The first 14C-based paleoseismic study of an active fault in the Philippines shows that a right-lateral fault on the northeast edge of metropolitan Manila poses a greater seismic hazard than previously thought. Faulted hillslope colluvium, stream-channel alluvium, and debris-flow deposits exposed in trenches across the northern part of the west Marikina Valley fault record two or three surface-faulting events. Three eroded, clay-rich soil B horizons suggest thousands of years between surface faulting events, whereas 14C ages on detrital charcoal constrain the entire stratigraphic sequence to the past 1300–1700 years. We rely on the 14C ages to infer faulting recurrence of hundreds rather than thousands of years. Minimal soil development and modern 14C ages from colluvium overlying a faulted debris-flow deposit in a nearby stream exposure point to a historic age for a probable third or fourth (most recent) faulting event.

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