Episodes of micro-rifting contribute to distributed extension in Afar: A case-study of a 2002 event
Abstract
Over two decades of InSAR observations of Central Afar have revealed rifting activity not resolved with GPS or seismological data. The region extends from Manda Hararo-Dabbahu in the west and Asal-Ghoubbet in the east, two major magmatic rift systems that accommodate most of the present-day diverging plate movement. However, the ~2 Myr basaltic floor of Central Afar is dissected by a dense network of normal faults, indicating that some deformation is distributed over time between these two major rift systems. Here we focus on a small rifting event, which occurred on a set of north-south faults, approximately 50 km east of the Dabbahu rift in June 2002. Surface deformation associated with the event was revealed from an ascending pass of RADARSAT-1 InSAR data in two different ways. Co-rifting interferograms showed phase cuts along faults activated in the event, but the phase delay produced by the turbulent troposphere masked the associated subsidence within the rift. A step function adjusted to a 10 year-time series of interferograms revealed the complete LOS surface deformation field within and around the rift system activated in the event. Ground subsidence (range increase) of up to 40 mm occurred in 3 sections of a 5 km-wide rift extending ~25 km in the north-south direction. Faults bounding the sections of subsidence exhibit down-dip slip of ~20 mm. Preliminary modeling of observed deformation indicates that extension must have taken place on a structure between depths of 4—8 km, with down-dip movement occurring on shallower faults. This pattern is similar to that produced by a subsurface dyke connecting with steep normal faults reaching the surface. The event released a geodetic potency of 3.18x108 m/m2 . Assuming a shear modulus of 3x1010 Pa, this value would correspond to a total geodetic moment of 95.5x1017 Nm, with strike-slip and down-dip components contributing geodetic moments of 1.85 and 31.2x1017 Nm, respectively. Deformation may have occurred at seismic speeds, though the regional earthquake catalogue does not record any seismicity associated with the event. Our results indicate shallow faults in Afar are in a critical state and can be activated in micro-slip events by subtle changes in magmatic conditions at depth. Similar behavior was observed on faults bounding the Asal Rift between 2000—2005.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMT024.0003S
- Keywords:
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- 7205 Continental crust;
- SEISMOLOGY;
- 8110 Continental tectonics: general;
- TECTONOPHYSICS;
- 8120 Dynamics of lithosphere and mantle: general;
- TECTONOPHYSICS;
- 8159 Rheology: crust and lithosphere;
- TECTONOPHYSICS