Flores aftershocks enter significant decay phase, BMKG reports

Labuan Bajo — Indonesia’s Meteorology, Climatology and Geophysics Agency (BMKG) says aftershock activity following the magnitude 7.7 Flores earthquake has entered a significant decay phase, with 15,722 aftershocks recorded in the 30 days to 05:00 WIB on 15 September 2026. According to a BMKG briefing from Jakarta, residents are urged to stay calm but remain alert as rehabilitation and reconstruction advance across affected areas of East Nusa Tenggara.

BMKG reports that the aftershocks range from magnitude 0.9 to 6.2, with 190 events felt by the public. The agency emphasizes that the number of earthquakes is declining over time, but seismic activity has not stopped and further tremors remain possible.

In its 15 September update, BMKG describes the sequence as entering a “peluruhan secara signifikan”, or significant decay, referring to the falling frequency of aftershocks over successive weeks rather than an abrupt end to the sequence.

Aftershocks by magnitude

BMKG’s analysis groups aftershocks into four broad magnitude bands, highlighting both their relative share of activity and likely impacts.

Events of magnitude 5.0 and above account for 26 earthquakes, or around 0.2 percent of the total sequence. BMKG notes that these stronger aftershocks can be felt over a wide area and may affect vulnerable structures, but they have become increasingly rare as the sequence progresses.

Earthquakes between magnitude 4.0 and below 5.0 make up 476 events, roughly 3 percent of the aftershocks. According to BMKG, these tremors are generally felt widely but have limited potential to cause major damage, and their frequency has declined after the first several weeks.

Magnitude 3.0 to below 4.0 aftershocks number 3,308, or about 20.9 percent of the total, while earthquakes below magnitude 3.0 account for 11,993 events, or 75.9 percent. BMKG indicates that the smallest tremors may continue for months, with most of them detectable only by instruments rather than being felt.

The agency’s weekly counts illustrate the decaying trend: approximately 5,011 aftershocks in the first week, 4,267 in the second, 3,158 in the third, 2,421 in the fourth and 864 in the fifth, as of 05:00 WIB on 15 September.

BMKG officials stress that these figures describe statistical behaviour over time and are not a prediction of the exact timing, location or magnitude of future earthquakes.

Field survey findings and local hazards

BMKG’s field survey across Flores and neighbouring districts identified strong shaking and ground effects that will be relevant for reconstruction. The agency reports maximum shaking intensity of VII on the Modified Mercalli scale at several locations in West Manggarai, Manggarai, East Manggarai, Nagekeo and Sikka.

Survey teams also documented indications of liquefaction, including sand boils, in Waekokak Village, Aesesa District, Nagekeo. BMKG notes that parts of Reok, Mbay and Marapokot sit on softer ground that can amplify earthquake shaking, while much of the surrounding area is categorized as medium ground.

On the basis of these findings, BMKG recommends that reconstruction and mitigation planning take local ground conditions into account and that engineering assessments be carried out before damaged buildings are reoccupied.

Mainshock, tsunami and precautions

The earthquake sequence began with a shallow mainshock on 15 August 2026. BMKG’s updated analysis places the epicentre at sea, around 30 kilometres northeast of Nagekeo, at a depth of about 15 kilometres, with a magnitude of 7.7.

BMKG attributes the event to the Flores Back-Arc Thrust, a thrust-fault system north of Flores that has previously generated significant earthquakes. The agency warns that this zone is capable of producing rapid-onset tsunamis along nearby coasts.

A tsunami warning was issued around two minutes and 16 seconds after the mainshock. BMKG reports that sea-level gauges detected rises at several coastal stations, prompting evacuations along the northern coastline of Nusa Tenggara.

Residents and visitors in coastal areas are advised to know their evacuation routes and move inland or to higher ground immediately if they feel a strong or long earthquake, or receive an official tsunami warning. BMKG continues to urge coastal communities around Flores to maintain preparedness, noting that tsunami waves in this region can arrive quickly after a major offshore quake.

Sources

• BMKG – Gempa Flores Meluruh, BMKG Ungkap Hasil Survei Lapangan sebagai Basis Mitigasi dan Rekonstruksi (press release)
• Okezone – Gempa Dahsyat M7,7 di NTT Disebabkan Aktivitas Sesar Back Arc Flores