Experts urge caution

Temsulemba Walling, a senior geologist and professor at Nagaland University, said more investigation was needed before Tamlu could be compared with Noklak.

He said the Mon and Tamlu highway landslides were triggered by heavy precipitation on July 19 and occurred on steep slopes with unfavourable geology and probably poor drainage. Noklak, by contrast, is a complex landslide in the Disang Group of shales, probably also influenced by neotectonic movements. “While the Tamlu slide happened in the Disang shales, the Mon landslide occurred in the Laisong Formation of the Barail Group,” he said.

Walling said the Noklak landslide involves toe erosion (wearing away of soil or rock at the base of a slope), debris movement, subsidence and tensional fractures across parts of the town. The two drainage systems, Khiamong and Lein rivers, are eroding debris, but the scale and persistence of movement suggest a deeper and more complex instability, possibly linked to active tectonism.

There were wet spells in the preceding week and heavy rain on July 18-19, but this does not qualify as a cloudburst. Sarat Phukan, a geologist at Gauhati University, said rainfall was the most likely trigger in Tamlu.

He said the upper Dikhow catchment received about 123.1 millimetres of rain between July 12 and 18, and 117.2 mm between July 18 and 19, according to NASA’s Integrated Multi-satellitE Retrievals for GPM data. Tamlu received about 165 mm and 94.2 mm over the same periods.

“The most likely explanation is rainfall-triggered local or compound shallow-slope deformation involving soil, colluvium, road fill and weathered shale,” he said.

Rain during the preceding week would have raised soil moisture and shallow groundwater levels, while heavy rain on July 18 and 19 increased pore-water pressure and weakened slope material, he said.

Phukan added that further movement at the same site was possible during future periods of prolonged or heavy rainfall, including future monsoons. But current information was insufficient to estimate the probability, depth or likely size of such movement.

Human factors questioned

Y Nuklu Phom, winner of the Whitley Award in 2021, said the Longleng disaster may have involved several factors: heavy rainfall, climate change, possible tectonic activity and human interventions. He said rainfall increased by 57 per cent in July, raising the water volume of the Dikhu and affecting the basin system. But he also pointed to road construction and landslides along the Dikhu and Yangmon river belts.

“For the past 10 to 12 years, the National Highways and Infrastructure Development Corporation Limited has been carrying out heavy road construction projects in the region, expanding to Mokokchung from Longleng,” he said. “I feel that hill cutting, deforestation and the use of heavy dynamite for blasting rocks have significantly contributed to the disasters, as these processes have weakened the slopes.”

“Sitting in my office, I could hear the sound of dynamite blasts 7-8 km away. The National Highway 702, which connects Longleng, runs along the Dikhu River, as does the Yongnyah-Tamlu road, which crosses the river. Major landslides occurred along the Dikhu and Yangmon river belts,” Nuklu added.

Nuklu also suggested that long-term use of salt and other chemicals by farmers as weedicides may have affected soil character, though this remains contested.

On the possibility of seismic activity, Phukan said the National Centre for Seismology catalogue recorded no nearby earthquake at the time of the July 19-20 ground movement. Earthquakes recorded on July 17 and 18 were too small, distant and deep to explain the observed damage, he said. “While a very small local earthquake cannot be completely ruled out due to the lack of nearby seismic stations, there is currently no evidence that an earthquake caused the ground movement in Tamlu,” he said.

Coal mining in the Bura Namsang area along the Dikhu has also been raised as a possible contributor. Nuklu said mining had increased in recent years and could have added to siltation and bank erosion.

Walling said there was not enough evidence to directly link coal mining to disruption of the Dikhu. Mining in Nagaland’s Schuppen Belt has existed for decades, he said, but its impact was more likely to be limited to smaller streams and tributaries. Any link to the recent events would require investigation using multi-temporal satellite data. “Available data also indicates a very low correlation between coal mining and landslides in Nagaland,” he said.

Whether Tamlu is witnessing the early stages of a prolonged geological process like Noklak or Tuensang remains unanswered. For now, residents are left watching cracks in the land and waiting for authorities to investigate before the next monsoon turns today’s warning signs into a larger disaster.

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