Seafloor Geodesy for Approaching Great Earthquakes Around Japan

Abstract:

Seafloor geodesy has been developed as an application of space geodetic technique for the purpose of investigating geodynamic phenomena having their major information in offshore regions. One of such targets is the occurrence of disastrous earthquakes in plate subduction areas. Japan, among others, has repeatedly experienced offshore megathrust earthquakes because of its tectonic location, where multiple plates interact with each other. Most recently, an earthquake of M9.0 occurred off the Pacific coast of east Japan in 2011 with a subsequent huge tsunami, which totally devastated coastal areas and claimed nearly 20,000 lives including those still missing.

We, the group of Japan Coast Guard (JCG), have developed a seafloor geodetic technique combining the GPS positioning and underwater acoustic ranging, which is able to measure the position of the seafloor reference point consisting of multiple acoustic transponders with a precision of a few centimeters. We have deployed our seafloor reference points over two regions on the Pacific side of Japan; one is the region along the Japan trench off the eastern coast where the huge 2011 event occurred and another is the region along the Nankai Trough off the southern coast where earthquakes of around M8 have repeated every 100-150 years.

With these measurements, we have so far successfully obtained important results providing exclusive information for elucidating the plate boundary behavior causing huge earthquakes. In particular, in the region off east Japan, we have revealed different phases of seafloor movements during the period between several years before and after the 2011 event. They include linear intraplate movements with several centimeters per year before the event, which were the first significant offshore geodetic signals detected around Japan, as well as the coseismic displacements of unprecedentedly huge amount over 20 m close to the epicenter and subsequent postseismic movements with various characteristics depending on the area. These seafloor data contribute to the understandings of physical processes relevant to the event beneath the seafloor in this region, which are unable to be clarified only by the terrestrial data.

The presentation summarizes efforts in the field of seafloor geodesy in Japan with a focus on the results obtained by the JCG group.