Eric Calais

GPS detection of ionospheric perturbations following the January 17, 1994, northridge earthquake

Sources such as atmospheric or buried explosions and shallow earthquakes producing strong vertical ground displacements are known to produce pressure waves that propagate at infrasonic speeds in the atmosphere. At ionospheric altitudes low frequency acoustic waves are coupled to ionospheric gravity waves and induce variations in the ionospheric electron density. Using Global Positioning System (GPS) data recorded by the permanent GPS network operating in southern California, we computed ionospheric electron content time series for several days preceding and following the January 17, 1994, Mw=6.7 Northridge earthquake. We observe an anomalous signal beginning several minutes after the earthquake with time delays that increase with distance from the epicenter. The signal frequency and phase velocity are consistent with results from numerical models of atmospheric-ionospheric acoustic-gravity waves excited by seismic sources as well as previous electromagnetic sounding results. We believe that these perturbations are caused by the ionospheric response to the strong ground displacement associated with the Northridge earthquake.

GPS geodetic constraints on Caribbean-North America plate motion

We describe a model for Caribbean plate motion based on GPS velocities of four sites in the plate interior and two azimuths of the Swan Islands transform fault. The data are well fit by a single angular velocity, with average misfits approximately equal to the 1.5–3.0 mm yr−1 velocity uncertainties. The new model predicts Caribbean-North America motion ∼65% faster than predicted by NUVEL-1A, averaging 18–20±3 mm yr−1 (2σ) at various locations along the plate boundary. The data are best fit by a rotation pole that predicts obliquely convergent motion along the plate boundary east of Cuba, but are fit poorly by a suite of previously published models that predict strike-slip motion in this region. The data suggest an approximate upper bound of 4–6 mm yr−1 for internal deformation of the Caribbean plate, although rigorous estimates await more precise and additional velocities from sites in the plate interior.

Geodetic Measurements of Crustal Deformation in the Western Mediterranean and Europe

Geodetic measurements of crustal deformation over large areas deforming at slow rates (<5 mm/yr over more than 1000 km), such as the Western Mediterranean and Western Europe, are still a challenge because (1) these rates are close to the current resolution of the geodetic techniques, (2) inaccuracies in the reference frame implementation may be on the same order as the tectonic velocities. We present a new velocity field for Western Europe and the Western Mediterranean derived from a rigorous combination of (1) a selection of sites from the ITRF2000 solution, (2) a subset of sites from the European Permanent GPS Network solution, (3) a solution of the French national geodetic permanent GPS network (RGP), and (4) a solution of a permanent GPS network in the western Alps (REGAL). The resulting velocity field describes horizontal crustal motion at 64 sites in Western Europe with an accuracy on the order of 1 mm/yr or better. Its analysis shows that Central Europe behaves rigidly at a 0.4 mm/yr level and can therefore be used to define a stable Europe reference frame. In that reference frame, we find that most of Europe, including areas west of the Rhine graben, the Iberian peninsula, the Ligurian basin and the Corsica-Sardinian block behaves rigidly at a 0.5 mm/yr level. In a second step, we map recently published geodetic results in the reference frame previously defined. Geodetic data confirm a counterclockwise rotation of the Adriatic microplate with respect to stable Europe, that appears to control the strain pattern along its boundaries. Active deformation in the Alps, Apennines, and Dinarides is probably driven by the independent motion of the Adriatic plate rather than by the Africa-Eurasia convergence. The analysis of a global GPS solution and recently published new estimates for the African plate kinematics indicate that the Africa-Eurasia plate motion may be significantly different from the NUVEL1A values. In particular, geodetic solutions show that the convergence rate between Africa and stable Europe may be 30–60% slower than the NUVEL1A prediction and rotated 10–30 counterclockwise in the Mediterranean.

Crustal motion in Indonesia from Global Positioning System measurements

Terrestrial Reference Frame 2000. We compute poles of rotation for the Australia, Eurasia, and Pacific plates based on our analysis of the global GPS data. We find that regional tectonics is dominated by the interaction of four discrete, rotating blocks spanning significant areas of the Sunda Shelf, the South Banda arc, the Bird’s Head region of New Guinea, and East Sulawesi. The largest, the Sunda Shelf block (SSH), is estimated to be moving 6 ± 3 mm/yr SE relative to Eurasia. The South Banda block (SBB) rotates clockwise relative to both the SSH and Australia plate, resulting in 15 ± 8 mm/yr of motion across the Timor trough and 60 ± 3 mm/yr of shortening across the Flores Sea. Southern New Guinea forms part of the Australia plate from which the Bird’s Head block (BHB) moves rapidly WSW, subducting beneath the Seram trough. The East Sulawesi block rotates clockwise about a nearby axis with respect to the Sunda Shelf, thereby transferring east-west shortening between the Pacific and Eurasia plates into north-south shortening across the North Sulawesi trench. Except for the Sunda Shelf, the crustal blocks are all experiencing significant internal deformation. In this respect, crustal motion in those regions does not fit the microplate tectonics model. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 1243 Geodesy and Gravity: Space geodetic surveys; 8150 Tectonophysics: Plate boundary—general (3040); 8158 Tectonophysics: Plate motions—present and recent (3040); 9320 Information Related to Geographic Region: Asia; KEYWORDS: crustal motion, Indonesia tectonics, GPS, current plate motions, Southeast Asia

Current strain regime in the Western Alps from continuous Global Positioning System measurements, 1996–2001

Four to six years of continuous measurements at 10 permanent Global Positioning System sites in the Western Alps show horizontal residual velocities of ,2 mm/yr with respect to stable Europe; uncertainties range from 0.3 to 1.4 mm/yr. These velocities and the associated strain-rate field indicate that the central part of the range is currently dominated by east-west extension, whereas the southern part shows north-south to northwest-southeast compression. The geodetic and seismotectonic data are consistent with a model where strain is essentially controlled by the counterclockwise rotation of the Adriatic microplate with respect to Eurasia. This rotation, together with the arcuate shape of the contact between the Adriatic microplate and the Alps, induces dextral shear kinematic boundary conditions across the Western Alps, with an additional divergence component in their central part and in Switzerland, and a convergence component in their southern part.

A kinematic model for the East African Rift

[1] The kinematics of the East African Rift (EAR) is the least well-known of all major plate boundaries. Here, we show that present-day data (a GPS+DORIS geodetic solution and earthquake slip vectors) are consistent with 3.2 Myr-average spreading rates and transform-fault azimuths along the Southwest Indian Ridge and support a kinematic model that includes three subplates (Victoria, Rovuma, and Lwandle) between Nubia and Somalia. Continental rifting in the EAR appears to involve localized strain along narrow rift structures that isolate large lithospheric blocks. Citation: Stamps, D. S., E. Calais, E. Saria, C. Hartnady, J.-M. Nocquet, C. J. Ebinger, and R. M. Fernandes (2008), A kinematic model for the East African Rift, Geophys. Res. Lett., 35, L05304, doi:10.1029/2007GL032781.

The use of Global Positioning System techniques for the continuous monitoring of landslides: application to the Super-Sauze earthflow (Alpes-de-Haute-Provence, France)

Recent researches have demonstrated the applicability of using Global Positioning System (GPS) techniques to precisely determine the 3-D coordinates of moving points in the field of natural hazards. Indeed, the detailed analysis of the motion of a landslide, in particular for a near real-time warning system, requires the combination of accurate positioning in three dimensions (infracentimetric) and fine temporal resolution (hourly or less). The monitoring of landslides with the GPS is usually performed using repeated campaigns, as a complement to conventional geodetic methods. Continuous monitoring of landslides with GPS is usually not performed operationally, mostly because of the cost of such a system compared to conventional deformation monitoring techniques. In addition, if GPS measurements can reach a millimetre-level accuracy for long observation sessions (typically 24 h), their accuracy decreases with the duration of the observation sessions, because of errors introduced by variations of the satellite constellation and multipath effects at the sites. This study aims at determining the experimental accuracy of GPS measurements for the continuous monitoring of landslides with GPS. In particular, we want to calibrate the variation of the measurement accuracy as a function of the duration of the observation sessions. The study was carried out on the Super-Sauze earthflow (Southern Alps, France) which evolves in a channelized flow with surface displacements reaching a few tens of centimetres to a few metres per year. The GPS data were acquired during two campaigns in May and October 1999 (two reference stations were installed outside the flow and four “moving” stations distributed on the flow). The maximal 3-D cumulative displacement reaches 2.1 m during 3 weeks in May 1999. The accuracy for a 1-h session reaches 2.7, 2.2 and 5.0 mm for the north–south, east–west and vertical components, respectively. The detectability threshold for a significant motion and a given temporal resolution stands between 3.5 mm/24 h and 8.5 mm/h in planimetry, between 6 mm/24 h and 19.5 mm/h in altimetry. Thus, the motion of the flow is clearly detected by the GPS measurements and the results have been compared with those obtained with conventional geodetic methods (theodolite and electronic distance-meters) or with a wire extensometer device. In addition, combination of periodical topometric measurements, continuous extensometric and GPS measurements allows us to identify seasonal and episodic transient variations in the surficial velocity of the flow. The analysis of the relationships between rainfall (and snowfall), groundwater level, and displacements permits us to understand the behaviour of the flow and to determine pore water pressures (PWP) thresholds initiating an acceleration of the movement. GPS therefore appears applicable to the continuous monitoring of geophysical objects or of man-made structures with small and slow displacements (∼5 mm/day). This technique does not require direct line of sight between the “moving” sites and the reference stations. Measurements can be carried out in all weather and at night. GPS processing can be performed in near real time without loss of accuracy. The use of GPS is, however, limited by the environmental characteristics of the geophysical object (mountains, vegetation), which can constitute masks limiting the visibility of the sky and create multipaths effects.

Oblique collision in the northeastern Caribbean from GPS measurements and geological observations

Abstract Previous studies along the Andean subduction zones of South America have shown that forearc basins can develop over shallow-dipping the subduction zone dips horizontally or up to 15°, and that these shallow-dipping subduction zones can alternate with more steeply dipping (>30°) subduction zones over distances of 400–1500 km (249–932 mi). This study describes the Cenozoic structural and depositional history of the Lower Magdalena Basin (LMB)—an Oligocene to Recent forearc basin covering an area of 42,000 km2 (16,216 mi2) and overlying a zone of shallow subduction (the depth to the top of the Caribbean slab ranges from 30 km to 90 km [19 to 56 mi] beneath the LMB). Using 7000 km (4350 mi) of two-dimensional (2-D) seismic reflection lines tied to 33 wells, we describe the initial Oligocene subsidence of the forearc basin along a radial array of 70°- to 110°-striking normal faults that remained active until the early Miocene. During this period, the LMB was underfilled by 1–3 seconds two-way-time (TWT) (1500 m [4921 ft]) of shallow-marine and deep-marine facies. During middle Miocene the LMB remained underfilled with marine sediments deposited in water depths of 200–2600 m (656–8530 ft). An angular unconformity spanning the interval of 11–7 Ma marks a shortening and uplift affecting the Sinu accretionary prism west of the LMB that became emergent to form a prominent forearc high along the western edge of the LMB. The regional structure of the LMB is a broad syncline that folds all units older than early Miocene and produces an asymmetrical shape—in profile—with the western edge of the LMB (against the Sinu accretionary prism), steeper than the eastern edge of the LMB. After the late Miocene–Pliocene, the forearc high continued to elevate and separate the LMB from the outer Sinu accretionary prism. During this period, the LMB overfilled with terrigenous sediments of shallow marine facies that spilled offshore into the Caribbean Sea to form the proto-delta of the Magdalena Fan; these spilled sediments led to rapid tectonic accretion and growth of the offshore Sinu accretionary prism from 5 Ma to present. During the period of Oligocene to middle Miocene, different structural styles and subduction-related magmatic intrusions suggest that the Caribbean slab was subducting at an angle greater than 30° with a discontinuous volcanic arc. The decrease in the dip of the Caribbean slab to its modern dip angles of 4–8° occurred during the late Miocene and is interpreted as the entry of thicker Caribbean oceanic plateau crust into the subduction zone. Comparison of the segmented dip of the 400-km-long (249-mi-long) subducting Caribbean slab is consistent with the upper, 220-km-long (137-mi-long) shallow-dipping part subducting at rates of 2 cm/yr (0.78 in/yr) from 11 Ma (late middle Miocene) to Recent. We propose that this change from the steeper to shallower-dipping slab in the middle Miocene led to (1) increasing elevation of the forearc high of the Sinu prism along the eastern edge of the LMB; (2) the regional synclinal structure of the LMB in profile; and (3) the possible elevation of the entire LMB after 11 Ma as it changed from underfilled, deep-water marine environments to overfilled, shallow-water marine and fluvial environments.

Evidence for a post-3.16-Ma change in Nubia–Eurasia–North America plate motions?

We combine updated GPS velocities from the Nubian (NU),Eurasian (EU),and North American (NA) plates with 500 new 3.16-Myr-average seafloor spreading rates and nine transform fault azimuths from the northern Atlantic and Arctic basin seafloor spreading centers to estimate and test for changes in the relative motion between these plates. The numerous new seafloor spreading rates and GPS velocities improve our ability to detect recent changes in the relative motions of these plates. The angular velocity vector that best fits the EU^NA GPS velocities lies significantly north of the 3-Ma-average pole,in accord with previously published geologic evidence that the EU^NA pole has migrated northward since V3 Ma. Although we also find evidence for a significant post-3-Ma change in NU^NA motion,it is less compelling because the Nubian plate GPS velocity field is sparse and NU^NA seafloor spreading rates appear to have remained steady within the 1 mm yr 31 uncertainties if we systematically decrease the seafloor spreading rates to correct for outward displacement of seafloor spreading magnetic lineations. The NU^EU pole derived from GPS site velocities lies more than 30 angular degrees south of the tightly constrained 3-Ma-average estimate and predicts significantly slower and more oblique present-day NU^EU convergence in the Mediterranean. Both models for NU^EU motion pass a key test for their accuracy,namely,they correctly predict strike-slip motion along the well-mapped Gloria fault east of the Azores. The change to more oblique NU^EU motion may reflect increasing difficulty in maintaining margin-normal convergence within this continent^continent collision zone. 8 2003 Published by Elsevier B.V.

Relative motion between the Caribbean and North American plates and related boundary zone deformation from a decade of GPS observations

Global Positioning System (GPS) measurements in 1986, 1994, and 1995 at sites in Dominican Republic, Puerto Rico, Cuba, and Grand Turk define the velocity of the Caribbean plate relative to North America. The data show eastward motion of the Caribbean plate at a rate of 21 ± 1 mm/yr (1 standard error ) in the vicinity of southern Dominican Republic, a factor of 2 higher than the NUVEL-1A plate motion model prediction of 11 ± 3 mm/yr. Independent measurements on San Andres Island, and an Euler vector derived from these data, also suggest a rate that is much higher than the NUVEL-1A model. Available data, combined with simple elastic strain models, give the following slip rate estimates for major left-lateral faults in Hispaniola: (1) the North Hispaniola fault offshore the north coast of Hispaniola, 4 ± 3 mm/yr; (2) the Septentrional fault in northern Dominican Republic, 8 ± 3 mm/yr; and (3) the Enriquillo fault in southern Dominican Republic and Haiti, 8 ± 4 mm yr. The relatively high plate motion rate and fault slip rates suggested by our study, combined with evidence for strain accumulation and historical seismicity, imply that seismic risk in the region may be higher than previous estimates based on low plate rate/low fault slip rate models and the relatively low rate of seismicity over the last century.