On the Origin of Orphan Tremors and Intraplate Seismicity in Western Africa

Abstract

On September 5–7, 2018, a series of tremors were reported in Nigeria’s capital city, Abuja. These events followed a growing list of tremors felt in the stable intraplate region, where earthquakes are not expected. Here, we review available seismological, geological, and geodetic data that may shed light on the origin of these tremors. First, we investigate the seismic records for parent location of the orphan tremors using a technique suitable when a single-seismic station is available such as the Western Africa region, which has a sparse seismic network. We find no evidence of the reported tremors within the seismic record of Western Africa. Next, we consider the possibility of a local amplification of earthquakes from regional tectonics, reactivation of local basement fractures by far-field tectonic stresses, post-rift crustal relaxation, landward continuation of oceanic fracture zones, or induced earthquakes triggered by groundwater extraction. Our assessments pose important implications for understanding Western Africa’s intraplate seismicity and its potential connection to tectonic inheritance, active regional tectonics, and anthropogenic stress perturbation.

Seismogenic Fault Reactivation in Western Central Africa: Insights From Regional Stress Analysis

“…Overall, several studies have speculated that preexisting fractures are hosting earthquakes along the continental margins and interior of western Africa but lack details of the ambient stress field and the evidence for coseismic surface fault rupture or presence of active fault scarps (Amponsah, 2002; Ayele, 2002; Blundell, 1976; Bouka Biona & Sounga, 2001; Kutu, 2013; Olugboji et al., 2021; Sykes, 1978). Here, with our stress analysis, we provide insight into the control of contemporary stress regimes on the occurrence of intraplate earthquakes in the region.…”

“…Aside from the Cameroon Volcanic Line and Bié Dome in Angola, where active mantle processes are driving magmatic activities and associated earthquakes (De Plaen et al., 2014; Tabod et al., 1992; Ubangoh et al., 1997), the origin of stress loading leading to seismogenic rupture of preexisting faults in the onshore areas of the western Africa's continental margin remains controversial and poorly understood (Olugboji et al., 2021). The proposed mechanisms include variations of lithospheric structure and gravitational potential energy associated with mid‐oceanic ridge (Mahatsente & Coblentz, 2015; Medvedev, 2016), the reactivation of local basement fractures by far‐field tectonic stresses from mantle processes along the Cameroon volcanic line, post‐rift crustal relaxation along the rifted margin, landward continuation of oceanic fracture zones, and induced earthquakes triggered by groundwater extraction (Olugboji et al., 2021).…”

“…Some passive rifted margins across the world are known to host pronounced distributed seismicity, among which are well‐instrumented regions such as the eastern Brazilian Atlantic margin (Assumpção, 1998), the southern Australian margin (Holford et al., 2011), and the eastern North American margin (Sbar & Sykes, 1973; Zoback, 1992). However, in poorly instrumented regions, such as Equatorial West Africa and western Central Africa where knowledge of the seismicity is scarce, the relation between the acting present‐day stress regime, the sources of stress perturbation, and the mechanics of reactivation of inherited structures are still poorly known (Olugboji et al., 2021). This knowledge gap hinders the development of viable early warning mechanisms for hazard mitigations in local communities located in such regions and therefore justify a special attention.…”

The Intraplate Stress Field of West Africa

“…The basement of West Africa is riddled with numerous faults that constitute regions of mechanical weaknesses that may explain the seismogenesis in the region. The co-location of earthquakes and previously mapped brittle discontinuities in West Africa argues for the influence of preexisting structures on the seismicity along the coast (e.g., Burke, 1969;Olugboji et al, 2021;Nkodia et al, 2022). For the newly resolved focal mechanisms within the interior of the continent, we argue that the seismicity may also be related to the weak state of brittle fault zones that were developed during the Precambrian orogenies as well as Precambrian and Mesozoic rifting episodes.…”

“…West Africa is a region with a long history of tectonic deformation that spans Precambrian and Phanerozoic tectonics. Far away from the active tectonic boundaries at the Mid-ocean ridges and subduction zones, its continental interior hosts a plethora of earthquakes that are challenging to explain using simple tectonophysics models (Mbossi et al, 2021;Olugboji et al, 2021). These earthquakes are observed in clusters within and across the Neoproterozoic intra-cratonic failed rifts (i.e the Gourma Aulacogen), the Mesozoic failed rift systems, the passive transform margins, and the Precambrian brittle shear zones across the intra-cratonic shields (Figure 1b).…”