Search

Crustal deformations associated with rifting episodes are often explained by both normal faulting and tensile opening in an elastic body, suggesting no significant rift-parallel displacements. In this study, using ALOS/PALSAR pixel-offset and multiple aperture interferometry (MAI) data, we detected similar to 1 m of northwestward block-like cumulative horizontal motion in the graben floor during the latter half of the 2005-2010 Afar rifting episode, Ethiopia. This horizontal block-like motion can be explained by additional shallower strike-slip on normal fault surfaces, which was presumably aseismic due to the very small number of strike-slip earthquakes and a significant gap between seismic and geodetic moment releases. We conclude that the rift-parallel movement can be interpreted by the strain accommodation of plate oblique spreading associated with dike intrusions, as has been demonstrated for the 2014 Baroarbunga dike intrusion episode. Although we do not propose direct evidence for the plate oblique spreading, the complicated tectonic setting caused by the rotation of the Danakil block and the upwelling plume beneath the Afar triple junction may be drivers of the rift-parallel motion associated with the Afar dike intrusion.

Crustal deformations associated with rifting episodes are often explained by both normal faulting and tensile opening in an elastic body, suggesting no significant rift-parallel displacements. In this study, using ALOS/PALSAR pixel-offset and multiple aperture interferometry (MAI) data, we detected similar to 1 m of northwestward block-like cumulative horizontal motion in the graben floor during the latter half of the 2005-2010 Afar rifting episode, Ethiopia. This horizontal block-like motion can be explained by additional shallower strike-slip on normal fault surfaces, which was presumably aseismic due to the very small number of strike-slip earthquakes and a significant gap between seismic and geodetic moment releases. We conclude that the rift-parallel movement can be interpreted by the strain accommodation of plate oblique spreading associated with dike intrusions, as has been demonstrated for the 2014 Baroarbunga dike intrusion episode. Although we do not propose direct evidence for the plate oblique spreading, the complicated tectonic setting caused by the rotation of the Danakil block and the upwelling plume beneath the Afar triple junction may be drivers of the rift-parallel motion associated with the Afar dike intrusion.

Crustal deformations associated with rifting episodes are often explained by both normal faulting and tensile opening in an elastic body, suggesting no significant rift-parallel displacements. In this study, using ALOS/PALSAR pixel-offset and multiple aperture interferometry (MAI) data, we detected similar to 1 m of northwestward block-like cumulative horizontal motion in the graben floor during the latter half of the 2005-2010 Afar rifting episode, Ethiopia. This horizontal block-like motion can be explained by additional shallower strike-slip on normal fault surfaces, which was presumably aseismic due to the very small number of strike-slip earthquakes and a significant gap between seismic and geodetic moment releases. We conclude that the rift-parallel movement can be interpreted by the strain accommodation of plate oblique spreading associated with dike intrusions, as has been demonstrated for the 2014 Baroarbunga dike intrusion episode. Although we do not propose direct evidence for the plate oblique spreading, the complicated tectonic setting caused by the rotation of the Danakil block and the upwelling plume beneath the Afar triple junction may be drivers of the rift-parallel motion associated with the Afar dike intrusion.

In July 2007, an earthquake swarm initiated in northern Tanzania near Lake Natron and lasted for about two months. Mt. Oldoinyo Lengai, located to the southwest of the swarm, began to erupt effusively about a month prior to the swarm, and increased its eruption intensity on September when the swarm almost ceased. Several previous studies have already reported the crustal deformation signals associated with the swarm using Interferometric Synthetic Aperture Radar (InSAR). However, nearly all the published data are based on the C-band ENVISAT/ASAR images acquired only from the descending path.We use the L-band ALOS/PALSAR images acquired fromboth ascending and descending paths,which allowus to examine the deformation signals in more detail. In addition to the InSAR data, we employ the offset-tracking technique to detect the signals along the azimuth direction. Using InSAR and offset-tracking data, we obtain the full 3D displacement fields associated with the episode. Besides the horizontal extension and subsidence signals due to the dike intrusion as already reported, the inferred full 3D displacements further indicate that the subsiding zone was horizontally moving by ~48 cm toward SSW. To explain the displacements, we performed fault source modeling, assuming an elastic half space. The fault slip distribution indicates that the contribution of the strike–slip component is about 20% of total moment release. Because almost all the focal mechanisms of earthquakes during the 2007 event indicate nearly pure normal faulting, aseismic strike–slip must have been responsible for the horizontal movement of the subsiding zone. The strike–slip at the shallowest depths suggests the presence of transtensive stress, which seems to be reasonable to generate the relay zones that are widely observed in the East African Rift. We also confirmed that the stress changes due to the dike intrusion were consistent with the inferred fault slip distributions.

In July 2007, an earthquake swarm initiated in northern Tanzania near Lake Natron and lasted for about two months. Mt. Oldoinyo Lengai, located to the southwest of the swarm, began to erupt effusively about a month prior to the swarm, and increased its eruption intensity on September when the swarm almost ceased. Several previous studies have already reported the crustal deformation signals associated with the swarm using Interferometric Synthetic Aperture Radar (InSAR). However, nearly all the published data are based on the C-band ENVISAT/ASAR images acquired only from the descending path.We use the L-band ALOS/PALSAR images acquired fromboth ascending and descending paths,which allowus to examine the deformation signals in more detail. In addition to the InSAR data, we employ the offset-tracking technique to detect the signals along the azimuth direction. Using InSAR and offset-tracking data, we obtain the full 3D displacement fields associated with the episode. Besides the horizontal extension and subsidence signals due to the dike intrusion as already reported, the inferred full 3D displacements further indicate that the subsiding zone was horizontally moving by ~48 cm toward SSW. To explain the displacements, we performed fault source modeling, assuming an elastic half space. The fault slip distribution indicates that the contribution of the strike–slip component is about 20% of total moment release. Because almost all the focal mechanisms of earthquakes during the 2007 event indicate nearly pure normal faulting, aseismic strike–slip must have been responsible for the horizontal movement of the subsiding zone. The strike–slip at the shallowest depths suggests the presence of transtensive stress, which seems to be reasonable to generate the relay zones that are widely observed in the East African Rift. We also confirmed that the stress changes due to the dike intrusion were consistent with the inferred fault slip distributions.