BMKG: 15,722 Flores Aftershocks Recorded as Sequence Enters Decay Phase

Labuan Bajo — Indonesia’s Meteorology, Climatology and Geophysics Agency (BMKG) says the aftershock sequence following the magnitude 7.7 earthquake in Flores has entered a clear decay phase, with 15,722 tremors recorded in the first 30 days after the main shock on August 15, 2026, most too small to be felt by residents, according to BMKG and media reports.

BMKG acting deputy for geophysics Ardhasena Sopaheluwakan presented the updated assessment at a press conference on Tuesday, September 15, 2026, in Jakarta, according to several outlets citing the briefing.

BMKG data show that residents felt around 190 of the aftershocks, while the remainder were detected only by instruments, according to reports on the agency’s monitoring.

The agency describes the current stage as a statistical decline rather than a clear end to the sequence, stressing that the downward trend does not provide an exact date for when aftershocks will stop or rule out further earthquakes.

Smaller tremors now dominate activity

According to BMKG figures cited by national and local media, the 15,722 recorded aftershocks ranged from magnitude 0.9 to 6.2 over the first 30 days after the main event.

Weekly activity fell sharply, with reports noting a drop from about 5,011 events in the first week to 864 in the fifth week, indicating that smaller tremors increasingly dominate the sequence.

BMKG’s Earthquakes and Tsunamis director, identified in coverage as Wijayanto, divided the sequence into four magnitude groups to illustrate the pattern:

• Magnitude 5 and above: 26 events, or about 0.2 percent of the total; none were recorded during the most recent week.
• Magnitude 4 to below 5: 476 events, around 3 percent, generally associated with limited damage potential.
• Magnitude 3 to below 4: 3,308 events, or about 20.9 percent, expected by BMKG to continue declining over the coming weeks.
• Below magnitude 3: 11,993 events, or roughly 75.9 percent, overwhelmingly detected by instruments and potentially continuing for months.

BMKG officials say the decline in larger aftershocks is the main reason they describe the sequence as being in a substantial decay phase, while cautioning that statistical trends are not guarantees that no damaging earthquake can occur.

The main earthquake was shallow, with BMKG placing its depth at about 15 kilometres and its epicentre roughly 30 kilometres northeast of Nagekeo, East Nusa Tenggara, attributed to movement on the Flores Back-Arc Thrust.

Survey findings to guide rebuilding

BMKG also highlighted findings from a joint field survey carried out between August 15 and 25, 2026, across seven affected regencies in East Nusa Tenggara.

According to BMKG’s director for engineering seismology, Teguh Rahayu, the survey was designed to support rehabilitation and reconstruction planning by mapping shaking intensity, soil conditions and tsunami impacts rather than only cataloguing immediate damage.

Rahayu said maximum shaking reached intensity VII on the Modified Mercalli Intensity scale in some locations, an upper-moderate to strong level capable of causing damage to vulnerable structures.

The team reported indications of liquefaction, including sand boils, in villages within Nagekeo Regency, identifying ground failures that can undermine building foundations.

Soil microzonation work found soft-soil classifications, known as SE in BMKG’s scheme, in areas such as Reok, Mbay and Marapokot, where local ground can amplify seismic waves and enhance shaking, a factor BMKG urges authorities to consider carefully in rebuilding plans.

The survey also documented tsunami impacts along parts of the north coast of Flores.

Field observations reported a maximum run-up of 2.68 metres in Selama village, with seawater reaching as far as 145 metres inland in Nampar Sepang and more than 100 metres in parts of Selama, according to BMKG’s tsunami survey summary.

BMKG said the tsunami damaged 23 homes and swept away two houses in Selama, while other dwellings in Nampar Sepang and nearby coastal settlements were also affected.

Officials noted that no tsunami-related deaths were recorded, attributing this to residents’ rapid self-evacuation following the earthquake and the agency’s early warning.

Early evacuation remains the key advice

BMKG reported issuing its tsunami early warning two minutes and 16 seconds after the August 15 main shock.

Sea-level monitoring stations in East Nusa Tenggara, West Nusa Tenggara and South Sulawesi recorded wave surges, including peaks of about 1.61 metres at Reo and 1.60 metres at Pota, according to the agency’s instrumentation data cited in media reports.

Rahmat Triyono, head of BMKG’s Centre for Instrumentation Standardisation, said near-shore tsunami waves can arrive very quickly after strong or prolonged shaking, reinforcing the agency’s core advice that coastal residents should evacuate immediately to higher ground and then follow verified official information.

BMKG says its earthquake and tsunami monitoring continues around the clock and urges residents not to interpret the reduction in aftershock numbers as a guarantee that all danger has passed.

The agency expects its survey results to inform local and national decisions on reconstruction standards, land-use planning and future risk reduction, particularly in areas with high shaking intensity, liquefaction-prone soils, soft ground and documented tsunami reach.

Sources

• CNBC Indonesia – BMKG Ingatkan NTT Waspada, Warga Pesisir Diminta Belajar Evakuasi
• ObserverID – Flores Aftershocks Subside, BMKG Unveils Field Survey to Guide Recovery
• Tempo.co (English) – 15,722 Aftershocks Recorded in 30 Days After Flores Earthquake
• NU Online – BMKG Temukan Indikasi Likuefaksi di Nagekeo dan Jejak Tsunami usai Gempa M7,7 NTT
• Kompas TV – BMKG Temukan Indikasi Likuifaksi dan Tanah Lunak Pascagempa Flores M7,7