**5. Conclusions**

140 Earthquake Research and Analysis – Seismology, Seismotectonic and Earthquake Geology

Fig. 11. Correlation between earthquake events at nine excavated sites on the Shihkang and Chelungpu faults, showing occurrence time and recurrence interval. The occurrence time estimates corresponds to the color rectangles based on radiocarbon dating. The horizontal black lines indicate the age range for each event at each site. Lines terminated by vertical bars have defined upper or lower age limits; arrowheads indicate undetermined limits. The

Our analysis of the configuration of paleoseismic excavations along the 1999 Chi-Chi earthquake rupture identified three structural features of the fault tip deformation associated with thrust-related folding, suggesting that the evolutionary path of thrustrelated folds in the excavation develops from fault-propagation folding to breakthrough fault-propagation folding and finally to fault-bend folding (Fig. 10). Stage I. The structural configuration of the Shijia and Siangong-temple excavations is interpreted to be the product of a monoclinal fold above the blind thrust fault during the earliest part of thrust-related fold development; this indicates the formation of a growth fault-propagation fold based on the trishear deformation model (Fig. 10a). The folding characters reveal onlap and overlap geometries, unconformity, fanning forelimb dip, and wedge-shaped deposits; each wedged depositional unit represents a surface-rupturing paleoearthquake event. Stage II. The structural configuration of the Pineapple-field excavation and the north wall of the Chushan excavation is interpreted to be a breakthrough fault-propagation fold showing a recumbent anticline with an overturned forelimb. The advancing thrust fault is found to have broken through the ground surface, and the continuing fault propagation rotates the forelimb in both the hangingwall and footwall. The folding characters yield a tightly folded hinge, a steep and narrow folded forelimb with pronounced bed thinning, and an anticlinal hinge with bed thickening (Fig. 10b). Stage III. The structural configuration of the south wall of the Chushan excavation is interpreted to be a fault-bend fold. The tip of the main thrust fault

breaks through the anticlinal hinge, forming a hangingwall-fixed fold (Fig. 10c).

Over the past tens years, we have excavated numerous trenches on the Chelungpu fault, where the surface rupture shows surface deformation within the Holocene deposits (Fig. 1). Here, we integrate paleoseismologic data with the data derived from excavations to understand the behavior of the Chelungpu fault. By excavating numerous sites along the strike of the Chelungpu fault, we document six surface-rupturing events that have occurred over the last 2000 years. These are the events that occurred in 1999 A.D. (E1), 430–300 yr BP (E2), 790–680 yr BP (E3), 950–790 yr BP (E4), 1550–1380 yr BP (E5), 1900–1710 yr BP (E6), and

locations of excavation sites (A-I) are shown in Figure 1.

The deformed configuration at the fault tip, resulting from the four excavated sites along the 1999 Chi-Chi earthquake rupture, showed three geometric patterns: fault-propagation, breakthrough fault-propagation, and fault-bend folds. The development deformation is modified by subsequent translation on the propagating fault tip. During the initial deformation, the thrust fault propagates upward through the undeformed units, producing a fault-propagation fold. Then, the fault tip breaks through the ground surface, resulting in a transition from breakthrough fault-propagation folding to fault-bend folding.

The paleoearthquake events are determined by radiocarbon dating of wedge-shaped colluvial deposits exposed in trenches. Evidence obtained from excavation across the Chi-Chi earthquake rupture indicates that there have been six large earthquake events during the past two millennia, which occurred in 1999 A.D. (E1), 430–300 yr BP (E2), 790–680 yr BP (E3), 950–790 yr BP (E4), 1550–1380 yr BP (E5), and 1900–1710 yr BP (E6), suggesting relatively short recurrence intervals in the order of 150–700 years.
