Modeling the impact on Hawaii of the 2011 tsunami

Modeling the impact on Hawaii of the 2011 tsunami. The earthquake and tsunami that struck Japan on 11 March 2011 killed or injured about 46 000 people and damaged or destroyed just over a million buildings (see PHYSICS TODAY, December 2011, page 33 ). Seven hours after the quake’s major tremor, the tsunami reached the Hawaiian Islands, where its energy took more than six hours to dissipate amid the islands’ reefs, channels, and harbors. Even after traveling 6600 km across the Pacific Ocean, the tsunami packed enough energy to destroy boats and damage docks (see photo). Thanks to a network of tide gauges, pressure gauges, and current profilers, the pattern of waves that developed once the tsunami arrived was measured with high resolution and broad coverage. Yoshiki Yamazaki of the University of Hawaii at Manoa and his collaborators used that data trove to test a model of the tsunami’s impact on the Honolulu coast. Besides the physics of wave propagation, their model has two ingredients: the profile of the megathrust that caused the tsunami and a high-resolution elevation map of the sea floor. The model calculates the wave field on four length scales—from 4 km, which suffices for the open ocean, down to 9 m, which suffices for reefs and harbors. The model not only matches the recorded data, it also provides a detailed view of the complex, multiperiod mix of standing waves that sloshed around the islands. In particular, the model could explain the observed damage: Although the maximum speed of the waves was a modest 1.3 m/s, the abrupt drop of 1.2 m when the waves retreated would have been enough to rip boats from their moorings. (Y. Yamazaki, K. F. Cheung, G. Pawlak, T. Lay, Geophys. Res. Lett. 39, L09604, 2012. Photo courtesy of Hawaii State Civil Defense.)