An Indonesian woman who lost her six-year old son in Monday's tsunami weeps as she sits on the rubble of a house in Pangandaran, West Java, Indonesia.© AP Photo/Dita Alangkara

A local-scale tsunami has killed at least five people in Indonesia. The recently installed warning system did issue an appropriate alert, authorities say, but it is unclear how many lives this warning saved.

A tsunami warning was issued at 11.07 GMT today (17.07 local time), just 4 minutes after a quake measuring 7.2 on the Richter scale occurred several hundred kilometres south of Jakarta, Indonesia, in the Sunda fault. Gauges deployed in the Indian Ocean measured a 7-centimetre sea-level rise, indicating that the quake had generated a tsunami.

Coastlines on Indonesia's island of Java shortly thereafter were struck by a 2-metre-high wave, according to eye-witness reports. The Indonesian president later confirmed on the radio that large waves had killed at least five people and caused severe destruction.

Indonesia has only recently been linked to the tsunami early warning system for the Indian Ocean, which the United Nations Economic, Scientific and Cultural Organization (UNESCO) is setting up together with nations in the region.

The instruments in the Indian Ocean were only just activated last month (see 'Indian Ocean tsunami warning system plugged in'). These send data to the pre-existing Pacific Tsunami Warning Center, which has its main centres in Hawaii and Japan. In this instance, the warning for Indonesia originated in Japan.

Indonesian President Susilo Bambang Yudhoyono, after a 2-metre tsunami struck Java.© DUDI ANUNG/AP/EMPICS

Authorities in Jakarta did receive that warning in real time, but it is as yet unclear whether they were able to convey the information to the local population. They would not have had much time to do so. Although reports vary, experts calculate that the waves probably arrived at the coast only 15 to 20 minutes after the earthquake.

Experts suspect that fresh memories of the Boxing Day tsunami from 2004 may have led many people to run inland on feeling the tremor. This simple response could have saved many lives.

"The good news is that the system has responded and seems to work fine," says Peter Koltermann, a UNESCO official who oversees the implementation of the Tsunami Early Warning System for the Indian Ocean. "It was a successful first test, but it is too early to say exactly how successful."

The earthquake itself was not abnormal in location or size. "This was a pretty standard quake, nothing unusual," says Sandy Steacy, a geophysicist and earthquake expert at the University of Ulster. The tsunami was generated as vertical motion lifted up the seafloor.

The risk of another ocean-wide tsunami, such as the one that killed more than 200,000 on 26 December 2004, was very small this time, says Steacy.

Seismologists expect the next big quake in the region to occur farther north than where this one struck, but they note there is a large earthquake potential everywhere along the Indonesian subduction zone.

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