Large earthquakes along the subducting plate boundary occur repeatedly in the area of an asperity that consists of a strongly coupled zone between two plates. Other areas along the subduction zone are considered to be stable-quasi-stable slip region, which is called a non-asperity, might release strain energy caused by oceanic plate subduction. The physical states of large asperities under the ocean are not well known at present because of the lack of offshore stationary observation networks (e.g., geodetic, seismic and electromagnetic networks).
Strong PP reflections from the subducting plate boundary were found in aseismic zones along the Japan Trench and in the slow slip region in the Tokai region. These features suggest the presence of low-Vp/soft materials and/or fluid along the subducting plate boundary. Such regions might cause continuous or intermittent aseismic slow-slips. If we can map areas of strong PP reflections from observations such as refraction-reflection studies using Ocean Bottom Seismometer (OBS) -airgun surveys, we will be able to obtain the distribution of asperities along the plate boundary.
Assuming that slip acceleration at non-asperity regions might trigger a large earthquake at adjacent asperities, a sudden change of physical states in a non-asperity region might suggest a high probability of plate-boundary earthquakes. Changes due to slip acceleration might be detected by continuously monitoring seismic reflection intensity at non-asperity regions. To perform continuous monitoring, we propose the Accurately Controlled Routinely Operated Signal System (ACROSS), with an integrated active monitoring method using continuously transmitting seismic and electromagnetic sinusoidal waves, which are accurately controlledby a GPS clock with a sophisticated signal analysis method.
The ACROSS seismic source at Toki city in central Japan has been operated continuously for more than 2 years. A field experiment in the Tokai region, central Japan, using this transmission method provided sufficient S/N ratios for the Pg phase traveling 60 km through stacking the data for one month.
Submarine cable OBS systems near the trenches enable us to continuously monitor seismic reflection signatures provided by ACROSS systems located on land. The planned submarine cable OBS in the Tonankaki region might be a good real-time receiver system.
The Exploration of Asperities-Reflectors System (EARS) is proposed for integrating the necessary research components-mapping, monitoring, and real-time continuous monitoring of the Earth's crust. In this paper, we describe the analytical method and important points in such a study.