1. Deformation along the north African plate boundary observed by InSAR Ian Hamling 1,2 Abdelkrim Aoudia 2 1.GNS Science, Avalon, New Zealand 2.ICTP, Trieste, Italy http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=79000

2. Outline Target areas: – Hun Graben, Libya – Oran, Chelliff and Mitijda Basins, Algeria InSAR methodology and ratemap generation using πrate. InSAR observations of the Hun Graben InSAR observations across northern Algeria Future work Conclusions

3. Hun Graben, Libya The 120 km long, 50 km wide graben, located in north eastern Libya, separates the Sirte Basin to the east from the Hamada al Hamra platform to the west

4. Hun Graben, Libya Border faults are considered to be seismically active. Faults are thought to have been responsible for the 1935 M7.1 earthquake

5. Hun Graben, Libya We use a total of 57 interferograms, constructed from 25 ERS and Envisat radar images, from 1992 - 2010

6. Ratemap formation using πrate Input: Geocoded unwrapped interferograms with errors corrected or removed. Orbital Correction: Best fitting plane and static shift is found for each epoch and removed. Wang et al. GJI 2012;

7. Ratemap formation using πrate Input: Geocoded unwrapped interferograms with errors corrected or removed. Orbital Correction: Best fitting plane and static shift is found for each epoch and removed. Atmospheric Correction: Remove atmospheric delays by assuming a linear correlation with topography Wang et al. GJI 2012;

8. Ratemap formation using πrate Input: Geocoded unwrapped interferograms with errors corrected or removed. Orbital Correction: Best fitting plane and static shift is found for each epoch and removed. Atmospheric Correction: Remove atmospheric delays by assuming a linear correlation with topography Wang et al. GJI 2012; Ratemap formation: LS inversion to estimate displacement rate at each coherent pixel. Inversion is weighted using temporal VCM. Pixels are discarded if there are fewer than 10 coherent epochs, the uncertainties are greater than 2σ of the rate or if the residuals at each pixel are greater than twice the variance of an ifm.

9. Deformation along the Hun Graben 25/04/92 - 29/12/98 23/12/03 – 29/06/10

10. Deformation is strongly controlled by the western border fault There is a strongly correlation between the location of known aquifer pump sites and subsidence Peak LOS displacement rates of 9 mm/yr ± 0.5 mm are observed between 2004 and 2010 with rates of 3 mm/yr between 1992 and 1999 Deformation along the Hun Graben

11. Due to the lack of information about the aquifer in the region, we assume that the observed deformation can be explained by the elastic compaction of a tensile crack (Okada; 1992) Aquifer is assumed to be horizontal at 3 km depth. Crack is discretized into 750 m x 750 m patches. We solve for the best fitting compaction model, m, for the specified geometry. Modelling of the deformation along the Hun Graben where A is are Greens functions representing displacements at observation points x and y, for 1 m of compaction on each of the patches. a, b and c are phase gradients in the x and y directions and c is the unknown 0 phase level in the data

12. Deformation along the Hun Graben The model suggests a maximum compaction of 15 mm/yr (Fig 4D). The model predicts a volume change of 1.2 10 6 m 3 yr -1 ±0.24. To see the effect of the aquifer pumping on the border fault we next calculate the Coulomb Failure Stress along the fault plane

13. Effect of pumping on bounding fault Coulomb failure stress along the western border fault of the Hun Graben assuming a normal (a) and strike-slip (b) mechanism after 12 years of pumping. Maximum stress change for a normal fault is ~0.2 bar/yr

14. Deformation in Northern Algeria Deformation along the North African boundary, related to the collision of the Nubian and Eurasian plates, is complex and poorly understood. The region has witnessed a number of large (Mw>6) earthquakes during the last century including the 2003 Zemmouri earthquake in Algeria. However, due to a lack of ground based instrumentation there is very little information on which structures are actively accumulating strain. Using the same method described earlier we generate LOS ratemaps over the Oran, Chelliff and Mitijda basins

15. Deformation in Northern Algeria Oran A decrease in the Line of Sight along the Oran anticline suggest uplift in the region. LOS displacements of ~4 mm/yr are observed Subsidence at rates of ~10- 12 mm/yr are observed near the Moroccan border. The source of the deformation here is most likely related to anthropogenic activities

16. Deformation in Northern Algeria Cheliff Deformation is concentrated within the Cheliff basin and is strongly controlled by mapped faults bounding the basin Largest displacement rates are suggest subsidence at rates of ~6 mm/yr within the basin Some is uplift observed to the north west of the basin. However, due to incoherence and noisy data this is hard to quantify

17. Deformation in Northern Algeria Mitijda Sahel anticline Deformation is concentrated along the coastal region Largest displacement rates are located within the Mitijda basin and suggest subsidence at rates of ~15 mm/yr Apparent uplift, at rates of ~3-4 mm/yr, is observed above the Sahel anticline

18. Deformation in Oran and Mitijda In both regions, deformation is concentrated above reverse faults along the coast. Pattern of uplift above fault does not fit with the interseismic model of Savage Back slip model of Savage showing the expected vertical deformation above the fault X = 0

19. Deformation in Oran and Mitijda Using an Okada type creeping dislocation produces uplift as observed above the Oran and Mitijda structures. Sample deformation profile for an Okada type creeping fault. X = 0

20. Conclusions We have identified a number of deforming areas across the north Africa plate boundary related to both anthropogenic activities and the convergence between the Nubian and Eurasian plates. Libya provides an ideal location for radar interferomerty due to the arid terrrain. Our data suggests that the border faults of the Hun Graben control the spatial pattern of subsidence associated with water extraction from deep aquifers. Loss of coherence in interferograms, large atmospheric artifacts combined with the direction of plate motion affect the interpretation of data over Northern Algeria. The available data suggests uplift along the Sahel and Oran anticlines consistent with a creeping fault and/or fold growth BUT… to better constrain the deformation, additional data is required in order to better estimate the errors and increase the signal to noise ratio