Recurrence of large earthquakes in magmatic continental rifts: Insights from a paleoseismic study along the Laikipia-Marmanet fault, Subukia Valley, Kenya rift

The seismicity of the Kenya rift is characterized by high-frequency low-magnitude events concentrated along the rift axis. Its seismic character is typical for magmatically active continental rifts, where igneous material at a shallow depth causes extensive grid faulting and geothermal activity. Thermal overprinting and dike intrusion prohibit the buildup of large elastic strains, therefore prohibiting the generation of large-magnitude earthquakes. On 6 January 1928, the M_S 6.9 Subukia earthquake occurred on the Laikipia-Marmanet fault, the eastern rift-bounding structure of the central Kenya rift. It is the largest instrumentally recorded seismic event in the Kenya rift, standing in contrast to the current model of the rift's seismic character in which large earthquakes are not anticipated. Furthermore, the proximity of the ruptured fault and the rift axis is intriguing: The rift-bounding structure that ruptured in 1928 remains seismically active, capable of generating large-magnitude earthquakes, even though thermally weakened crust and better oriented structures are present along the rift axis nearby, prohibiting any significant buildup of elastic strain. We excavated the surface rupture of the 1928 Subukia earthquake to find evidence for preceding ground-rupturing earthquakes.We also made a total station survey of the site topography and mapped the site geology. We show that the Laikipia-Marmanet fault was repeatedly activated during the late Quaternary. We found evidence for six ground-rupturing earthquakes, including the 1928 earthquake. The topographic survey around the trench site revealed a degraded fault scarp of ≈ 7:5 m in height, offsetting a small debris slide. Using scarp-diffusion modeling, we estimated an uplift rate of U = 0:09-0:15 mm/yr, constraining the scarp age to 50-85 ka. Assuming an average fault dip of 55°-75°, the preferred uplift rate (0.15 mm/yr) accommodates approximately 10%-20% of the recent rate of extension (0.5 mm/yr) across the Kenya rift.