We now have the use of a ‘stethoscope’ and an ‘endoscope’

Developments leading up the Tonankai earthquake

　It is thought that before a giant earthquake occurs there is increased micro-earthquake activity at shallow subseafloor depths, a phenomenon which is normally not seen. We expect DONET to be able to capture these micro-earthquake changes.

　Changes also occur to the seafloor depth. Normally, the seafloor where DONET is set up is dragged along by the sinking Philippine Sea plate, and sinks deeper. Immediately before the earthquake occurred however this phenomenon was reversed, and the seafloor depth was seen to decrease. The DONET water pressure gauges also caught a 1 cm change in water pressure. By capturing changes of the seafloor with the 20 observation stations, we can obtain ‘surface’ data.

　“DONET is like a stethoscope. The long-term borehole observatory now connected as well will make these data more detailed,” explains Kaneda. The strain gauges will make it possible to get a sense of how deep subseafloor forces change, and thermometers enable a sense of the movements of fluids deep beneath the seafloor.

　If DONET is a stethoscope, then the long-term borehole observatory can be likened to an endoscope, and by combining the two we are able to better understand the earth’s ‘health situation’. We expect these detailed data to also play a very important role in the prediction of earthquakes and tsunamis.

Spreading the net wider and deeper

　The Tonankai earthquake which we fear will occur in the future is said to be linked to the Nankai earthquakes. In fact, there are many examples in the past where they occurred in conjunction. To further the investigation into this link DONET2 was set up to connect the cabling from Kaifu region, Tokushima prefecture to Muroto city in Kochi prefecture, and roll out the system towards the south-east of Shikoku island. Kaneda says that in future, an observation network like DONET also needs to be deployed toward southwest area.

　The aim for the deep subseafloor observatory on the other hand is eventually to be installed at a depth of several thousands of meters beneath the seafloor, in what is called the earthquake nest. Kaneda explains that the connections made this time are the first step towards that goal. First the equipment will be tested at a depth of 1,000 m, and a new observatory will be built to reflect those data. When this is installed at even greater depths, we will also be counting on the efforts of the CHIKYU.