Indonesia quake too weak for oceanwide tsunami

SYDNEY, July 18 (Reuters) A tsunami that hit Indonesia's Java island killing at least 230 people remained localised because the earthquake that caused it was dramatically smaller than the 2004 quake that generated an Indian Ocean-wide tsunami.

Seismologists also said the earthquake's physical area was significantly smaller and caused only a small change in the ocean floor, limiting the size of the tsunami that hit Java yesterday.

The two quakes were not linked, except that they occured on the Eurasian fault line which runs along the Indonesian archipelago and is part of the ''Pacific Ring of Fire'', they said.

The US-based Pacific Tsunami Warning Center put the latest earthquake's magnitude at 7.2 while the US Geological Survey estimated it at 7.7. Indonesia's state meteorology and geophysics agency said the quake's strength was 6.8 on the Richter Scale.

''The possibility of a 7.2 magnitude earthquake producing an ocean crossing tsunami is very dim, quite negligible. That is why it was more localised,'' geophysicist Victor Sardina at the Pacific Tsunami Warning Center in Hawaii, told Reuters.

''In terms of tsunami generating potential it is not even close to the worst case scenario,'' Sardina today said.

Australia's Seismology Research Centre said a quake would need to be at least 8.5 to generate an oceanwide tsunami.

The 2004 quake off Indonesia's Sumatra was 9.3 on the Richter scale, releasing far more energy, and sending a tsunami across the Indian Ocean that left 230,000 people dead or missing.

The Java tsunami only hit 300 kms of coast around Pangandaran beach near the town of Ciamis on the southern shore.

''The current magnitude estimate of 7.7 makes the total energy released (by the Java quake) something of the order 1,000 times less than the big earthquake,'' said Gary Gibson, senior seismologist at Australia's Seismology Research Centre.

''The rupture on this one would only be about 70 km long, whereas the length of the rupture of the Sumatra earthquake was about 1,200 km,'' Gibson said.

''More importantly, the (ocean floor) displacement on the big one was about 10 to 20 metres (30 to 60 feet) in parts, whereas in this one it would be no more than a few metres.'' The Java quake produced smaller waves than the 2004 tsuanmi.

Indonesian television reported waves up to 1.5 metres high pummelled Pangandaran beach. Officials said nearly 130 people are missing after waves washed away buildings, wooden cottages and kiosks lining the shoreline facing the Indian Ocean.

Seismologists said another factor limiting the size of the Java tsunami was the physical change in the ocean floor.

Unlike the Sumatra quake's vertical thrust, where one tectonic plate lifted dramatically above the other, in the Java quake, the northern plate slipped under the southern plate.

''It is not a simple surface rupture, it is almost certainly a horizontal rupture and that will minimise the movement you get,'' explained Gibson.

The Pacific Tsunami Warning Center's Sardina said the only connecton between the latest quake and the one in 2004 may be that the big Sumatra quake could have altered the tectonic layout, which was still adjusting.

''Events of that magnitude can rearrange whole faulting systems in an area and upset tectonic activity,'' said Sardina.

''If you look at a tectonic map of that area, in terms of faults it is a mess. There are thousands of faults.'' REUTERS SB VA DS1200