BMKG says Flores aftershocks may ease to low level by late September

Labuan Bajo — — Indonesia’s Meteorology, Climatology and Geophysics Agency (BMKG) expects aftershock activity following the magnitude 7.7 Flores earthquake to continue weakening over the coming weeks, potentially reaching a relatively low level around late September 2026, based on temporary modelling that remains subject to revision, according to BMKG and statements by its Deputy for Geophysics Nelly Florida Riama.

BMKG reported that 3,394 aftershocks had been recorded by 05:00 WIB on Thursday, 20 August 2026, with magnitudes ranging from 1.1 to 6.2.

The agency has not identified a precise day when aftershocks will stop. Riama has stressed in briefings cited by national and regional media that the outlook is based on current data and modelling and will be updated as new observations become available.

That distinction matters for residents watching the end of the forecast window. The expectation of “relatively low” activity is not the same as a guarantee that earthquakes will cease. Riama has emphasised that aftershocks may still occur even after the period when activity is forecast to have declined, and that the forecast remains dynamic.

What the forecast means

BMKG’s public assessments describe a trend over the weeks following the 15 August mainshock rather than a fixed end date. In a 19 August briefing, Riama said monitoring showed a general decay in aftershock activity and that BMKG’s calculations suggested activity would continue to weaken over roughly 21 to 30 days.

According to reports citing Riama, BMKG’s model indicates that aftershock activity could reach a relatively low level within one to three months of the main earthquake, or around late September 2026, assuming the observed decay trend continues.

Riama and BMKG have underlined that this outlook is provisional. They have described it as an estimate derived from ongoing monitoring and statistical modelling, to be refined as more data are collected, and have cautioned residents not to treat late September as a deadline for ending precautions.

Earthquake sequence and tectonic setting

The main Flores earthquake occurred on Saturday, 15 August 2026, at 05:58:23 WITA, with a magnitude of 7.7 and a depth of 15 km, according to BMKG’s initial InSAR report.

BMKG places the epicentre at approximately 121.39° east longitude and 8.41° south latitude and describes the event as a shallow tectonic earthquake associated with reverse-fault movement along the Flores Back Arc Thrust north of the Lesser Sunda Islands.

In an article on the dynamics of the Flores sequence, BMKG reports that 3,394 aftershocks had been recorded by 20 August 2026 at 05:00 WIB, with magnitudes between 1.1 and 6.2. Independent coverage by several Indonesian outlets, citing BMKG data, gives the same total and magnitude range and notes that 26 of these events had magnitudes above 5 and 99 were felt by residents.

Recorded sequence

• Main earthquake: magnitude 7.7 on Saturday, 15 August 2026, at 05:58:23 WITA, depth 15 km, epicentre near 121.39°E and 8.41°S.
• Aftershocks recorded by 19 August: 3,002 events with magnitudes between 1.1 and 6.2, including 86 felt and 25 above magnitude 5, according to BMKG data cited in national media.
• Aftershocks recorded by 20 August: 3,394 events with magnitudes between 1.1 and 6.2, according to BMKG.
• Felt aftershocks: 86 by 19 August, based on BMKG figures reported by national outlets.
• Aftershocks above magnitude 5: 25 by 19 August, with 26 above magnitude 5 by 20 August, according to BMKG data cited by media.

BMKG’s initial InSAR analysis used satellite radar images acquired at different times to map ground deformation caused by the 15 August earthquake, describing the technique as a way to measure changes in the distance between the ground and the satellite. BMKG has not publicly released a detailed daily activity-rate table or a technical description of the specific statistical model underpinning the late-September outlook.

What residents should continue doing

BMKG states that it is maintaining round-the-clock seismic monitoring in Flores and has deployed field teams to conduct macroseismic and microseismic surveys, assess ground conditions and support tsunami analysis as part of mitigation and reconstruction planning.

Residents are advised to remain calm but alert, particularly when entering or staying near damaged buildings. BMKG and disaster management authorities recommend following structural assessments by officials where available, rather than relying solely on a building’s outward appearance.

People are urged to continue monitoring official information from BMKG, Indonesia’s National Disaster Management Agency (BNPB), regional disaster management agencies (BPBD) and local governments, and to be familiar with evacuation routes in case strong shaking occurs again.

BMKG has warned the public not to rely on unofficial predictions of the timing or magnitude of future earthquakes. Riama has reiterated that specific forecasts circulating online should not be treated as authoritative unless they come from official sources and are based on scientific monitoring.

Earthquake experts interviewed by Indonesian media have noted that most aftershocks typically occur within tens of kilometres of the mainshock’s epicentre and have encouraged communities in East Nusa Tenggara to remain cautious until activity has clearly subsided.

Sources

• BMKG – Laporan Awal Analisis InSAR Gempa Flores 2026 M7.7
• NU Online – Tercatat 3.394 Gempa Susulan Flores hingga Kamis Pagi, Korban Meninggal Jadi 71 Jiwa
• Bisnis.com – Gempa Bumi Susulan Flores Capai 3.394 Kejadian, 26 Kali Bermagnitudo di Atas 5
• RRI – 3.002 Gempa Susulan Tercatat di Flores, BMKG Sebut Aktivitas Mulai Menurun
• Media Indonesia – BMKG Catat 3.002 Gempa Susulan di Flores, Aktivitas Diperkirakan Meluruh 21-30 Hari
• Kupang Tribun – Gempa Susulan di Flores jadi 14.236 Kali hingga Kamis 10 September 2026, Kapan Gempa NTT Berhenti?
• Radar Cirebon – Gempa Susulan Flores Tembus 3.002 Kali, BMKG Ungkap Tren Aktivitas hingga Akhir September