Ground tilt changes in whole Japan caused by the 2010 Maule, Chile, earthquake tsunami

Oscillating ground tilt perturbations accompanied by the 2010 Maule, Chile, earthquake tsunami were observed in a broad inland area along the Pacific Ocean coast of Japan by a high-density tiltmeter network. In the Japanese islands, the National Research Institute for Earth Science and Disaster Prevention operates approximately 800 Hi-net stations whose average spacing is approximately 20 km and most Hi-net stations are equipped with high-sensitivity accelerometers (tiltmeters), which could record these tsunami-induced ground tilt changes. This is the first case where the tsunami-induced geodetic or seismic signals in inland area are detected by such a high-density wide area observation network as Hi-net. From these data, we obtain a detailed spatiotemporal distribution of tsunami-induced tilt changes, and reveal an obvious relationship between the tilt amplitude and distance from the coast. At distances of 1 km or less, the peak tilt amplitudes are 5 x 10^{-2} μrad, and almost constant against the distance. At distances larger than 3 km, amplitudes decrease inversely with the distance, and reach 5 x 10^{-3} μrad approximately 50 km away from the coast. These observed ground tilt changes were successfully modeled with a loading deformation caused by sea level variations accompanied by the tsunami. From these tilt records observed by the dense and wide-area network, it is possible to evaluate spatial characteristics in the tsunami off the coast.