Flores: Living on a Moving Giant
written by: Anne Sirait and Yudhiakto Pramudya
affiliation: Universitas Indonesia, Universitas Ahmad Dahlan
Have you ever imagined that the ground beneath your feet is constantly moving? On Flores, that movement is very real — and at times, terrifyingly powerful.
Flores, the “Child” of the Lesser Sunda Islands
Flores is far more than a tourist destination with stunning lakes, Komodo dragons, and colorful coral reefs. Geologically, this island sits at one of the most active crossroads on Earth where two massive tectonic plates collide, shove, and grind against each other.
Picture two giants wrestling. To the north lies the sturdy Eurasian Plate. To the south, the Indo‑Australian Plate is diving beneath Flores at a rate of 5–6 cm per year (DeMets et al., 2010) as fast as your fingernails grow! This collision creates one of the most complex tectonic “stages” in Indonesia.
Understanding the Flores Back‑Arc Thrust
This plate drama creates a series of major fault lines. The one that keeps scientists awake at night is the Flores Back‑Arc Thrust also known as the Sesar Naik Flores or Flores Thrust.
What Exactly Is It?
The Flores Back‑Arc Thrust is a thrust fault — a type of fracture where one section of the Earth’s crust is pushed up and over another due to immense compressional forces. This fault system formed in the back‑arc region, which is the area behind the volcanic arc of Flores, located beneath the floor of the Flores Sea, north of the island.
A Giant in the Sea
The fault stretches over an impressive distance — approximately 1,500 kilometers (Silver et al., 1983, 1986) — running along the northern coast of Flores, from north of Bali in the west to the Wetar region in the east. It is a continuous fault system, with varying characteristics along its length. The dip angle (the tilt of the fault plane) varies by region. In eastern Java to Bali, it can be as shallow as 26 ± 8° to the south (Felix et al., 2022), 7°-38° in eastern Bali to western Sumbawa, 31°-25 ° and 47° in eastern Sumbawa to Komodo, 41°-54° in western Flores, and 31°-38° in eastern Flores (Andikagumi & Bradley, 2024).
How It Works
The creation of the Flores Back‑Arc Thrust is a direct result of the tectonic collision. Geologists believe it may represent the initial stage of a polarity reversal of the eastern Sunda Arc, a process where the direction of subduction flips (McCaffrey & Nábělek,1984). As the Indo-Australian plate pushes northward, it doesn’t just slide smoothly. Immense pressure builds up, and the fault acts like a release valve, accommodating about 10 mm of total plate convergence each year (Yang et al., 2026). When the stress becomes too great, the fault ruptures, triggering powerful earthquakes.
The 1992 Flores Earthquake: A Grim Reminder
The danger of the Flores Back-Arc Thrust is not theoretical. On December 12, 1992, a devastating earthquake struck off the coast of Flores, with a magnitude of Mw 7.8 to 7.9. The earthquake, caused by a rupture on the thrust fault, generated a massive tsunami that devastated Maumere and other coastal communities (Beckers & Lay, 1995).
The tsunami waves reached heights of up to 26 meters and swept inland as far as 300 meters (Imamura et al, 1995; Tsuji et al, 1995). Over 2,500 people lost their lives, and more than 18,000 homes were destroyed (Yeh et al, 1993). This event remains one of Indonesia’s deadliest natural disasters and is a direct product of the Flores Back-Arc Thrust system.
The August 15, 2026 Flores Earthquake
History repeated itself just days ago.
The Event
At 4:58 AM local time on August 15, 2026, a powerful Mw 7.7 earthquake struck off the northern coast of Flores. The epicenter was located approximately 30 km northeast of Nagekeo, at a shallow depth of 10 to 15 kilometers. The BMKG (Indonesia’s Meteorology, Climatology, and Geophysical Agency) confirmed that the earthquake was caused by the activity of the Flores Back‑Arc Thrust (CNN Indonesia, 2026).
The shaking was intense, reaching a maximum Modified Mercalli Intensity of VIII (Severe) in Manggarai Regency and VII (Very Strong) in West Manggarai Regency and Bima Regency. Buildings collapsed, including a waiting room at Maumere’s port terminal, and a hospital in Ende had to move patients outside. Initial reports indicate at least 17 fatalities, with the death toll potentially rising.
The Tsunami Warning
The BMKG issued a tsunami warning just two minutes after the earthquake struck, as the fault is offshore and any large rupture could displace water. Sea levels rose significantly in several locations:
· Pota, Manggarai Timur: 1.605 metres
· Maurole, Ende: 0.94 metres
· Bulukumba: 0.54 metres
· Labuan Bajo: 0.36 metres
· Bakalang, ALor: 0.14 metres
The BMKG recorded tsunami occurred in 16 locations of NTT, NTB, Sulawesi Selatan, and Sulawesi Tenggara. The tsunami warning was lifted at 07:30 AM local time — about two and a half hours later — when sea levels returned to safe conditions.
Maximum Potential
The Flores Back-Arc Thrust has a maximum earthquake potential of up to Mw 7.8 (ANTARA News, 2026). The fact that the 2026 event released energy at Mw 7.7, so close to its estimated maximum, is a significant relief, as it suggests the fault may have released much of its stored stress.
Aftershocks: The Unwelcome Guests
Major earthquakes are almost never alone. They are typically followed by a series of smaller earthquakes called aftershocks — and the 2026 Flores event is no exception.
What Are Aftershocks?
Aftershocks are smaller earthquakes that occur in the same general area as the main shock, caused by the sudden redistribution of stress along the fault. Think of it like this: when the main earthquake ruptures, it releases massive amounts of energy, but it also changes the stress on nearby sections of the fault. These adjacent areas may now be “overstressed” and must adjust to their new conditions — and they do so by producing smaller quakes.
The 2026 Aftershock Sequence
As of two hours after the main shock, BMKG had recorded at least 37 aftershocks, with magnitudes ranging from 3.6 to 6.2. The largest of these, a magnitude 6.2 aftershock, occurred about 40 minutes after the main quake and caused additional panic among residents who had already evacuated to higher ground. Other notable aftershocks included:
· A magnitude 5.7 event around 45 minutes after the main shock
· A magnitude 5.2 event about one hour later
· Numerous smaller tremors ranging from magnitude 3.6 to 4.8
How Long Will They Last?
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Scientists warn that aftershocks could continue for days, weeks, or even months. The frequency and magnitude of aftershocks typically decrease over time, following a pattern known as Omori’s Law — but they can be unpredictable. Some aftershocks can be almost as large as the main shock, causing further damage to already weakened structures and triggering landslides in steep terrain. The BMKG have recorded at least 2768 aftershocks as of August 19th, 2026.
The Danger of Aftershocks
While aftershocks are generally smaller than the main quake, they pose significant risks:
· Building Collapse: Structures damaged by the main shock may collapse during strong aftershocks.
· Landslides: Steep hillsides loosened by the main quake can slide during aftershocks, blocking roads and burying villages.
· Psychological Impact: Constant shaking keeps communities in a state of fear and anxiety, making recovery efforts more difficult.
· Hindered Rescue Operations: Aftershocks can make it dangerous for rescue workers to enter damaged buildings or reach remote areas.
Scientist have tried utilizing Artificial Intelligence to estimate the Flores Sea Aftershock Rupture Data. This effort can be useful to enhance the knowledge about aftershock in the area (Jufriansah et al, 2023).
Will There Be a Bigger One?
One of the most common questions asked after an earthquake is: “Was that the main shock, or was it a foreshock to something even bigger?” In the case of the 2026 Flores earthquake, BMKG scientists are confident that the Mw 7.7 event was the main shock, given that the fault’s maximum potential is estimated at Mw 7.8. However, there remains a small possibility of another large event on a different segment of the fault, as the Flores Back-Arc Thrust stretches over 1,500 kilometers. Residents are advised to remain vigilant and follow official updates.
Why the Flores Thrust is So Dangerous
Several factors make this fault particularly deadly:
· Shallow Depth: Most of the earthquakes generated by the Flores Thrust occur within the upper 40 kilometers of the crust. Shallow earthquakes release their energy closer to the surface, causing more intense shaking and greater damage.
· Offshore Location: Because the fault lies beneath the sea, large earthquakes can displace massive volumes of water, generating tsunamis that strike coastlines within minutes — often too fast for effective evacuation.
· High Activity: Since 1900, the area has experienced 79 earthquakes of magnitude 6.0 or greater, with 11 of them generating tsunamis. The system has previously triggered deadly quakes in 1815, 1836, 1857, 1976, and the devastating 1992 event.
· Long Fault System: With a length of over 1,500 kilometers, a rupture on one segment can sometimes trigger breaks in neighboring segments, creating even larger and more destructive events.
The Science Behind the Shaking
When the Indo‑Australian Plate dives beneath the Eurasian Plate, it doesn’t slide smoothly. Instead, it gets stuck at certain points where friction holds it in place. Meanwhile, the rest of the plate continues moving. This builds up enormous elastic energy like stretching a rubber band tighter and tighter.
Eventually, the stress overcomes the friction, and the fault suddenly slips. That sudden slip sends seismic waves rippling outward, shaking the ground for hundreds of kilometers. If the slip occurs under the ocean floor, it pushes water upward, creating a tsunami that races across the sea at speeds of up to 800 km/h — as fast as a jet plane.
The aftershocks that follow are the fault’s way of “settling down” after the main rupture — like the creaking and groaning of a building after a heavy load has been placed on its structure.
Scientists Keep a 24/7 Watch on Flores
Researchers from Indonesia and international institutions closely monitor this complex tectonic environment. They study thousands of earthquake records, decoding the “language” of ground vibrations — hoping to understand when and where the next threat might strike. Modern earthquake monitoring includes:
· Real‑time seismic networks that detect quakes within seconds
· Sea‑level sensors (tide gauges) that confirm or cancel tsunami warnings
· GPS stations that track minute ground movements to measure stress buildup
Their goal is noble: to save lives.
Final Thought: Living on a Giant That Never Sleeps
Flores teaches us that our planet is not a dead stone. The events of August 15, 2026, are a powerful reminder that the Flores Back‑Arc Thrust is not a relic of the past — it is an active fault that will continue to shape the lives of those who call this beautiful island home. The aftershocks that continue to rattle the region are a sobering reminder that the Earth’s work is not yet done.
But with science, we are not helpless. We can learn to read the Earth’s whispers — and by doing so, coexist more safely with its power. Disaster mitigation measures have been implemented in schools and universities to raise awareness while preserving the memory of the younger generation. Furthermore, greater attention must be paid to structural mitigation in buildings, particularly in light of the damage sustained during the 2026 earthquake.
Because in the end, Flores is not only about its beauty, but also about how we respect and understand the hidden forces beneath our feet.
References
Andikagumi, H., & Bradley, K. (2024). The Flores Thrust and its interplay with volcanism. Tectonics, 43, e2024TC008269. https://doi.org/10.1029/2024TC008269
ANTARA News. (2026, August 15). 7.7-magnitude quake in Indonesia linked to Flores fault: BMKG. https://en.antaranews.com/news/427193/77-magnitude-quake-in-indonesia-linked-to-flores-fault-bmkg
Beckers, J., and T. Lay (1995), Very broadband seismic analysis of the 1992 Flores, Indonesia, earthquake (Mw = 7.9), J. Geophys. Res., 100(B9), 18179–18193, doi:10.1029/95JB01689.
CNN Indonesia. (2026, August 15). BMKG ungkap penyebab gempa M7 guncang NTT hingga picu tsunami. https://www.cnnindonesia.com/teknologi/20260815082915-199-1392594/bmkg-ungkap-penyebab-gempa-m7-guncang-ntt-hingga-picu-tsunami
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Jufriansah, A., Khusnaini, A., Pramudya, Y., Afriyanto, M. (2023) Estimation of Flores Sea Aftershock Rupture Data Based on AI. Indonesian Review of Physics (IRiP), 6(1), 45–55, doi: 10.12928/irip.v6i1.6705
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