POTENTIAL SEISMIC AND TSUNAMI HAZARD FROM THE PALAU TRENCH, AS VIEWED FROM MOLLUSCAN GRAZING NOTCHES IN THE UPLIFTED CORAL ATOLLS OF PALAU

The islands of Palau sit on the western edge of the Yap-Palau trench system in the central, mid-Pacific, an area which today displays remarkably little seismicity. However, the bathymetry and regional geology of the area suggests that it is active tectonically, with the V-shaped trench bottom reaching depths of ~ 8 km only 40 km from the truncated eastern edge of the atoll. Geological studies of Pleistocene rocks in the area suggest that the main island (Babeldaob) is being actively uplifted, despite the lack of modern seismicity.

A curious physiographic feature of Palau is the presence of numerous mushroom-shaped ‘rock islands’, around the base of which the solidified ancient limestone of the reef is cut back precisely at the intertidal zone by as much as 7m. Once thought to be due to the action of waves, the late Prof. Heinz Lowenstam of Caltech recognized over 50 years ago that these notches were the active result of molluscan grazing by chitons and limpets (Polyplacophoran and Archaeogastropod mollusks), who cap their radular teeth with hardening layers of the biogenic iron minerals, magnetite and goethite. Observations of these animals feeding on endolithic algae suggested erosion rates on the order of a mm/year, implying the notches have been forming for a few thousand years.

Of paleoseismic concern is the presence of an occasional remnant notch seen on many of the rock islands, estimated to be at elevations of 15 to 20 m above present sea level, and occasional hints of even earlier notches at an equivalent distance above those. These are probably not remnants of the MIS 5e sea-level highstand at 120 Kyr, as the erosion rate of 1 m/Kyr would have removed them long ago. Another alternative is that they represent sudden seismic uplifts, followed by several thousands of years of stasis. Numerical simulations for a hypothetical tsunami scenario in which the seafloor is uplifted by 20 meters due to a megathrust earthquake near the Palau trench suggests that the coastlines of the Philippines and Papua New Guinea could be hit by tsunami of more than 5 meters in height within one hour, comparable to the Tohoku-oki case, with waves reverberating around the Pacific. These paleo-notches need to be dated accurately (perhaps with U/Th decay techniques on flowstone), and compared with improved paleo-tsunami records for the surrounding regions.