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GPS refutes prior geological beliefs

Reversing the long-held belief that the Japanese archipelago has been rising, six years of global positioning system (GPS) observations have shown that many areas along the “spine” of the archipelago have in fact been subsiding, a government institute said Wednesday.

The Geographical Survey Institute of the Construction and Transport Ministry said ground level changes of the entire Japanese archipelago had been precisely measured for the first time. The institute has been conducting the GPS observations since 1996 as part of nationwide studies on how the movement of tectonic plates affects the surface of the land.

One of the institutes’ senior researchers, Hiroshi Murakami, reported that the findings were “of considerable interest” at a meeting of the Seismological Society of Japan on Wednesday in Yokohama.
The GPS analyses revealed that ground levels along many coastal regions of the Pacific Ocean, the Sea of Japan and the Seto Inland Sea have been rising, Murakami said at the meeting.

However, the mountainous spine of the Japanese archipelago has been sinking, he said.

The changes in the ground levels are very minute, about one-hundredth of a millimeter over the six-year observation period, according to the findings.

Before the availability of GPS technology, it was thought that the inland regions of the archipelago, in particular the area ranging from Itoi River in Niigata Prefecture to Shizuoka, Shuzuoka Prefecture, and another stretching from Niigata Prefecture to Kobe, were rising. The truth is quite the opposite, Murakami said.

The ground in the littoral areas around the Suruga Bay–the Tokai region that experts are predicting could experience a very powerful earthquake–also were found to be sinking, according to Murakami.

Geographical observations using a GPS are highly accurate horizontally, but they are less useful in gauging vertical ground level changes, he said.

To ensure the accuracy of its findings, the institute carried out the GPS measurements for the six-year period from a network of observation spots nationwide.