Reconnaissance survey of coastal boulders in the Moro Gulf (Philippines) using Google Earth imagery: initial insights into Celebes Sea tsunami events

A recent study has suggested that palaeo-tsunami events in the South China Sea may have attained wave heights of up to 15 m. One method for investigating palaeo-tsunami along coastlines is the study of boulders that, through measurement and the use of hydrodynamic equations, may assist in understanding wave heights. An area with known high magnitude tsunami is the eastern coast of Moro Gulf on the island of Mindanao (Philippines) in the Celebes Sea, where a damaging tsunami of up to 9 m high occurred in 1976. This study undertakes a reconnaissance of the coastline using Google Earth imagery and, appreciating limitations, identifies two potential coastal sites with presumed wave-transported boulders near Namat and Sedem, c. 10 and 55 km from the 1976 earthquake epicentre respectively. Using Google Earth tools, 12 boulders were measured and, with some estimated parameters, heights calculated for both tsunami and storm waves required to transport the boulders. This preliminary analysis indicates that four of the boulders may have been transported by storm activity, but all 12 boulders could have been transported by the 1976 tsunami. Indeed, the orientation of the majority of boulders indicates a flow direction consistent with the location of the 1976 earthquake epicentre. Furthermore, there is no evidence presented in this preliminary analysis to suggest that a tsunami of a magnitude greater than the 1976 event has affected this coastline. An hypothesis is concluded that could be used to guide future fieldwork in this regionally important area of natural hazard studies