August 16, 2026 | 12:32 pm
TEMPO.CO, Jakarta - A powerful magnitude 7.7 earthquake in Flores, East Nusa Tenggara (NTT), Indonesia, on the morning of Saturday, August 15, 2026, triggered landslides reaching the peaks and craters of several mountains. The Geological Agency observed these secondary effects at Mount Ebulobo in the southern part of Nagekeo Regency, Mount Kelimutu in Ende Regency, and Anak Ranakah in Manggarai Regency.
The landslides on these three mountains add to the list of impacts from the earthquake, which has already claimed more than 40 lives. The quake also triggered minor tsunamis along coastal areas of northern Flores and South Sulawesi.
According to the Head of the Geological Agency, Lana Saria, the landslides on these Flores mountains were caused by the instability of the rock masses, particularly on slopes ranging from steep to nearly vertical.
Factors included the influence of fractures and planes of discontinuity within the volcanic rock, as well as localized hydrothermal alteration and weathering processes. "These conditions act as predisposing factors, resulting in relatively low stability for some rock masses," Lana stated on Saturday, August 15, 2026.
She explained that on very steep slopes, planes of discontinuity developing within the rock mass can create rock blocks; the stability of these blocks is heavily influenced by the orientation, dip, continuity, and surface condition of those planes.
Meanwhile, hydrothermal alteration in volcanic areas can alter the mineralogy and the physical-mechanical properties of the rock. In specific alteration zones, the transformation of primary minerals into secondary minerals—including clay minerals—can degrade the quality of the rock mass and weaken the planes of discontinuity. "This is especially true if the material experiences an increase in water content," Lana noted.
She cited the example of rock zones that are relatively hard due to silicification; even in such cases, rock stability can be compromised if the rock contains intensive fracturing that creates isolated blocks. “Dynamic shaking caused by earthquakes can induce stress changes within rock masses and along planes of discontinuity; consequently, rock blocks or masses that were already in a critical state may detach and move downslope.”
On crater walls characterized by very steep to near-vertical slopes, Lana added, the movement mechanisms can include rockfalls, block detachment, toppling, wedge failures, translational slides, or a combination thereof.
According to Lana, landslides or rock mass collapses in these summit and crater areas are best understood as a gravitational response by rock masses already predisposed to instability. Earthquake shaking serves merely as a trigger.
“In this context, the earthquake acts as the triggering factor, while factors contributing to slope susceptibility include the very steep slope morphology, rock fractures, weathering, and localized hydrothermal alteration,” she stated.
Lana also confirmed that, based on visual and instrumental monitoring of several volcanoes in the NTT region, there are no indications of significant changes in volcanic activity parameters that would suggest an escalation in eruptive activity. Post-mainshock seismicity remains dominated by tectonic earthquakes and aftershocks, while monitored volcanic parameters remain within normal ranges.
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