characters of surface rupture distribution and the seismic reflection profiles indicate that the Chi-Chi earthquake occurred on a thrust nappe structure with a straight fault plane [40]. However, for those great earthquakes taking place on subducting interplate boundary like the 2010 Chile earthquake and the 2011 Japan earthquake, the ruptured surface on subduction fault usually has significant curvature regarding the characteristic rupture length (≥200-300 km) of great earthquakes [41].…”