Underwater landslide may have caused localized increase in 2011 tsunami height

Science : When the 2011 Tōhoku earthquake struck, the resulting tsunami caused a 10-m surge along much of Japan’s coast. Along a 100-km stretch, where the surge reached 40 m, some 4500 people were killed—one quarter of the total death toll. Why that small stretch experienced such a huge surge has been the subject of extensive study. Now Stephan Grilli of the University of Rhode Island, Narragansett Bay, and his colleagues propose that an underwater landslide may have been responsible. To determine ocean surface motion the day of the quake, they examined data from gauges placed along the shore. That data led them to conclude that the earthquake caused a 20 km × 40 km slab of sediment nearly 2 km thick to slide 300 m down the Japan Trench near the northern end of the fault line, 170 km from shore, and 4.5 km below the ocean’s surface. If such a landslide were involved, it would make it more difficult to predict the size of future tsunamis when they occur in regions prone to landslides. However, sea-floor mapping has not shown any evidence of such a slide, and other seismologists say that the answer may be simpler, suggesting that a secondary quake could have been the cause.