A 6.1 quake off Sumba shakes three districts and raises no tsunami flag

A magnitude 6.1 earthquake struck off Southwest Sumba in East Nusa Tenggara at 5:55 a.m. local time on Sunday, 14 kilometres southwest of Kodi, at a depth of 10 kilometres. Indonesia's disaster agency said it had no tsunami potential, and by mid-morning it had no report of damage or deaths.

Berton SP Panjaitan, acting head of the data and communications centre at the National Disaster Mitigation Agency, gave the coordinates as 9.70 degrees south and 118.92 degrees east. The epicentre was at sea. Preliminary checks found strong shaking of about one minute in Southwest Sumba, West Sumba and West Manggarai. Some residents felt it. Panjaitan said there was no panic. Local disaster offices were still in the field on Sunday and had received no immediate damage report.

The agency told residents to ignore rumours and to use official updates. If aftershocks come, it said, people should stay out of buildings that have cracked and keep away from the coast for the time being. That advice is standard after a shallow offshore quake. It is also a recognition that a 10-kilometre depth under water can still crack masonry inland even when the tsunami flag stays down.

Why the depth and the place matter

A magnitude 6.1 is large enough to damage weak buildings if it is shallow and close. This one was shallow, 10 kilometres, and the nearest named place, Kodi, is 14 kilometres from the epicentre. The combination is why BNPB put people in the field before it had a casualty number. Southwest Sumba's housing includes traditional raised houses and a large stock of unreinforced masonry in the district towns. A one-minute shake is long enough to open cracks that a shorter jolt would not. The absence of a damage report on Sunday morning is provisional. It is not a finding that nothing cracked.

The no-tsunami call is the other half of the bulletin. BMKG, the meteorology and geophysics agency, is the body that issues that flag in Indonesia. BNPB repeated it. A quake of this size can still displace water if the rupture moves the seabed vertically. Officials judged that it had not, or not enough to warn the coast. The advice to stay off the shoreline is the residual caution after that judgment, aimed at aftershocks rather than at a wave already ruled out.

The island's recent record

Sumba and the wider East Nusa Tenggara chain have taken harder hits than this in the last decade. A 7.7 quake in the province left survivors in temporary housing that BNPB was still building out in later seasons. Flores has had its own sequence, with aid groups trucking water to camps. Sunday's event is not that. It is a 6.1 offshore, felt on three districts, with the casualty line still empty at the first bulletin.

The useful comparison is the speed of the second report. If BPBD teams in Kodi and the district capital find collapsed walls or injured people, the morning bulletin will be revised before evening. If they do not, the event stays a felt quake with a recorded magnitude and no tsunami. Either outcome depends on a field check that, as of Panjaitan's statement, had not finished.

What residents were told to do

The instruction set is short. Stay calm. Do not forward unverified claims about a wave or a larger quake coming. Use BNPB and BMKG, not a forwarded message. After a shock, leave a cracked building and do not stand on the beach. Those four lines are the whole public product of the Sunday bulletin, along with the numbers: 6.1, 10 kilometres deep, 14 kilometres from Kodi, 5:55 a.m., no tsunami flag.

Until a damage count exists, that set of numbers is the story. A shallow quake of this size, this close to an inhabited coast, is the kind of event that produces either a quiet afternoon or a revised toll. Sunday's first official word was the quiet version, with teams still walking the districts.