Multiple large earthquakes in the past 1500 years on a fault in metropolitan Manila, the Philippines

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