Mozambique Earthquake Sequence of 2006: High-Angle Normal Faulting in Southern Africa

Mozambique Earthquake Sequence of 2006: High-Angle Normal Faulting in Southern Africa (right) Comparison between observed (solid traces) and synthetic P waveforms (dashed traces) of the main shock. The synthetic seismograms are calculated with a rupture history (source time function) represented by a symmetric triangle of 7.5 s in duration. Notice excellent azimuthal distribution of observations which are well matched by synthetic seismograms. (left) Comparison of synthetic seismograms based on the Harvard CMT solution with selected observations whose polarities of first motions are violated in several cases because a dip angle of 65° is too low. Open circles in the plot of fault plane solution are poles of nodal planes.
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We report source mechanisms for the six largest shocks of the Mozambique earthquake sequence of 22 February 2006. The main shock of this sequence is one of the largest (Mw ~ 7.0) to occur in Africa over the past 100 years and its P waveforms alone are sufficient to show that north‐south trending normal faulting near the surface continues to depths of more than 15 km along an exceptionally steep dip of 76° ± 4°. This new result shows that globally seismogenic normal faulting spans a wide range of dips, from about 30° to 75°. S waveforms, when combined with a minor component of left‐lateral slip observed in the field, indicate the rake to be between −80° and −89° which, in turn, places a new constraint on relative plate motions: Complications from a nonspecific Rovuma microplate notwithstanding, our results favor the Euler pole between the Somalia and Nubia plates to lie southward of the epicenters.
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Acknowledgements: We thank E. Calais, C. Collettini, C. Hartnady, R. Sibson, S. Stein, and Y. D. Zheng, for helpful discussions. In particular, S. Marshak suggested antithetic faulting in a listric master system. The data management centers of IRIS and GEOSCOPE kindly pro- vided seismograms to us, and D. Doser, C.J. Ebinger, and B.W. Stump made careful reviews of the manuscript. This work is supported by the U.S. National Science Foundation grant EAR9909362 (Project Hi-CLIMB: An Integrated Study of the Himalayan-Tibetan Continental Lithosphere during Mountain Building, contribution I06). Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the NSF.</p>

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