The Korea Meteorological Administration (KMA) said it has drilled 100 metre-deep holes (330 feet) for the equipment in three locations -- Ganghwa Island, west of Seoul, Yeoncheon, north of Seoul, and in the northeastern border area of Inje.

The agency said the main purpose was to detect quakes in North and South Korea. "They also can be used to monitor any North Korean nuclear tests," spokesman Lim Jang-Ho told AFP.

The agency said the sensors were being installed deep underground to avoid interference from surface noise.

It said they could also detect any attempts to dig cross-border invasion tunnels, like those the North dug during the Cold War.

Currently, South Korea has only one earthquake observation centre near Ulleung Island in the East Sea (Sea of Japan).

North Korea tested an atomic weapon in October 2006, sparking international alarm and United Nations sanctions.

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