Japan “Megaquake” focal regions double in size from previous estimates

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The government’s disaster prevention panel on Tuesday revised the basic plan for disaster preparedness in the wake of the March 11 earthquake and tsunami, seeking to reinforce measures against tsunami waves that caused most of the damage in the disaster in northeastern Japan, officials said.

A separate panel of experts compiled an interim report the same day doubling the focal area of potential megaquakes that could strike Japan and provisionally raising the magnitude of such temblors from 8.7 to 9.0.The Central Disaster Prevention Council, headed by Prime Minister Yoshihiko Noda, created a section devoted to tsunami countermeasures in the revised basic disaster prevention plan.

The Nankai megathrust earthquakes are great earthquakes that occur along the fault that forms the plate interface between the subducting Philippine Sea Plate and the overriding Amurian Plate (part of the Eurasian Plate), which dips beneath southwestern HonshuJapan. All of these great earthquakes have given rise to damaging tsunami.

The Nankai Trough is the near-surface extension of a zone of active seismicity that dips beneath SW Japan. The rupture zone has been subdivided into five areas with respect to seismic modelling.

In 1999, the likelihood of the occurrence of a great earthquake in the Tokai area in the period 2000-2010 was estimated to be in the range of 0.35.–0.45.

Despite the uncertainty of when such an earthquake will occur, local authorities are already taking action to prepare residents for what they regard as an inevitability.

Possible focal regions for a megaquake that experts say could occur along the Nankai Trough off the southern coast should be doubled in size from previous estimates, according to an interim report compiled Tuesday by a Cabinet Office panel.

With the expansion of possible focal regions, the magnitude of the megaquake could be 9 or higher should the three earthquakes occur concurrently, according to the report.

The revision was based on the results of recent studies, with the newly estimated possible focal regions covering an area of about 110,000 square kilometers, expanding to the north and southwest.

The newly added region on the north side has been struck by earthquakes that generated low-frequency seismic waves at a depth of about 30 kilometers or deeper.

The report also included areas off Miyazaki Prefecture as part of the newly estimated possible focal regions because of the likelihood that a magnitude-7.5 quake could occur in the area in conjunction with the three quakes.

The newly estimated possible focal regions have also been expanded to the east to cover the plate boundaries in Suruga Bay, as well as a belt of fault lines centered on the mouth of the Fujikawa river, which flows into the bay.

The interim report stated that tsunami would be triggered by not only the newly estimated possible focal regions, but also an area south of the regions at a depth of about 10 kilometers or less.

The Nankai Trough 南海トラフ is a submarine trough located south of Japan‘s island of Honshū, extending approximately 900 km offshore. In plate tectonics, the Nankai Trough marks a subduction zone that is caused by subduction of the Philippine Sea Plate beneath Japan, part of the Eurasian plate .

This plate boundary would be an oceanic trench except for a high flux of sediments that fills the trench. Within the Nankai Trough there is a large amount of deformed trench sediments (Ike, 2004), making one of Earth’s best examples of accretionary prism.

Furthermore, seismic reflection studies have revealed the presence of basement highs that are interpreted as seamounts that are covered in sediments.

The Nankai Trough is actively deforming and marks a region of seismic activity. Deformation is concentrated in the outermost imbricate zone, with a significant amount of “out of sequence” thrusting occurring landward.

Based on the work of Operta et al., 2006, several areas of intense tectonic activity in the Nankai Trough were identified using full waveform tomography. The upper portion of the upper accretionary prism and the underlying backstop are currently undergoing a great deal of compressional pressure. Several thrust faults were identified by Operta et al., 2006, of which the thrust faults closest to the subduction zone are active.

Furthermore, Pisana et al., 2006, identified protothrusts and decollement surfaces along the Nankai Trough. Recently there has been an increase in interest in the release of water from illite clays in subducting sediments.

Source: Mainichi

Source: Yomiuri

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