5-7. Oceanographic study and global scale monitoring at ocean bottom using submarine cables

 

 Using GeO-TOC, GOGC, GPT decommissioned submarine cables, we have carried out seismological and electro-magnetic studies. We monitored earthquakes using IZU station attached to the GeO-TOC cable deployed at the forearc slope of the Izu-Bonin Trench since 1997. We collected 5-year dataset for earthquakes occurring along the Izu-Bonin subduction zone. However, it is pity that it abruptly terminated on October 1, 2002, just after a huge typhoon passed over the western edge of Sagami Bay.

We also carried out an experimental observation (VENUS project) using the GOGC submarine cable between Guam and Okinawa (Fig. 1). A broadband seismometer was used (Fig. 2). It observed Ms6.1 aftershock of Chichi earthquake in Taiwan　(Fig. 3).  We found extremely time-dependent ocean bottom noise around 100-200 seconds　(Fig. 4).

Detectability of the electric potential variation originated in Earth's core is re-evaluated.  It is confirmed theoretically that the electric potential variation can be observed using the cables which are now operating for electric observations(Fig. 5).

Because there are great technological and science achievements since the previous workshop, we are going to stage an “International workshop on Scientific use of Submarine Cables and Related Technologies” in Tokyo during June 25 and 27, 2003.

 

 

Fig. 1. GeO-TOC　(Guam-Ninomiya) and GOGC(Guam-Okinawa) cable and locations of IZU and VENUS test site.

 

Fig. 2 . A snapshot of the broadband seismometer a the VENU site ( 2100m deep).

Fig. 3. Aftershocks of Chichi Earthquake observed by VENUS  broadband seismometer (on Nov. 1, 1999 , Ms6.1).

Fig. 4 . Noise level variation observed by VENUS broadband seismometer.

Fig. 5.  Expected electric potential variation due to torsional oscillation in Earth's core.