Solving the Mystery of Orphan Tremors Detected in Western Africa on Sept
5-7, 2018
Abstract
On September 5-7, 2018, a series of puzzling tremors were reported in
the capital city of Nigeria in Mpape, Abuja. This event follows a
growing list of tremors experienced in a stable intra-plate region not
expected to be earthquake-prone. Initial speculation suggested that the
shaking resulted from anthropogenic activity related to ground-water
extraction, although no corroborating geophysical, or seismic evidence
exists to confirm such a hypothesis. Here, we identify the parent
location of this orphan tremor by developing a technique for the
single-station location of seismic sources, an approach particularly
suited for regions with sparse seismic networks. Our procedure
identifies the spatial origin of incoming seismic waves by using a
combination of energy and polarization filters to robustly identify
seismic phases and extract differential time (epicentral distance) and
arrival azimuth. We demonstrate that our technique can discriminate
between local, regional or teleseismic events. We apply this technique
to the 3-day seismic record obtained from select stations around
Nigeria. We rule out western-Africa as the origin of the reported
tremors. A comprehensive scan of the available seismic record suggests
that the source of the felt tremors was coincident with the
earthquake which was reported to have triggered massive landslides. We
hypothesize that these landslides generated enough low-frequency
single-force surface-collapse, strong enough to be preferentially
amplified by the local geologic structure of the Abuja area.