New seismic hazard spotted in Japan’s 2011 quake, Pg12

Key Highlights:

• Fifteen minutes after the 2011 Tōhoku earthquake, the ground across Japan shifted eastward by up to 6 mm.
• Scientists, utilizing satellite data, identified this movement as being triggered by ScS waves.
• ScS waves are seismic waves that travel from the earthquake source, reflect off the Earth's core-mantle boundary, and return to the surface.
• This phenomenon, where deep-Earth reflected waves cause ground displacement, has been identified as a new seismic hazard.

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Detailed Insights:

• The 2011 Tōhoku earthquake, also known as the Great East Japan Earthquake, was a magnitude 9.0-9.1 undersea megathrust earthquake, the most powerful ever recorded in Japan.
• The earthquake occurred off the northeast coast of Honshu, Japan, lasting approximately six minutes and generating a devastating tsunami.
• ScS waves are a type of shear wave (S-wave) that cannot propagate through the Earth's liquid outer core, causing them to reflect off the core-mantle boundary (CMB).
• This is the first documented instance where an ScS wave was powerful enough to cause measurable, permanent ground displacement and reactivate tectonic plate boundaries near the surface.
• The strong main shock of the Tōhoku earthquake destabilized the tectonic plate boundaries, making them susceptible to the additional stimulus from the returning ScS waves.
• Japan is situated at the complex junction of four major tectonic plates: the Pacific, Philippine Sea, North American (or Okhotsk), and Eurasian (or Amurian) plates, contributing to its high seismic activity.

Scientific/Technical Concepts Involved:

• ScS waves: Shear waves that travel through the Earth's mantle, reflect off the core-mantle boundary, and return to the surface.
• Seismic Hazard: The probability of an earthquake occurring in a specific area within a given timeframe, with ground motion exceeding a certain intensity.
• Tectonic Plates: Large, rigid segments of Earth's lithosphere that move and interact, causing earthquakes, volcanic activity, and mountain formation.
• Core-Mantle Boundary (CMB): The interface between Earth's liquid outer core and its solid mantle, located at approximately 2,890 kilometers depth.