Japan, one of the most tremor-prone countries in the world, started work Thursday to beef up its undersea earthquake monitoring system.

The system will be placed 1,000 to 2,000 metres (3,280-6,562 feet) beneath the ocean surface to gather data on seismic activity that will be transmitted by fiber-optic submarine cable to an above-ground observation centre.

The cable system will provide the Japan Meteorological Agency with an additional five earthquake monitoring devices and three tsunami monitoring devices on the ocean floor, according to contractor NEC Corp.

The equipment left the port of Yokohama near Tokyo on Thursday after a purification ceremony by a Shinto priest.

"It's an important operation that needed a lot of preparation. Just to put the cable on board required two days," said NEC engineer Kenji Hishiki.

After the system is put in place there will be nine ocean bottom earthquake monitoring devices in the region and four tsunami monitoring devices.

The system will be placed on the seabed off the Pacific coast of central Japan to support monitoring devices already in place.

The equipment will enable the Meteorological Agency to gather seismic information from even the smallest of ocean trench earthquakes, NEC said.

Japan, which lies at the crossing of four tectonic plates, experiences 20 percent of the world's powerful earthquakes and is constantly striving to protect itself against major tremors.

Japan has started a world-first early warning system for earthquakes by monitoring seismic waves.