Assessment of Unrecorded Intraplate Seismicity: Empirical Magnitude Determination of the Shaki Earthquake, Southwest Nigeria

Abstract

Recent seismic events at Shaki, an old town in Southwest Nigeria, have generated concern regarding the possible reactivation of old fault lines in the area. Two events of note occurred on August 21, 2016 and September 8, 2021, both of which were followed by a sequence of aftershocks lasting several weeks. Unfortunately, the events were not instrumentally recorded, restricting data for hazard characterization. To resolve this problem, we carried out a study to estimate the local magnitude (ML) of the events through an integration of site visitations and questionnaires administered to the residents of the localities. The study adopted a systematic method founded on the Modified Mercalli Intensity (MMI) scale that relates instrumental intensity to perceived shaking, object movement, and possible building damage. One hundred respondents were interviewed in the areas where the tremors were most strongly felt. The data collected were processed with a weighted statistical approach, and the magnitude was computed from the empirical relations obtained. The results gave a consistent magnitude estimate of 4.7 ML for both events, with a narrow 95% confidence limit of 4.6 - 4.8 ML. The results offer useful information on the seismic hazard in Shaki and the need for instrumental monitoring and additional investigations to clarify the geological and tectonic processes responsible for the seismic activity in the area. The research contributes to seismic risk assessment and the formulation of proper hazard mitigation strategies in Shaki and the surrounding areas.