a b s t r a c tWe document regional pore-fluid pressures in the active Taiwan thrust belt using 55 deep boreholes to test the classic HubberteRubey hypothesis that high static fluid pressures (depth normalized as l ¼ P f / r r gz) account for the extreme weakness of thrust faults, since effective friction m * f ¼ m f ð1 À lÞ. Taiwan fluid pressures are dominated by disequilibrium compaction, showing fully compacted sediments with hydrostatic fluid pressures at shallow depths until the fluid-retention depth z FRD z 3 km, below which sediments are increasingly undercompacted and overpressured. The HubberteRubey fault weakening coefficient is a simple function of depth (1 À l) z 0.6z FRD /z. We map present-day and pre-erosion fluid pressures and weakening (1 À l) regionally and show that active thrusts are too shallow relative to z FRD for the classic HubberteRubey mechanism to be important, which requires z !~4z FRD z 12 km to have the required order-of-magnitude HubberteRubey fault weakening of (1 À l) ~0.15. The bestcharacterized thrust is the Chelungpu fault that slipped in the 1999 (M w ¼ 7.6) Chi-Chi earthquake, which has a low effective friction m * f z0:08e0:12, yet lies near the base of the hydrostatic zone at depths of 1e5 km with a modest HubberteRubey weakening of (1 À l) z 0.4À0.6. Overpressured Miocene and Oligocene detachments at 5e7 km depth have (1 À l) z 0.3. Therefore, other mechanisms of fault weakening are required, such as the dynamical mechanisms documented for the Chi-Chi earthquake.
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