Uplifted marine terraces in Davao Oriental Province, Mindanao Island, Philippines and their implications for large prehistoric offshore earthquakes along the Philippine trench

We conducted systematic mapping of Holocene marine terraces in eastern Mindanao Island, Philippines for the first time. Raised marine platforms along the 80-km-long coastline of eastern Davao Oriental Province are geomorphic evidence of tectonic deformation resulting from the westward subduction of the Philippine Sea plate along the Philippine trench. Holocene coral platforms consist of up to four terrace steps: T1: 1–5 m, T2: 3–6 m, T3: 6–10 m, and T4: 8–12 m amsl, from the lowest to highest, respectively. Terraces are subhorizontal, exposing cemented coral shingle and eroded coral heads, while terrace risers are 1–3 m high. Radiocarbon ages, 8080–4140 cal yr BP, reveal that erosional surfaces were carved onto the Holocene transgressive reef complex which grew upward until ∼8000 years ago. The maximum uplift rate is ∼1.5 mm/yr based on the highest Holocene terrace at <11.4 m amsl. The staircase topography and meter-scale terrace risers infer that at least four large earthquakes have uplifted the coast in the past ∼8000 years. The deformation pattern of the terraces further suggests that seismic sources are probably located offshore. However, historical earthquakes as large as MW 7.5 along the Philippine trench were not large enough to produce meter-scale coastal uplift, suggesting that much larger earthquakes occurred in the past. A long-term tectonic uplift rate of ∼1.3 mm/yr was also estimated based on Late Pleistocene terraces.

► Uplifted marine terraces were recognized in eastern Mindanao Island, Philippines.
► Uplifted Holocene marine terraces were formed by large coseismic uplift events.
► Probably source faults are thrust faults close to and along the Philippine trench.
► Prehistoric earthquakes should be larger than MW 7.5 to produce meter-scale uplift.