In March 2018, a large fissure suddenly appeared across the Nairobi-Narok highway near Mai Mahiu town in the southwestern part of Kenya. The alignment of the fissure closely along the eastern branch of the East African Rift (EAR) system led to a debate regarding its linkage with the rifting process, which is believed to split the African continent slowly. Widening and deepening of fissures were also recently reported in and around Kaptemwa and Nakuru towns, ~100 km northeast of Mai Mahiu, in May 2024, which caused damages to infrastructures and agricultural lands. The emergence and/or widening and deepening of the fissures may be attributed to: (1) tectonic origin by episodic rifting through a series of normal faults due to the extensional force operating across the EAR system between the lithospheric plates (say, Nubian and Somalian plates), (2) denudational origin by roof collapse at the advanced stage of subsurface cavity formation due to high infiltration of rainwater through volcaniclastic alluvium during prolonged torrential rainfall in March–April 2018. A thorough and insightful analysis based on land surface deformation, rainfall pattern and denudational processes was conducted to adjudge the cause of the aseismic fissures.

In conventional and advanced differential interferometric SAR, the neighbouring volcanoes (e.g., Mt Suswa and Mt Longonot) show consistent uplift motion with a further acceleration since March-April 2018, coincident with the prolonged torrential rainfall. Besides, transient uplift indicative of magmatic inflation was observed in the affected area. From the present study, it is inferred that episodic magmatic inflation and increased hydrological loading led to elevated extension, fracturing of the magma chamber, and dyke intrusion along the existing NNW-SSE faults. It was responsible for reactivation of the faults and subtle rifting on WSW dipping fault plane(s). The slow rift motion was also observed in the N-S and E-W components of the neighbouring IGS stations. Additionally, prolonged torrential rainfall and internal erosion in porous and permeable volcaniclastic soil led to cavity formation along the reactivated weak plane(s) and roof collapse, which emerged as several kilometres long aseismic fissure across the Nairobi-Narok highway near Mai Mahiu town in Kenya.

Detailed observations and analyses of this study are documented in the following article:

Chatterjee, R.S., Kumar, K., Pranjal, P., Kannaujiya, S., Chauhan, P., 2022. Magmatic inflation, miniature dyke intrusion and prolonged torrential rainfall led to the emergence of the 2018 aseismic fissure in Kenya, Natural Hazards, 115, 565-591. DOI: 10.1007/s11069-022-05566-7