Deformation along the north African plate boundary observed by InSAR

Ian Hamling1,2

Abdelkrim Aoudia2

1. GNS Science, Avalon, New Zealand2. ICTP, Trieste, Italy

http://earthobservatory.nasa.gov/NaturalHazards/view.php?id=79000

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

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

Hun Graben, Libya

• Border faults are considered to be seismically active.

• Faults are thought to have been responsible for the 1935 M7.1 earthquake

Hun Graben, Libya

We use a total of 57 interferograms, constructed from 25 ERS and Envisat radar images, from 1992 - 2010

Ratemap formation using πrateInput:

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;

Ratemap formation using πrateInput:

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 using πrateInput:

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.

Deformation along the Hun Graben

25/04/92 - 29/12/98 23/12/03 – 29/06/10

• 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

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

525

530

535

540

545

3280

3285

3290

3295

3300

3305

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

Deformation along the Hun Graben

The model suggests a maximum compaction of 15 mm/yr (Fig 4D).

The model predicts a volume change of 1.2 106 m3 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

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

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

Deformation in Northern AlgeriaOran

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

Deformation in Northern AlgeriaCheliff

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

Deformation in Northern AlgeriaMitijda

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

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

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

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