Publication Date:
2013-02-15
Description:
The process of dike emplacement changes the stress field in the intruded region, causing swarms of migrating earthquakes. We determine source mechanisms of the largest earthquakes ( M L ≥ 3.5) induced by the emplacement of two large volume dikes along an incipient seafloor spreading segment in Afar, Ethiopia to determine their space-time relations. Given the possibility of complex source mechanisms during dike emplacement, we solved for four different source models: double couple (DC), DC + isotropic (DC + ISO), deviatoric (DVMT), and full moment tensor (FMT). The solutions obtained using the different models indicate that the earthquakes have non-double couple components. The best double-couple mechanisms, which are mainly normal faulting with small strike-slip components, have slip planes nearly perpendicular to the geodetically determined plate opening direction. Most of these earthquakes are low-frequency earthquakes with peak frequencies ≤2 Hz, and they occurred during the propagation phase of the dikes. The space-time distribution of the source mechanisms with respect to the migrating patterns of seismicity during dike emplacement, the shallow source depths estimated, the lack of mechanisms with ~90º rotated P-axes from the regional maximum compressive stress and the non-double couple nature of the mechanisms indicate that the largest magnitude earthquakes are generated mainly by normal faulting above the dikes probably with some component of tensile opening under the influence of dike-related fluids. These observations suggest that normal faulting above dikes is the main process of seismic energy release during dike intrusions. Assuming that faults above dikes follow length-displacement relations found for tectonic earthquakes, total seismic slips of 0.8 m and 1.3 m are estimated for the November 2007 and October 2008 dikes, respectively. Similarity of the total slip estimates from the largest earthquakes and from elastic dislocation model estimates of normal fault slips and the larger total seismic versus geodetic moment deficits indicates that most of the plate opening by dike intrusion is accommodated aseismically. Our results show that during dike intrusions surface faulting above dikes is the main process that controls the development of rift zone geomorphology and topography.
Print ISSN:
0956-540X
Electronic ISSN:
1365-246X
Topics:
Geosciences
Published by
Oxford University Press
on behalf of
The Deutsche Geophysikalische Gesellschaft (DGG) and the Royal Astronomical Society (RAS).
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