REEF-PLATFORM BOULDERS AS SIGNATURES OF STORMS AND TSUNAMIS ON TROPICAL COASTLINES (Invited Presentation)

Reef-platform boulders are a subset of coastal boulder deposits (CBDs). In this presentation, we use large coral and limestone boulders deposited on intertidal reef flats and beaches on tropical Pacific coastlines to illustrate the value and significance of these high-energy-wave signatures. Many Pacific islands are prone to strong waves generated by tropical cyclones and tsunamis, while coral reefs that fringe island shores provide a source of clasts for the quarrying of coastal boulders. As a result, impressively-sized megaclasts and boulder fields consisting of numerous large reef boulders have been reported from around the Pacific, sometimes by local communities in the form of ancient legends, suggesting these conspicuous landforms have been used as hazard warnings in traditional knowledge for many generations. When boulders were detached by storm or tsunami waves from a living coral reef, age-dating of corals comprising the fabric of boulders can also provide an opportunity to reconstruct the long-term extreme event history.

There is increasing attention on landslide and volcanic tsunamis in recent years. Here we include our research on the characteristics of a reef-top boulder field created by a local submarine landslide tsunami in Fiji (the 1953 Suva tsunami). The boulder distribution confirms that the shorter-period-than-normal tsunami waves generated by submarine landslides would create a boulder field resembling typical storm boulder fields due to relatively short transport distances. We suggest this boulder field can be used as a reference set in identifying possible prehistorical local submarine landslide tsunami occurrences elsewhere and that it will be particularly relevant for tropical Pacific island coasts where submarine slope failures are relatively common and past hazards are not well documented.

This presentation is supported by the NSF-funded ISROC—Inundation Signatures on Rocky Coastlines—Research Coordination Network. Their support is gratefully acknowledged.