East African Seismicity
The February 22nd 2006 Mw 7 earthquake in Machaze, Mozambique is probably the largest event ever recorded instrumentally within continental Africa. Prior to that, ten earthquakes with approximately M7 associated with the East African Rift have been reported in the past century. The location of this particular earthquake was somewhat surprising given that it is not associated with any known large-scale fault. We investigated the physics of this earthquake and found that it possibly occurred on a previously unknown active segment of the East African Rift.
The earthquake (red star) was recorded in high quality at teleseismic distances globally. We selected seismograms from 175 stations illustrated by white triangles to study its source properties.
The location of the Machaze earthquake with respect to other geological features in East Africa is shown in this figure. The red-dashed line indicates Eastern and Western branches of the East African Rift (EARS), which appears to terminate to the North of the location of the Machaze earthquake.
Left: Example high quality P waveforms (blue) at three different teleseismic distances relative to noise (red). Upper panel is time domain and the bottom panel is spectral domain. Right: Moment rate spectral density functions estimated from seismic P waveforms such as those shown in the left panel. After correcting for wave propagation effects, we estimated a scalar moment of 3.98x1e19 Nm (Mw 7) for this earthquake.
We inverted this high quality waveform data set for an azimuthally averaged source time function (STF) that shows moment release evolution. In the process, we also inverted for the focal mechanism of the earthquake. We demonstrated that using an accurate crustal structure model is important for the precision of these inversions. Our inversion took into account the effect of higher order reverberations generated when seismic waves interact with discontinuities in the crust.
A schematic P (blue) and S (green) ray path diagram showing waves interacting with discontinuities. These interactions generate higher order reverberations.
The fit of predicted waveforms (blue and red) to observed ones (black). The blue waveforms are predicted for a complex crustal model, whereas red waveforms are for a simple crustal model.
This figure shows the results of the inversion. On the left is the focal mechanism and on the right is the STF.
We found that this earthquake occurred on an active structure previously not known. In the figure below, small earthquakes (black dots, M < 3) detected in the MOZART experiment clearly delineates an active structure connecting the presumed termination of the East African Rift (EARS) and the Machaze earthquake region with an offset. This newly detected structure might be the southernmost extent of the East African Rift. The Machaze event is shown with a red star and the white dashed-line shows the general trend of seismicity, extending the EARS further to the south.
Attanayake, J. and J. B. D. Fonseca, 2016. The Intraplate Mw 7 Machaze Earthquake in Mozambique: Improved Point Source Model, Stress Drop, and Geodynamic Implications, J. Afr. Earth Sci., 117, 252-262. http://dx.doi.org/10.1016/j.jafrearsci.2016.01.027 (pdf)