A "silent earthquake" in Hawaii caused a 190-square-kilometre slab of the Kilauea Volcano to slip nearly 9 centimetres into the sea.

And no one noticed.

No one, that is, except American researchers using global positioning system (GPS) devices to measure the slow earth movements.

A GPS device measures "silent earthquakes" in Hawaii

Geophysicists with the U.S. Geological Survey and Stanford University recorded the silent earthquake on Kilauea's southern flank in November 2000.

The earthquake measured 5.7 on the Richter scale, but moved so slowly the ground didn't shake.

A mass of earth 19 by 10 kilometres in area, and eight kilometers thick, slid down the volcano about nine centimetres over a 36-hour period.

The researchers said a tropical storm that dumped a metre of rain on the Big Island of Hawaii may have triggered the slide.

The authors of the study, which appears in Thursday's issue of the journal Nature, said their work could be used to model how a much faster massive land slide could occur.

In an article accompanying the study, geophysicist Steven N. Ward of the University of California at Santa Cruz said a land mass that size suddenly collapsing into the ocean would cause a massive tsunami that would threaten coastal areas in California, Chile and Australia.

A tsunami, or seismic sea wave, can travel up to 800 km/h on the open ocean and can build up to 20 metres high in shallow water.

But the sudden collapse of a volcano such as this occurs only about once every 10,000 years, Ward said.

The last such collapse in Hawaii occurred roughly 200,000 years ago.