Taiwan's destructive Chi-chi earthquake of September 21, 1999, was a dramatic expression of active tectonic processes at a complex collisional plate boundary. It resulted in more than 2,400 causalities and tens of billions of dollars in property loss. During the earthquake, an 80-km stretch of the country's mountainous backbone moved upward and westward along the range-bounding Chelungpu thrust fault (Figure la). A team of earthquake geologists from the United States, in collaboration with geoscientists from Academia Sinica, National Taiwan University and the Central Geological Survey of Taiwan, worked together to address questions concerning the recurrence of large-magnitude earthquakes along reverse faults in Taiwan.