9:00 AM - 9:15 AM
[SSS05-01] Seismic and Tsunami Observation around the Japan Islands
★Invited Papers
Keywords:cable-type seafloor observation network for earthquakes and tsunamis, earthquake observation, tsunami observation, early warning
Real-time observation of earthquakes and tsunamis that occur in the ocean in the vicinity of their source area is important both for academic and disaster mitigation purposes. In Japan, not only are land areas monitored by a nationwide dense network of seismic observation stations, but real-time observation of earthquakes and tsunamis in the ocean is also carried out on the world's largest scale.
There are several methods of tsunami observation, depending on the distance from the coast. In coastal area, tide gauges directly measure sea level using floats and other devices, but they can only be installed in coastal areas. For a little further offshore area, buoys equipped with GPS (NOWPHAS) are used to measure sea level fluctuations. This system uses buoys to directly measure sea level changes, but as the buoys need to be moored, they can only be installed at sea depths of 100 to 400 m, limiting their placement to about 20 km from the coast. To extend the observation network further offshore, cable-type seafloor observation network, in which the ocean bottom observatories are connected to a landing station using ocean bottom fiber optic cables for power supply and data transmission, is the only practical method. For earthquake observation, which requires higher sampling rates than tsunami observation, cable-type observation is also generally adopted for real-time observation.
The area where giant earthquakes repeatedly occur due to the subduction of the Pacific and the Philippine Sea plates to the Japan Islands is covered by such cable-type observation network. Along the Japan and Kuril Trenches, NIED operates S-net, and along the Suruga and Nankai Troughs, JMA and NIED operate the Tokai/Tonankai cable ocean bottom seismographs systems and DONET. Furthermore, a new cable-type observation network, N-net, is currently being constructed in the western half of the anticipated source area of a Nankai Trough earthquake, which has remained a blank area for real-time observation. Once completed, the offshore areas of the Pacific Ocean, from Hokkaido to Kyushu will be densely covered by these observation networks.
There are two main designs for the cable-type observation network for the ocean bottom seismic and tsunami. One is an in-line system in which the observatories are hardwired to the main cable in a row. This system is adopted by many systems, including S-net, and allows for the simultaneous installation of cables and observatories by cable ship, thus enabling wide coverage in a short-time construction at a relatively low cost. The other is a plug-in node system in which the observation instruments have connectors and can be attached/detached. DONET employs this system, and it is designed to allow replacement and expansion of observation instrument. N-net adopts a hybrid system that combines the merits of the in-line and the plug-in node systems, and is designed to enable the rapid and relatively inexpensive installation of a wide-area observation network and to ensure its expandability in the future.
In this presentation, we will introduce an overview of seismic and tsunami observation networks around the Japan Islands as well as the observed data from the new observation network, N-net, which is currently being constructed in the western half of the anticipated source area of the Nankai Trough earthquake.
There are several methods of tsunami observation, depending on the distance from the coast. In coastal area, tide gauges directly measure sea level using floats and other devices, but they can only be installed in coastal areas. For a little further offshore area, buoys equipped with GPS (NOWPHAS) are used to measure sea level fluctuations. This system uses buoys to directly measure sea level changes, but as the buoys need to be moored, they can only be installed at sea depths of 100 to 400 m, limiting their placement to about 20 km from the coast. To extend the observation network further offshore, cable-type seafloor observation network, in which the ocean bottom observatories are connected to a landing station using ocean bottom fiber optic cables for power supply and data transmission, is the only practical method. For earthquake observation, which requires higher sampling rates than tsunami observation, cable-type observation is also generally adopted for real-time observation.
The area where giant earthquakes repeatedly occur due to the subduction of the Pacific and the Philippine Sea plates to the Japan Islands is covered by such cable-type observation network. Along the Japan and Kuril Trenches, NIED operates S-net, and along the Suruga and Nankai Troughs, JMA and NIED operate the Tokai/Tonankai cable ocean bottom seismographs systems and DONET. Furthermore, a new cable-type observation network, N-net, is currently being constructed in the western half of the anticipated source area of a Nankai Trough earthquake, which has remained a blank area for real-time observation. Once completed, the offshore areas of the Pacific Ocean, from Hokkaido to Kyushu will be densely covered by these observation networks.
There are two main designs for the cable-type observation network for the ocean bottom seismic and tsunami. One is an in-line system in which the observatories are hardwired to the main cable in a row. This system is adopted by many systems, including S-net, and allows for the simultaneous installation of cables and observatories by cable ship, thus enabling wide coverage in a short-time construction at a relatively low cost. The other is a plug-in node system in which the observation instruments have connectors and can be attached/detached. DONET employs this system, and it is designed to allow replacement and expansion of observation instrument. N-net adopts a hybrid system that combines the merits of the in-line and the plug-in node systems, and is designed to enable the rapid and relatively inexpensive installation of a wide-area observation network and to ensure its expandability in the future.
In this presentation, we will introduce an overview of seismic and tsunami observation networks around the Japan Islands as well as the observed data from the new observation network, N-net, which is currently being constructed in the western half of the anticipated source area of the Nankai Trough earthquake.