Asymmetric Opening and Episodic Faulting in the Asal Rift, Djibouti

Gilles Peltzer (1,2) and Cécile Doubre (1) (1) University of California, Los Angeles (2) Jet Propulsion Laboratory, California Institute of Technology

Located at the western end of the Aden Ridge, the Asal Rift is the first subaerial opening segment of the ridge propagating into Afar (Fig. 1a). In November 1978, the rift was the locus of a major seismo-volcanic crisis producing a fissural eruption, 2 m of horizontal extension between the rift shoulders, 0.7 m of subsidence of the inner floor and a number of signs of ground deformation, including up to one meter of slip on large bounding faults and numerous open fissures (1-3). After the 1978 crisis, the rate of opening across the rift remained high (6±1 cm/yr) until 1987 and then returned to a value of ~1.5 cm/yr, comparable to the long-term plate motion. Small earthquakes (M

We used the interferometry method (InSAR) with RADARSAT-1 scenes acquired between 1997 and 2005 to survey the surface deformation field in the Asal Rift (Fig. 1b). The turbulent atmosphere in this subtropical region produces phase delay errors in the data exceeding the tectonic signal we seek. Data averaging and time series analysis allowed us to mitigate this signal.

A 2-component surface displacement field was constructed using interferograms from 15 ascending and 15 descending image pairs. We assume that the horizontal movement of the crust in and around the rift is approximately perpendicular to the main faults and solve for the ~N35˚E horizontal and vertical components from radar observations along ascending and descending lines of sight. The average displacement field shows the following features (Fig. 2). A ~30 km-wide zone centered 4 km north of the rift axis is inflating at a rate of up to 8 mm/yr. The ~8 km-wide central rift subsides asymmetrically (down-tilted to the north) with respect to the rift shoulders as a result of slip on Fault g to the north and Fault E to the south, both of which were active during the 1978 crisis. The horizontal velocity shows extension across the central rift at a rate of ~13 mm/yr, gradually decreasing in the far field. A local maximum of 16 mm/yr in the horizontal velocity occurs on the northern shoulder of the rift and coincides with the area of maximum uplift. This divergent velocity between both shoulders of the rift exceeds the 11 mm/yr, far field motion between the Arabia and Nubia plates (4), suggesting that magmatic activity is currently driving the opening of the Asal rift. Elastic models show that a 4 km-deep, dyke system, expanding both laterally and upward, combined with down-dip slip and horizontal opening of Fault g and Fault F, accounts for most of the observed velocity across the rift.

The horizontal extension measured across the opening faults suggests that the subvertical faults at the surface have shallower dipping planes at depth. For example, the mean vertical throw rate on Fault E is ~1.8 cm/yr and its opening rate is ~3.8 cm/yr, indicating that the fault has a dip of ~26˚ at depth. Vertical throw and horizontal extension rates are approximately equal on Fault g indicating a dip of ~45˚ for the deeper part of the fault.

We constructed an 8 year timeline of the surface deformation in the Asal Rift from the data acquired from RADARSAT-1 descending passes using the small-baseline subset approach (5). The time-series shows that slip rates on Faults g and E vary in time with slow and steady creeping periods, interrupted by accelerated slip events of a few millimeters. These events are coeval with bursts of micro-earthquakes in the area of the creeping faults, although the seismic moment released during these bursts is three orders of magnitude lower than a rough estimate of the geodetic moment associated with the slip events. This shows the aseismic nature of the slip events on Faults g and E.

The coeval occurrence of fault-slip events and bursts of micro-earthquakes distributed around the slipping faults suggests that both phenomena respond to a sudden change in the stress condition in the upper crust. Fluid pressure changes in the magmatic/hydrothermal system can explain these observations. If the fluid pressure increases in the fissures connecting to faults at depth, it results in the decrease of the normal stress on the fault, hence bringing it closer to failure. In the extensional stress regime, prevailing in and around the rift, such pressure changes are likely to trigger small earthquakes and control aseismic slip events on normal faults.

These preliminary results demonstrate the interest of data averaging and time-series construction to track micro-deformation and fault-slip events associated with transient processes. Future work will include completion of a two-dimensional time series, including data from both ascending and descending passes and the development of mechanical models.

References:
(1) Abdallah, A., et al., Afar seismicity and volcanism: relevance to the mechanics of accreting plate boundaries. Nature 282, 17-23 (1979).

(2) Le Dain, A.-Y., Robineau, B. & Tapponnier, P. Les effets tectoniques de l’événement sismique et magmatique de Novembre 1978 dans le rift d’Asal-Ghoubbet. Bull. Soc. Géol. France 7, 817-822 (1979).

(3) Ruegg, J.-C., Lépine, J.-C. & Tarantola, A., Geodetic measurements of rifting associated with a seismo-volcanic crisis in Afar. Geophys. Res. Lett. 6, 817-820 (1979).

(4) Vigny, C., et al., 25 years of geodetic measurements along the Tadjoura-Asal Rift system, Djibouti, East Africa. In review for J. Geophys. Res. (2006).

(5) Berardino, P., Fornano, G., Lanari, R. & Sansosti, E., A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms. IEEE Trans. Geosc. Remote Sens. 40, 2375-2383 (2002).

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