JAKARTA, Indonesia — A powerful, shallow earthquake struck near Flores Island in eastern Indonesia early Saturday, setting off a tsunami warning across four provinces and generating small waves recorded at several coastal monitoring stations.
Indonesia’s Meteorology, Climatology and Geophysics Agency, known as BMKG, said the earthquake occurred at 4:58 a.m. Western Indonesian Time — 5:58 a.m. in Flores — about 30 kilometers northeast of Mbay in Nagekeo Regency. The agency revised its initial estimate of magnitude 7.0 to 7.7 and placed the depth at 15 kilometers.
The United States Geological Survey, cited in early international reports, also measured the earthquake at magnitude 7.7 but estimated a depth of 10 kilometers and placed the epicenter roughly 68 kilometers northwest of Ende. The European-Mediterranean Seismological Centre upgraded its initial estimate from 6.9 to 7.6, also at a depth of 10 kilometers. Such differences are common during the first hours after a large earthquake, as monitoring agencies incorporate additional seismic data.
Evacuations Ordered Along Vulnerable Coasts
BMKG extended its tsunami warning to East and West Nusa Tenggara, South Sulawesi and Southeast Sulawesi. Northern coastal districts of Manggarai, Ngada, West Manggarai, Ende and Sikka, along with Selayar, Jeneponto and Bantaeng in South Sulawesi, were placed under “Siaga,” or warning status.
Under Indonesia’s warning system, that designation represents modeled waves of 0.5 to three meters and calls for evacuation. Lower-level advisories covered other parts of Flores, Bima, Dompu, Bau-Bau and coastal South Sulawesi, where residents were instructed to leave beaches, harbors and riverbanks.
By 5:49 a.m. Western Indonesian Time, sea-level instruments had detected waves of 0.36 meters at Labuan Bajo, 0.19 meters at Sikka and 0.17 meters at Dompu. Although those initial measurements were substantially below the highest modeled levels, BMKG had not canceled the warning as of 6 a.m.
The agency continued to advise residents to follow local evacuation orders. Tsunamis can arrive in successive waves, and the first recorded surge is not necessarily the largest.
A Rapid and Powerful Aftershock Sequence
The earthquake was followed by a concentrated series of strong aftershocks. BMKG’s real-time feed recorded a magnitude 6.5 shock about 15 minutes after the main earthquake, followed by events measuring 6.2, 5.9 and 5.8, as well as numerous tremors in the magnitude 4 range. That sequence was more extensive than the two aftershocks described in the earliest international reports.
No authoritative casualty or damage assessment had been released by Indonesia’s National Disaster Management Agency, or BNPB, during the first hour. Communications, roads and buildings in remote coastal communities were still being assessed, leaving the initial absence of reports inconclusive.
A Hazardous Tectonic Corridor
Flores lies in one of Indonesia’s most complicated seismic zones. Compression created by the interaction of the Australian and Sunda-Banda plates is partly accommodated by the Flores back-arc thrust system, an extensive fault zone running along the northern side of the Lesser Sunda Islands.
The earthquake’s shallow depth and location are consistent with the broader hazard posed by that system, although an official focal-mechanism analysis will be required before scientists can identify the fault that ruptured. Research has shown that earthquakes along the Flores back-arc system can produce local tsunamis with very short arrival times.
The warning also carried particular historical weight. In 1992, a magnitude 7.7 earthquake north of Flores produced a destructive tsunami that reached some nearby shores within minutes, killing about 1,690 people and destroying roughly 18,000 homes, according to the International Tsunami Information Center.
Saturday’s early sea-level readings were far smaller. But officials cautioned that residents should remain on high ground and away from the coast until BMKG formally declares the threat over.
Sources: BMKG, Reuters