Multiple large earthquakes in the past 1500 years on a fault in metropolitan Manila, the Philippines
The first 14 C-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 14 C ages on detrital charcoal constrain the entire stratigraphic sequence to the past 1300–1700 years. We rely on the 14 C ages to infer faulting recurrence of hundreds rather than thousands of years. Minimal soil development and modern 14 C 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.