Numerical estimation of maximum possible sizes of paleo-earthquakes and tsunamis from storm-derived boulders

Estimating the magnitudes of potential large earthquakes and associated tsunami hazards along a subduction zone is important for risk evaluation. Coastal boulder deposits can serve as valuable geological indicators in intertidal zones with high wave velocity, even though finer sediments are generally washed away. This report presents a new method to constrain the maximum possible sizes of paleo-earthquakes and tsunamis using boulders of storm wave origin as a constraint in numerical simulations. The fundamental assumption is that presently observed storm-derived boulders have not moved a great distance by past tsunami waves. This illustrative case study uses the sizes and spatial distributions of storm wave boulders on the reef at Kudaka Island, Okinawa Prefecture, which faces the Ryukyu Trench. In all, 47 tsunami scenarios with different locations and fault parameters were assumed. Results show that tsunami waves with a maximum water level of 2.2 m and maximum flow speed of 1.5 m/s at a beach of Kudaka Island did not strike the island. Under this condition, earthquakes of Mw≥8.3 are unlikely to have occurred in the central Ryukyu Trench if one obeys an empirical scaling law for subduction earthquakes or ≥2 m of maximum possible slip near the trench if the fault size is fixed as 100 km × 50 km. Those tsunami and earthquake sizes could not have been generated during this period if one assumes that the reef of Kudaka Island and emplacement of storm boulders began more than 3500 years ago. This finding implies that the seismogenic mechanism of the Ryukyu Trench and the risk of a giant earthquake differ from those of other trenches such as the Japan Trench and the Chilean Trench, where giant earthquakes of Mw≥9.0 have occurred repeatedly during the past. The proposed method would enable researchers to estimate the maximum possible sizes of the paleo-earthquakes and tsunami waves in any region, even in regions with a vague or unknown history of past events because of a lack of sandy tsunami deposits or seismic records.