Academic journal article The Science Teacher

Ultrasound of Earth's Crust

Academic journal article The Science Teacher

Ultrasound of Earth's Crust

Article excerpt

Research by a team of U.S. and Japanese geoscientists may help explain why part of the seafloor near the southwest coast of Japan generates such devastating tsunamis, such as the 1944 Tonankai event, which killed at least 1,200 people. The findings will help scientists assess the risk of giant tsunamis in other regions of the world.

Geoscientists from The University of Texas (UT) at Austin and colleagues collected three-dimensional (3-D) seismic data that reveals the structure of Earth's crust below a region of the Pacific seafloor known as the Nankai Trough. The resulting images are akin to ultrasounds of the human body. The results address a longstanding mystery as to why earthquakes below some parts of the seafloor trigger large tsunamis, while earthquakes in other regions do not.

The 3-D seismic images allowed researchers to reconstruct how layers of rock and sediment have cracked and shifted over time. They found two things that contribute to big tsunamis. First, they confirmed the existence of a major fault that runs from a region known to unleash earthquakes about 10 km deep right up to the sea-floor. When an earthquake happens, the fault allows it to reach up and move the seafloor up or down, carrying a column of water with it and setting up a series of tsunami waves that spread outward.

Second, and most surprising, the team discovered that the recent fault activity, probably including the slip that caused the 1944 event, has shifted to landward branches of the fault, becoming shallower and steeper than it was in the past. …

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