Kurt L. Feigl

papers published in the peer-reviewed literature

December, 2006

1.        Feigl, K. L., R. W. King and T. H. Jordan (1990), Geodetic measurement of tectonic deformation in the Santa Maria Fold and Thrust Belt, California, J. Geophys. Res. 90, 2679–2699. abstract at AGU

2.        Feigl, K. L., R. W. King, T. A. Herring, and M. Rothacher (1991), A scheme for reducing the effect of Selective Availability on precise GPS carrier phase measurements, Geophys. Res. Lett. 12, 1289-1292. abstract at AGU

3.        Feigl, K. L., D. C. Agnew, Y. Bock, D. Dong, A. Donnellan, R. W. King, B. H. Hager, T. A. Herring, D. D. Jackson, T. H. Jordan, S. Larsen, K. M. Larson, M. H. Murray, Z. Shen, and F. H. Webb (1993), Space geodetic measurement of crustal deformation in central and southern California, 1984–1992, J. Geophys. Res. 98, 21,677–21,712. abstract at AGU

4.        Massonnet, D., M. Rossi, C. Carmona, F. Adragna, G. Peltzer, K. Feigl, and T. Rabaute (1993), The displacement field of the Landers earthquake mapped by radar interferometry, Nature 364, 138–142. PDF

5.        Ruegg, J.-C., P. Briole, K. L. Feigl, A. Orsoni, C. Vigny, M. A. Abdallah, O. Bellier, J.‑B. de Chaballier, P. Huchon, E. Jacques, S. A. Kurbash, A. Laike, N. d'Oreye, and M. Prévot (1993), First epoch geodetic GPS measurements across the Afar plate boundary zone,
Geophys. Res. Lett. 20, 1899–1902.
 abstract at AGU

6.        Massonnet, D., K. L. Feigl, M. Rossi, and F. Adragna (1994), Radar interferometric mapping of deformation in the year after the Landers earthquake, Nature 369, 227–230. PDF

7.        Peltzer, G., K. W. Hudnut, and K. L. Feigl (1994), Analysis of surface displacement gradients using radar interferometry: New insights into the Landers earthquake,
J. Geophys. Res. 99, 21,971–21,981.

8.        Feigl, K. L., A. Sergent, and D. Jacq (1995), Estimation of an earthquake focal mechanism from a satellite radar interferogram: application to the December 4, 1992 Landers aftershock, Geophys. Res. Lett. 22, 1037-1048. PDF at AGU

9.        Massonnet, D., and K. L. Feigl (1995), Discriminating geophysical phenomena in satellite radar interferograms, Geophys. Res. Lett. 22, 1537-1540.  PDF at AGU

10.     Massonnet, D., and K. L. Feigl (1995), Satellite radar interferometric map of the coseismic deformation field of the M = 6.1 Eureka Valley, California earthquake of May 17, 1993, Geophys. Res. Lett. 22, 1541-1544.  PDF at AGU

11.     Sylvander, M., K. L. Feigl, A. Souriau, and P. L. Blelly (1995), Absolute ionospheric vertical electron content inferred from transmissions of the Global Positioning System, C. R. Acad. Sci. Paris série II a 320, 793-799, 1995.

12.     Massonnet, D., K. L. Feigl, H. Vadon, and M. Rossi (1996), Coseismic deformation field of the M = 6.7 Northridge, California earthquake of January 17, 1994 recorded by two radar satellites using interferometry, Geophys. Res. Lett. 23, 969-972. PDF at AGU

13.     Massonnet, D., and K. L. Feigl (1998), Radar interferometry and its application to changes in the Earth's surface, Rev. Geophys. 36, 441-500. PDF

14.     Ferhat, G., K. L. Feigl, J. F. Ritz, and A. Souriau (1998), Geodetic Measurement of tectonic deformation in the Southern Alps and Provence, France, 1947-1994, Earth Plan. Sci. Lett. 159, 35-46  PDF

15.     Feigl, K. L., and E. Dupré (1999), RNGCHN: a program to calculate displacement components from dislocations in an elastic half-space with applications for modeling geodetic measurements of crustal deformation, Computers and Geosciences 25, 695-704. This paper won the Best Paper Award for “the paper judged outstanding in Computers and Geosciences” awarded by the International Association of Mathematical Geology.  PDF

Duong, C. C., and K. L. Feigl (1999), Geodetic measurement of horizontal strain across the
Red River fault near Thác Bà, Vietnam, 1963-1994, J. Geodesy 73, 298-310.  PDF

Reilinger, R.E., S. Ergintav, R. Bürgmann, S. McClusky, O. Lenk., A. Barka, O. Gurkan, L. Hearn, K. L. Feigl. R. Cakmak B. Aktug. H. Ozener, and M. N. Toksoz (2000), Coseismic and postseismic fault slip for the 17 August 1999, M=7.4,
Izmit, Turkey earthquake, Science 289, 1519-1524. PDF

18.    Feigl, K.L., J. Gasperi, F. Sigmundsson, and A. Rigo (2000), Crustal deformation near Hengill volcano, Iceland 1993-1998: coupling between volcanism and faulting inferred from elastic modeling of satellite radar interferograms,J. Geophys. Res. 105, 26,555-25,670.  PDF

19.     Pedersen, R., F. Sigmundsson, K.L. Feigl, and T. Árnadóttir (2001), Coseismic interferograms of two MS=6.6 earthquakes in the South Iceland Seismic Zone, June 2000, Geophys. Res. Lett. 28, 3341-3344.


20.     Vigny, C., J. Chéry, T. Duquesnoy, F. Jouanne, J.-C. Ruegg, J. Ammann, M. Anzidei, J.-P. Avouac, F. Barlier, R. Bayer, P. Briole, E. Calais, F. Cotton, F. Duqenne, K.L. Feigl, M. Flouzat, J.F. Gamond, A. Geiger, A. Harmel, M. Kasser, M. Laplanche, M. Le Pape, J. Martinod, B. Meyer, J.-M. Scheubel, O. Scotti, and G. Vidal (2002), GPS network monitors the western Alps over a five-year period: 1993-1998, J. Geodesy 76, 63-76. PDF

Feigl, K. L. (2002), Measurement of coseismic deformation by satellite geodesy, in International Handbook of Earthquake and Engineering Seismology, volume 81A, edited by W. H. K. Lee, H. Kanamoori & P. C. Jennings, and coordinated by the International Association of Seismology and Physics of the Earth's Interior (IASPEI), pp. 607-620, Academic Press
.  PDF

22.     Feigl, K.L., F. Sarti, H. Vadon, P. Durand, S. Mclusky, S. Ergintav, R. Bürgmann, A. Rigo, D. Massonnet, and R. Reilinger (2002), Estimating slip distribution for the Izmit mainshock from coseismic GPS, ERS-1, RADARSAT and SPOT measurements, Bull. Seism. Soc. Amer. 92, 138-160. PDF

23.     Clifton, A.E., F. Sigmundsson, K. Feigl, G. Guðmundsson, and T. Árnadóttir (2002), Surface effects of faulting and deformation resulting from magma accumulation at the Hengill triple junction, SW Iceland, 1994 - 1998, J. Volcanology Geothermal Res. 115, 233-255.  PDF

Abstracts and Links may not be complete or accurate above this line.....

24.     Kohlhase, A.O., K.L. Feigl, and D. Massonnet (2003), Applying differential InSAR to orbital dynamics: a new approach for estimating ERS trajectories, J. Geodesy 77, 493-502.  PDF

A new approach for tuning the trajectories of the European remote sensing (ERS) satellites is devel- oped and assessed. Differential dual-pass interferometry is applied to calculate interferograms from the phase difference of synthetic aperture radar (SAR) images acquired by the ERS satellites over the site of the 1992 earthquake in Landers, California. These interferograms contain information about orbital trajectories and geophysical deformation. Beginning with good prior estimates of the orbital trajectories, a radial and an across-track orbital adjustment is estimated at each epoch. The data are the fringe counts along distance and azimuth. Errors in the across-track and radial compo- nents of the orbit estimates produce fringes in the interferograms. The spacing between roughly parallel fringes gives the gradients in distance and azimuth coordinates. The approach eliminates these fringes from interferometric pairs spanning relatively short time intervals containing few topographic residuals or atmo- spheric artefacts. An optimum interferometric path with six SAR acquisitions is selected to study post- and inter- seismic deformation fields. In order to regularize the problem, it is assumed that the radial and across-track adjustments both sum to zero. Applying the adjustment approach to the prior estimates of trajectory from the Delft Institute for Earth-Orientated Space Research (DEOS), root mean squares of 7.3 cm for the across- track correction components and 2.4 cm for the radial ones are found. Assuming 0.1 fringes for the a priori standard deviation of the measurement, the approach yields mean standard deviations of 2.4 cm for the across-track and 4.5 cm for the radial components. The approach allows an ‘interval by interval’ improve-ment of a set of orbital estimates from which post-fit interferograms of different time intervals spanning a total 3.8-year inter-seismic time interval can be created. The interferograms calculated with the post-fit orbital estimates compare favorably with those corrected with a conventional orbital tuning approach. Using the adjust- ment approach, it is possible to distinguish between orbital and deformation contributions to interferometric SAR (InSAR) phase gradients. Surface deformation changes over an inter-seismic time interval longer than one year can be measured. This approach is, however, limited to well-correlated interferograms where it is possible to measure the fringe gradient.

25.     Pagli, C., R. Pedersen, F. Sigmundsson, and K.L. Feigl (2003), Triggered seismicity on June 16, 2000 on the Reykjanes Peninsula, SW Iceland captured by radar interferometry, Geophys. Res. Lett. 30 (6), 1273, doi:10.1029/2002GL015310.  PDF at AGU

Dynamically triggered seismicity followed shortly after a Ms 6.6 earthquake in Iceland on June 17, 2000. Smaller earthquakes occurred on the Reykjanes Peninsula up to 100 km from the mainshock rupture. Using interferometric analysis of Synthetic Aperture Radar images (InSAR), we measure crustal deformation associated with three triggered deformation events. The largest of these occurred at Lake Kleifarvatn, 85 km west of the mainshock epicenter. Modeling of the InSAR data reveals strikeslip on a north-striking fault, with a geodetic moment of 6.2 × 10e17 Nm, equivalent to magnitude Mw5.8 earthquake. A seismological estimate of the moment is not yet available, because the seismic signature of this event is partly hidden by the mainshock waveform. The paucity of aftershocks on the triggered rupture plane suggests some aseismic slip there, compatible with a thin seismogenic crust, high heat-flow, hydrothermal alteration and the presence of fluids in the area.

26.     Pedersen, R., S. Jónsson, T. Árnadóttir, F. Sigmundsson, and K.L. Feigl (2003), Fault slip distribution derived from joint inversion of InSAR and GPS measurements of two June 2000 Mw 6.5 earthquakes in South Iceland, Earth Plan. Sci. Lett., 213, 487-502.  PDF

We present the first detailed estimates of co-seismic slip distribution on faults in the South Iceland Seismic Zone (SISZ), an area of bookshelf tectonics. We have estimated source parameters for two M(w)6.5 earthquakes in the SISZ on June 17 and 21, 2000 through a joint inversion of InSAR and GPS measurements. Our preferred model indicates two simple 15 km long, near vertical faults extending from the surface to approximately 10 km depth. The geometry is in good agreement with the aftershock distribution. The dislocations experienced pure right-lateral strike-slip, reaching maxima of 2.6 m and 2.9 m for the June 17 and 21 events, respectively. We find that the distribution of slip with depth may be correlated to crustal layering, with more than 80% of the total geometric moment release occurring in the uppermost 6 km. According to the distributed slip model the middle and upper crust appears to be more apt to generate large displacements than the lower crust. The geodetic estimates of seismic moments are 4.4 X 10(18) Nm (M(w)6.4) and 5.0 X 10(18) Nm (M(w)6.5). The total moment released by the two events equals that generated by several decades of plate motion in the area, but is only a fraction of the moment accumulated in the area since the last major earthquake in 1912.

27.     Chlieh, M., J.-B. de Chabalier, J.-C.,Ruegg, R. Armijo, R. Dmowska, J. Campos, and K. L Feigl (2004) Crustal deformation and fault slip during the seismic cycle in the North Chile subduction zone, from GPS and InSAR observations, Geophysical Journal International 158, 695-711. PDF

The different phases of the earthquake cycle can produce measurable deformation of the Earth's surface. This work is aimed at describing the evolution of that deformation in space and time, as well as the distribution of causal slip on the fault at depth. We have applied GPS and synthetic aperture radar (SAR) interferometry (InSAR) techniques to northern Chile, where fast plate convergence rates are associated with large subduction earthquakes and extensive crustal deformation. The region of northern Chile between 18°S and 23°S is one of the most important seismic gaps in the world, with no rupture having occurred since 1877. In 1995, the Mw= 8.1 Antofagasta earthquake ruptured the subduction interface over a length of 180 km in the region immediately to the south of this 450 km long gap. The coseismic deformation associated with this event has been documented previously. Here we use GPS position time-series for 40 benchmarks (measured between 1996 and 2000) and ERS SAR interferograms (for the interval between 1995 and 1999) to map both the post-seismic deformation following the 1995 event and the ongoing interseismic deformation in the adjacent gap region. In the seismic gap, the interseismic velocities of 20-30 mm/yr to the east with respect to South America are mapped. Both the GPS and the InSAR measurements can be modelled with 100 per cent coupling of the thrust interface of the subduction to a depth of 35 km, with a transition zone extending down to 55 km depth. The slip rate in that zone increases linearly from zero to the plate convergence rate. South of the gap, the interferometric map shows interseismic deformation superimposed with deformation following the 1995 earthquake and covering the same area as the coseismic deformation. Some 40 per cent of this deformation is related to seismic activity in the 3.3 yr following the 1995 event, in particular slip during a Mw= 7.1 earthquake in 1998. However, most of the signal (60 per cent) corresponds to post-seismic deformation resulting from widespread aseismic slip in the subduction interface. The afterslip appears to have occurred down-dip in the transition zone at 35-55 km depth and to have propagated laterally northwards at 25-45 km depth under the Mejillones Peninsula, which is a prominent geomorphological feature at the boundary between the 1877 and 1995 rupture zones. We propose a simple slip model for the seismic cycle associated with the Antofagasta earthquake, where the transition zone alternates between aseismic shear and seismic slip.

28.     Catita, C., K. L. Feigl,, J. Catalao., J. M. Miranda, J. M., and L. Victor-Mendez (2005), InSAR time series analysis of the 9 July 1998 Azores earthquake, International Journal of Remote Sensing 26, 2715-2729. PDF

The 9 July 1998 Mw 6.1 Pico-Faial earthquake was one of the largest events recorded in the Azores (North Atlantic) in recent years. It generated significant co-seismic deformation that was captured by a GPS network on Faial Island. On the other islands, where no such networks were available, the co-seismic surface displacement field was heretofore unknown. To measure it on Pico Island, we analysed Synthetic Aperture Radar (SAR) images using interferometry. Our dataset includes 17 images acquired by the ERS-1 and ERS-2 satellites in descending passes between June 1992 and November 2000. The interferograms computed from the available image pairs show poor correlation, particularly over the dense vegetated area of Faial Island and the flanks of Pico Volcano. However, a well-correlated fringe pattern remains over 33 months for barren parts of NW Pico Island. We analysed phase profiles across this fringe to distinguish the relative contributions of the co-seismic signal and the tropospheric noise, observing a co-seismic step of 29 +/- 10mm in range.

29.     Árnadóttir, T., S. Jónsson, F. Pollitz, W. Jiang, K. L. Feigl, E. Sturkell, and H. Geirsson (2005), Post-seismic deformation following the June 2000 earthquake sequence in the south Icelandic seismic zone, J. Geophys. Res. 110, B12308. PDF at AGU

We observe post-seismic deformation on two spatio-temporal scales following Mw=6.5 earthquakes in the south Iceland seismic zone on June 17 and 21, 2000. We see a rapidly decaying deformation transient lasting no more than 2 months and extending about 5 km away from the two mainshock ruptures. This local, month-scale transient is captured by several radar interferograms (InSAR) and is also observed at a few campaign GPS sites located near the faults. A longer-scale transient with a characteristic time scale of about a year is only detected by GPS measurements. The month-scale deformation pattern is caused by poro-elastic rebound due to post-earthquake pore-pressure changes [Jónsson et al., 2003]. In contrast, the year-scale deformation can be explained by either afterslip at 8–14 km depth or viscoelastic relaxation of the lower crust and upper mantle in response to the coseismic stress changes. Afterslip models provide a better fit to the first year of observations (2000–2001), whereas viscoelastic relaxation is the more plausible mechanism to explain the deformation observed during 2001–2004. The optimal viscoelastic models have lower crustal viscosities of  ~10e19 Pa s and upper mantle viscosity ~ 10e18 Pa s. Both types of post-seismic deformation models suggest that the areas of large co-seismic stress increase east of the June 17 and west of the June 21 ruptures, continue to be loaded by the post-seismic deformation. 

30.    Berthier, E., H. Björnsson, F. Pálsson, M. Llubes, K. L. Feigl, and F. Rémy (2006), The level of the Grímsvötn subglacial lake, Vatnajökull, Iceland, monitored with SPOT5 images, Earth and Planetary Science Letters, 243, 293–302. http://dx.doi.org/10.1016/j.rse.2004.11.005

We describe the vertical displacement field of an ice shelf floating on a subglacial lake, Grímsvötn, located underneath the Vatnajökull ice cap (Iceland). The uplift is measured using the correlation of two satellite optical SPOT5 images acquired 5 days apart with similar, non-vertical incidence angles. This is the first time correlation of optical images has been used to measure vertical displacements. Our technique is suitable for mapping short-term elevation changes of glaciers. If the surface features are preserved, vertical displacements can be measured every 25 m with an accuracy of about 0.5 m. The uplift map of Grímsvötn shows that 10.9 (±1) km2 of ice was floating between 11 and 16 August 2004. The ice shelf rose by 1.7 (±0.6) m indicating that the volume of liquid water in the lake increased by 0.018 (±0.007) km3. Our field observations show that surface melting due to meteorological processes contributed 70% of the accumulated water, hence, the rest originated from ice melted by the subglacial geothermal activity. The power required to melt 0.005 km3 (water equivalent) of basal ice in 5 days is 4000 MW. The applicability of the technique can be extended to volcanology and seismology, and even landslides or subsidence, when finer-resolution optical images become available. Applied to two pairs of images, it could solve for the 3- dimensional displacements of the Earth's surface. 

31.    Fadil, A., P. Vernant, S. McClusky, R. Reilinger, F. Gomez, D. Ben Sari, T. Mourabit, K. Feigl, and M. Barazangi (2006), Active tectonics of the western Mediterranean: Geodetic evidence for rollback of a delaminated subcontinental lithospheric slab beneath the Rif Mountains, Morocco, Geology, 34, 529-532. http://dx.doi.org/10.1130/G22291A.1

Surface deformation in Morocco, derived from five years of global positioning system (GPS) survey observations of a 22-station network, four continuously recording GPS (CGPS) stations, and four International GNSS Service (IGS) stations in Iberia, indicates roughly southward motion (~3 mm/yr) of the Rif Mountains, Morocco, relative to stable Africa. Motion of the Rif is approximately normal to the direction of Africa-Eurasia relative motion, which is predominantly strike slip, and results in shortening of the Rif and subsequent crustal extension of the adjacent Alboran Sea region. The sense, and the N-S asymmetry of the observed deformation (i.e., no evidence for north-directed shortening in the Betic Mountains north of the Alboran Sea) cannot be easily explained in terms of crustal plate interactions, suggesting that dynamic processes below the crust are driving the recent geologic evolution of the western Mediterranean. The model that best fits the observations involves delamination and southward rollback of the African lithospheric mantle under the Alboran and Rif domains. 

32.    Árnadóttir, T., W. Jiang, K. L. Feigl, H. Geirsson, and E. Sturkell (2006), Kinematic models of plate boundary deformation in southwest Iceland derived from GPS observations, J. Geophys. Res., 111, B07402. http://dx.doi.org/doi:10.1029/2005JB00390732.

We use data from GPS campaign and continuous measurements from 1992 to 2004 in SW Iceland to map the surface velocity field from the Reykjanes Peninsula to the Eastern Volcanic Zone. We divide the time series into preseismic (July 1992 to June 2000) and postseismic (June 2000 to May 2004) time intervals, and we estimate GPS station velocities for each interval as well as coseismic offsets due to the June 2000 earthquake sequence in the south Iceland seismic zone (SISZ). In addition to the plate spreading, the preseismic velocity field shows the effects of inflation at Hengill and Hekla volcanoes, whereas the postseismic velocities show deformation following the June 2000 earthquakes. We consider several kinematic models to explain the preseismic velocities. Our preferred model includes several dislocations and point sources in an elastic half-space, with left-lateral slip along the plate boundary on the Reykjanes Peninsula and below the SISZ, opening across the Reykjanes Peninsula, and the Western and Eastern rift zones. The optimal model has a locking depth of about 8 km in the central and eastern part of the Reykjanes Peninsula with a deep slip rate of about 17 mm/yr and an opening of about 9 mm/yr. This locking depth is in agreement with the thickness of the seismogenic crust on the Reykjanes Peninsula, which appears to vary between 7 and 9 km. For the SISZ, we obtain a deep slip rate of about 19 mm/yr below 16 km depth, which is considerably deeper than the earthquake hypocenter depths in the area. 

33.    Feigl, K. L., and W. Thatcher (2006), Geodetic observations of post-seismic transients in the context of the earthquake deformation cycle, Comptes Rendus - Geoscience, 338, 1012 - 1028.  PDF

Satellite geodetic techniques that can measure displacements with millimeter-level accuracy reveal transient signals in the de-formation fields produced by both moderate and large earthquakes. These post-seismic signals exhibit characteristic time scales  ranging from weeks to decades and distance scales from hundreds of meters to hundreds of kilometers. By considering them in the  context of the earthquake deformation cycle, we can test hypotheses about the processes driving them and constrain the rheology  of the lithosphere. We discuss three broad categories of mechanism: afterslip in the plane of the co-seismic rupture (analogous to a rubber eraser), fluid flow in the fault zone (analogous to a water-laden sponge), and ductile flow in a weak substrate (analogous  to a pot of honey).

34.         Trota, A., N. Houlie, P. Briole, J. L. Gaspar, F. Sigmundsson, and K. L. Feigl (2006), Deformation studies at Furnas and Sete Cidades Volcanoes (Sao Miguel Island, Azores). Velocities and further investigations, Geophysical Journal International, 166, 952-956. doi:10.1111/j.1365-246X.2006.03039.x PDF

The Azores archipelago is located near the triple junction between the Nubian (NU), North America (NA), and Eurasia (EU) plates. It is characterized by a relatively strong seismicity and active volcanism. The best estimate of the current plate velocities in the area over geologic timescales comes from the NUVEL1-NNR model (DeMets et al.) . At the geodetic timescale, plate motion models (Altamimi et al., Sella et al.) including GPS, DORIS, SLR and VLBI data currently do not yet include data from the Azores. In the framework of a research project focused on the seismotectonics and volcanotectonics of Sao Miguel island, we have analysed GPS data collected there in 1993, 1994, 1997, 2000 and 2002. Our analysis determines both the velocity in the REVEL (Sella et al.) reference frame of Sao Miguel and its internal deformation. The comparison of the 2000 and 2002 coordinates indicates that Sao Miguel accommodates moderate WNW-ESE extension at a rate slower than 5 mm/yr.

35.        Pagli, C., F. Sigmundsson, T. Árnadóttir, P. Einarsson, E. Sturkell, T. Arnadottir, W. Jiang, K. L. Feigl, H. Geirsson, and E. Sturkell (2006), Deflation of the Askja volcanic system: Constraints on the deformation source from combined inversion of satellite radar interferograms and GPS measurements, Journal of Volcanology and Geothermal Research, 152, 97-108. DOI: 10.1029/2005JB003907. PDF

The Askja central volcano in northern Iceland has been continuously subsiding at least since 1983. GPS and optical leveling tilt measurements show subsidence of at least 75 cm from 1983 to 1998 in the center of the Askja caldera, without any eruptive activity. We have performed an interferometric analysis of Synthetic Aperture Radar images (InSAR) of the area, utilizing data from the ERS satellites. We observe subsidence of the Askja caldera and its fissure swarm, up to a distance of 25 km from the volcano. We evaluate the geometry of the magma chamber at Askja, from a combined inversion of satellite radar interferograms and GPS measurements. Several models were tested, including a Mogi point source as well as an ellipsoidal source. The use of an ellipsoidal source instead of a Mogi source gives an estimate of the dimensions of the magma chamber and its deflating pressure, whereas these parameters are not independently resolved if a Mogi source is used. Two-source models were also considered in order to explain the additional subsidence observed along the Askja fissure swarm. We tested a model using two Mogi sources at different depths, a shallow ellipsoidal cavity with a deeper Mogi source, and then a shallow Mogi source with a deeper elongated ellipsoid, oriented along the fissure swarm. Results indicate that an ellipsoidal source at about 3 km depth can accommodate most of the deflation occurring in the caldera. Residual subsidence occurs along the Askja fissure swarm suggesting the existence of a deeper source of contraction. We interpret this signal in terms of subsidence of the plate boundary.

36.    W    Vigny, C., J.-B. d. Chabalier, J.-C. Ruegg, P. Huchon, K. Feigl, R. Cattin, L. Asfaw, and K. Kanbari (2004), 25 years of geodetic measurements along the Tadjoura-Asal rift system, Djibouti, East Africa, J. Geophys. Res., in press [accepted Tue, 19 Dec 2006]. Preprint PDF

Since most of Tadjoura-Asal rift system sits on dry land in the Afar depression near the triple junction between the Arabia, Somalia and Nubia plates, it is an ideal natural laboratory for studying rifting processes. We analyze these processes in light of a time series of geodetic measurements from 1978 through 2003. The surveys used triangulation (1973), trilateration (1973, 1979 and 1981 to 1986), leveling (1973, 1979, 1984-1985, and 2000), and the Global Positioning System (GPS, in 1991, 1993, 1995, 1997, 1999, 2001, and 2003). A network of about 30 GPS sites covers the Republic of Djibouti. Additional points were also measured in Yemen and Ethiopia.  Stations lying in the Danakil block have almost the same velocity as Arabian plate, indicating that opening near the southern tip of the Red Sea is almost totally accommodated in the Afar depression. Inside Djibouti, the Asal-Ghoubbet rift system accommodates 16±1 mm/yr of opening perpendicular to the rift axis, and exhibits a pronounced asymmetry with essentially null deformation on its southwestern side and significant deformation on its northeastern side. This rate, slightly higher than the large-scale Arabia-Somalia motion (13 ± 1 mm/yr), suggests transient variations associated with the relaxation processes following the Asal-Ghoubbet seismo-volcanic sequence of 1978. Inside the rift, the deformation pattern exhibits a clear two-dimensional pattern. Along the rift axis, the rate decreases to the northwest, suggesting propagation in the same direction. Perpendicular to the rift axis, the focus of the opening is clearly shifted to the northeast, relative to the topographic rift axis, in the “Petit Rift”, a rift-in-rift structure, containing most of the active faults and the seismicity. Vertical motions, measured by differential leveling, show the same asymmetric pattern, with a bulge of the northeastern shoulder. Although the inner floor of the rift is subsiding with respect to the shoulders, all sites within the rift system show uplift at rates varying from 0 to 10 mm/yr with respect to a far-field reference outside the rift.