Preliminary geological evidence for multiple tremors in Kwoi, Central Nigeria

Preliminary field assessment of the geological evidence for the September 2016 multiple tremors was felt as far as 10 km in Nok and Chori villages in the north and northwest of Kwoi respectively. This involved systematic studies of the crack pattern and intensity on buildings, rocks and hanging walls of slopes around the area with the highest shock. The structural mapping was complemented by composite images of SPOT 5, supplementary subsurface investigations utilized aeromagnetic data, seismic data, and Digital Terrain Model (DTM) for the area. Results show that the major evidence that proved a possible link to the area of highest impact is the tremor triggered displacement of a 4 by 3 m diameter rock boulder situated about a kilometre from the Kwoi town, some 3 km from the nearest epicentre that fell through a cumulative distance of 25 m, splitting the fresh granite boulder into two and creating a high impact scar on its path. Seismic vibrations that shook the residents of Kwoi and environs (with epicentres located along a NE-SW linear traverse and remotely coinciding with buried fractures) appear to have been generated and propagated laterally from the rocks along a northwest-southeast profile. Despite the non-homogenous strengths of the impacted buildings, the near-consistency of the E-W striking walls being the most fractured gives a remote connection with the major 345° to 015° fracture patterns on the granitic plutons that shields Kwoi town to the north. Additionally, the intensity of the fractures and collapse in the buildings increased south-westwards from the perceived area of highest impact. A possible tectonic origin related to stress build up in the rocks of the area for the tremors can be insinuated thereby foreclosing the theory of a non-tectonic origin being considered in some quarters including the recent Mpape Abuja tremor of 2018. All these epicentres plot along an extrapolated trend that coincides with the Chain Fracture Zone of the North Atlantic.


Introduction
Tremors are increasingly becoming common in Nigeria which hitherto had been erroneously considered to be aseismic (Alaneme and Okotete 2018). The recent tremor in Mpape Abuja, an area with high concentration of quarries where researchers had thought it is the stress build-up as a result of blasting activities there. In recent years however, these tremors are beginning to occur along a constant rate (Campbell et al. 2018) but with inconsistent structural trajectory. Divergent attempts to give their source has led to speculations on whether they are of tectonic or non-tectonic origin. In central Nigeria especially where seismicity have been relatively low, tremors and ground shaking activities were initially thought to be due to non-tectonic processes such as hill slope instability and avalanches (Oluyide and Okunola 1993). But the current repeated occurrences of incontrovertible ground movements and tremors of low magnitude within this belt which until its occurrence was in 2009 in southwestern Nigeria (Akpan et al. 2014) including Abuja has prompted renewed interest in researches geared towards establishing its tectonic origin or otherwise. This renewed interest in its monitoring led to the establishment of new seismic monitoring stations including one in the Nasarawa State University Keffi Campus. Statistics show that prior to the Kwoi occurrence, at least 37 tremors have occurred since 1919 to 2016(Tsalha et al. 2015Nwankaola and Orji 2018;Campbell et al. 2018;Ananaba 1991;Ajakaiye et al. 1987;Onuoha 1989) These occurrences cover nearly all the different geopolitical zones in Nigeria, with the southwest having the highest statistics of 25 while the central Nigerian zone has recorded only 3. This low frequency in central Nigeria has made the new occurrences, and especially in the Kwoi area a major point of focus.
Active faulting within the West African tectonic terrain have been given as a reason for seismicity (Burke 1969), (Akpan and Yakuba 2010). The model for earthquake occurrence in West Africa have been summarized to include so many deep seated chain fracture systems (including St. Paul's FZ, Romanche FZ, Chain FR, Charcot FZ and the Cameroun Volcanic Line whose intersections at the West African Margins are responsible for the seismicity in this region (Campbell et al. 2018). In this model, mapped Faults are confined between the West African coast from Ghana and the NNE-SSW Ifewara -Kalangai systems. The Chain Fracture Zone from the Atlantic (Burke 1969) (Campbell et al. 2018) which though not substantiated by geological evidence is spatially connected to the epeirogenic plume related to the Jurassic Magmatism manifested in the Jos Plateau.
Major earthquakes are generally known the world over to be generated principally in connection with plate tectonics resulting in the concentration of epicentres along plate boundaries, especially along the circum-Pacific belt. Others are due to natural forces and therefore called tectonic earthquakes, volcanic earthquakes and a few caused by human activity (Referty 2011). Tectonic earthquakes are those that result from sudden release of strain built up in rocks to a point where they can no longer be accommodated. These are usually related to faults within the earth. Earth tremors usually are earthquakes of milder intensities. Their generation are essentially the same. Seismologically, tremors represent arrivals of unique P-or S-waves generated by the rupture in the earthquake source from deep or shallow crusts. Traditionally, tremors associated with volcanic eruptions, either due to fluids, which may be magmas or hot water, or steam are called volcanic tremors. While it is easy to understand such volcanic tremors, seismic tremors are by no means understood and therefore least predictable.
The seismicity history of Nigeria shows that tremors, whose magnitudes rarely exceed 6.0 have very incoherent pattern (Tsalha et al. 2015) and cannot be traceable to any major tectonic systematics.
This regional geological scenario shows that the vicinity of the tremor has a post Quaternary history of quietude.
The central Nigerian basement in this region host pockets of extinct or dormant volcanic members in form of ash cones and pyroclastic basalts as well as rhyolites during the emplacement of the Younger Granites.
The occurrence of tremors in this relative seismically quite terrain therefore will justify the interest given to this research.

Location and geomorphology
It is evident that Kwoi town, the suspected loci of the source of the tremor is located within the table land that appears relatively and tectonically less active and deformed than the adjoining areas. The small hill upon which the small globe is located (Fig. 1) and the Nok hill to the north are the only geomorphological uplifts that truncate this plateau-like landscape.

Regional geological context
The Abuja -Kwoi axis lies in the Central Nigerian Basement Complex fringed to the west and east by rift-bounded basins, were probably genetically initiated by deep-seated lineaments of the basement. The whole architecture of the Intra Cratonic Benue Trough to the east has been traced to the Chain Fracture Zone of the Gulf of Guinea (Benkhelil 1989) which is also spatially related the other fracture trends of the Atlantic Ocean. To the west, the consensus is that the NW-SE Nupe/Bida basin is generated by rifting initiated by flexures in the underlying basement. Still west but within the basement of the Kwoi area are major fracture systems such as the Kalangai Fault and related fractures which orthogonally cut the Bida basin and displaced the major schists and metasediments in the northwest and southeast creating displacements and shear systems in the quartzites and major gold-hosted quartz veins (Fitches et al. 1985).
West Africa, apart from the Cameroun volcanic line has been seismically inactive in most part. This is because west Africa is underlain by stable and cratonized shields apart from the eastern fringes where the oregons created by the 600 Ma Pan-African event rejuvenated the older rocks. The major heat flows that could possibly have created tremors are not unconnected with the quaternary volcanicity that are predominantly related with the Cameroun volcanic line, the volcanic flows related to the central epeirogenic uplift of the Jos Plateau and the Biu basalts (Fig. 2). Because of this history of stability, any form of seismicity becomes an invaluable asset and an addition to geodynamics of this relatively stable Oregon. Locally, extensive flood basalts also occur near the Kagoro Younger Granite Complex in Kafanchan, some 70-80 km north eastof Kwoi.

Materials and methods
This study involved a general assessment and investigation of geological evidences for the episodic tremors recorded in Kwoi, central Nigeria. On the field visits to houses and structures affected by the tremor was carried out by measuring the intensity of damage on the buildings, styles of cracks and the orientations of the crack patterns.
This field investigation also covers a detailed study of rocks and boulders for possible systematic displacements and breakages. Structures including fractures, joints and strikes of fault planes were measured on the rocks around the area and plotted to get the general trends. Surface structural patterns were assessed using Spot 5 composite images processed using ArcGIS 10.1. Lineaments were digitized and the attribute table generated the orientations from which the rosette gave the general trends. In order to assess the depths to which these surficial fractures penetrated the sub-surface, the aeromagnetic data of the area was acquired from the Nigerian Geological Survey Agency and processed in Oasis Montage. The Source Parameter Imaging (SPI) was utilized to obtain the magnetic basement over the area. Tilt derivatives gave good results for mapping buried fractures and faults. This was studied for any possible link to the surficial cracks as well as their proximity to the possible source of tremor. The interpreted seismic data was obtained from the Centre for Geodesy and Geodynamics Toro Nigeria. This was used to assess the dept. and locations of the focus and epicentres respectively.

Geology and structural features
The Kwoi area is underlain by migmatized gneisses, schist, quartzites and minor amphibolites. These metasedimentary rocks have been intruded by granitic rocks ranging from the Pan-African suites to the Jurrassic suites which form undulating rolling whalebacks (Fig. 3). The extreme weathering makes mapping of the highly deformed rocks difficult except for the granitic plutons that surround the area and stretches into the Kafanchan and Jema'a sheets. To the north of the Kwoi Town, the Nok areas are intruded by Younger Granites that are principally granites, granodiorites with minor syenites and volcanic dykes.
The metamorphosed gneisses and schists are foliated with a generally NW-SE and NE-SW strike and dips sub-vertically to the northeast. The granitoids have been intensely fractured with principal orientations of NW-SE (Figs. 3, 4 and 5).
An attempt to systematically categorize them was not very helpful even though the cracks Fig. 6 on the E-W walls which were sub-horizontal to the generally NW-SE trajectory of the effect suggestively implicated.

Aeromagnetic lineaments and seismic data
The raw and processed aeromagnetic data for the area are shown in Figs. 7 and 8, the shaded relief  (Fig. 9) and the composite lineament map (Fig. 10) while the interpreted seismic data are shown in Figs. 9 and 12 respectively. The general trends of the structure seem to tally while the tremor epicentres plotted on them define a trend that seem to be consistent with the general buried lineaments from the TMI data (Table 1).
Geologically, what appeared to be major fractures are clearly depictable from Kafanchan, Jema'a and Gitata sheets (NE, SE and SW quadrants of the mosaiced sheets).
The area appears to be tectonically active especially at depth because the depth of penetration of the aeromagnetic data goes beyond the surface to about 100 m. From the magnetic data, lineaments whose lengths range in kilometres are apparent. Major lineaments that may be fault lines at depths include the over 80 km traceable from Kachia to near the northeastern fringes of Gwarimpa district in Abuja in the Gitata sheet along a NNE -SWW traverse (west of Fig. 10). This is also obliquely cut by another 40 km lineament along a NWW -SEE lineament with intersection north of Kubacha town (SE   Fig. 10). This later trend is particularly significant because though it disappears beneath Kwoi town, and coincides with the trend of increasing seismic vibrations and effects on buildings. Another tectonically significant lineament is a 40 km long stretch along a NNE -SSW traverse from the northern end of the Nok hills that terminates north of Kwoi town. An extrapolation of the two lineaments coincide exactly 6.5 km NNW of Kwoi town. This NNE trend also aligns with the trajectory of the epicentres obtained from the seismic stations. Visual ispection to as far as Nok, Chori and Fori illages (Fig. 3) shows that there is no physical damages but oral testament through interviews shows the impacts were felt there.
The mapped fracture trends (Fig. 11) define a NW-SE trend which also coincide with the NW-SE trajectory of the impacted buildings and in some cases also spatially coincident with the crack patterns on the buildings. This probably shows that the surface waves may have been orthogonal to the trend of the trajectories of the epicentres (Fig. 8). While it is obvious that the epicenters do  not fall directly on the mapped major lineaments, the NNE-SWW trend defined by their trajectory obviously signify that a deeper investigation of their path in the subsuface may define a direct link and therefore infer a relationship bewteen the fractures and nature or path of propagation.
A close inspection of the paths of impact on the building which are defined along a NW-SE channel coincide with the major mapped fractures on the field. This however contrasts sharply with the lineaments as well as the NE-SW trajectory of the plotted epicenters. This is also shown on the rossette plots ( Fig. 11 a & b).
The scope of this research mainly utilized the seismographs to the extend of establishing their location and depths. It is to be acknowledged that detailed propagation in relation to the other sources of data for structural patterns are difficult but their trajectory show some remote connection. This ascertion is particularly instructive because for long, researchers have been speculating the possible link between the trans-Atlantic fracture zones.
The seismograms (Figs. 12 and 13) gave an indication of the depth to source to be about 10 km and picked along a NE-SW trend. These trends coincided with the major deep-seated fractures/faults (Figs. 7, 8 and 10) that cuts through the study area though not exactly on the fractures. It is the conclusion of this research that these geological evidences point to a tectonic origin for the seismicity in this region. Across the regional picture, there is no specific evidence of the link of this tremor centers with the Chain Fracture Zone, but they definitely fall along the northeasterly extrapolation of these established fracture zones including the Charcot F.Z. that cuts through the Gulf of Guinea and connects to the Benue Trough. While it may not be definitely concluded that the tremor is tectonic because of the paucity of the seismic data, all the geological observations which includes field observations of the structural patterns, inferred major fractures and faults obtained from the aeromagnetic and remote sensed data, and consequently placing them in the light of the regional geological context (the North Atlantic fracture systems), it is logical to infer that these trends does not just merge by coincidence. The tremor epicentres, including that of the recent Mpape Tremor in Abuja which is southwest of the Kwoi area and even though not included in this study also fall along the major fractures and when projected, seem to align with the Chain Fracture Zone in the Atlantic. The tremors therefore may be linked to reactivation of faults systems related to these Atlantic Fractur Zones.

Conclusions
There were multiple tremors of magnitudes 2.9 to 3.1 which took place on the following days: There was displacement of a 4 m by 3 m diameter fresh granite boulder that fell through a cumulative distance of 25 m. The effect on buildings which were mostly cracks on walls of buildings were not random but define a NW-SE trajectory, and those on the NNE -SSW oriented walls were more pronounced.
Seismic vibrations that shook the residents of Kwoi and environs appear to have been propagated laterally from the rocks along a south west profile.
A possible tectonic origin that is not well understood related to stress build up in the rocks of the area can be insinuated and spatially related to the Chain Fracture Zone (CFZ).