Slip zone and energetics of a large earthquake from the Taiwan Chelungpu-fault Drilling Project

Caught in the slips The magnitude 7.7 Chi-Chi earthquake of 1999 caused widespread destruction across central Taiwan and left behind a large slip of up to 10 metres at or near the surface. This provided a rare opportunity to drill boreholes to sample material from a fault that had recently undergone a large slip. By combining results from the fault core analyses with seismological observations, earthquake efficiency can be estimated, and important constraints placed on understanding of earthquake faulting mechanisms. Determining the seismic fracture energy during an earthquake and understanding the associated creation and development of a fault zone requires a combination of both seismological and geological field data 1 . The actual thickness of the zone that slips during the rupture of a large earthquake is not known and is a key seismological parameter in understanding energy dissipation, rupture processes and seismic efficiency. The 1999 magnitude-7.7 earthquake in Chi-Chi, Taiwan, produced large slip (8 to 10 metres) at or near the surface 2 , which is accessible to borehole drilling and provides a rare opportunity to sample a fault that had large slip in a recent earthquake. Here we present the retrieved cores from the Taiwan Chelungpu-fault Drilling Project and identify the main slip zone associated with the Chi-Chi earthquake. The surface fracture energy estimated from grain sizes in the gouge zone of the fault sample was directly compared to the seismic fracture energy determined from near-field seismic data 3 , 4 . From the comparison, the contribution of gouge surface energy to the earthquake breakdown work is quantified to be 6 per cent.