. RIGIDITY OF NUBIA AND KINEMATICS OF THE EAR FROM COMBINED GPS AND DORIS SOLUTIONS: IMPLICATION TO AFREF Saria Elifuraha ([email protected]), Calais Eric, (Purdue University, IN - USA), Altamimi Zuheir (ENSG, IGN, Paris, France), Willis Pascal (IPGP, IGN, Paris, France), Farah Hussein (RCMRD Nairobi, Kenya Fernandes R.M.S. (University Beira Interior, IDL, CGUL, Covilhã, Portugal), D.Sarah Stamps (Purdue University, IN - USA), CONCLUSION. This study provides the first continent-wide position/velocity solution for Africa, expressed in ITRF2008, and is therefore a first contribution to the upcoming African Reference Frame (AFREF). Except for a few regions, the Afri- can continent remains largely under-sampled by continuous space geodetic data. Efforts are in order to augment the geo- detic infrastructure and openly share existing data sets so that the objectives of AFREF can be fully reached. In ad-dition to the geophysical implications of this work, we argue that using a static frame for territories lo-cated on stable Nubia is acceptable because of its low level of internal deformation. However, we also argue that deforming territories in northern and eastern Africa should consider using a semi-dynamic (or prefer-ably dynamic) datum in order to avoid network distor- tions that would be easily detected given the deforma-tion rates involved and the level of preci-sion currently achievable by well-equipped surveyors. The kinematic model proposed here for East Africa can serve to define such a datum. Close up on the East African Rift and the Somalian plate. A. Re-sidual velocities with respect to each plate (only showing sites whose velocity uncertainties are less than 1.5~mm/yr). Residuals are small (less than 1~mm/yr) and show no systematic trend. Grey arrow at site ABPO in Madagascar shows ve-locity with re- spect to So-malia, white arrow shows velocity with respect to Nubia. Site ABPO does not fit either plate. B = Pre-dicted ve-locities with respect to Nubia for the Somalia, Victoria, and Rovuma plates. 0 1 2 Velocity standard deviation (mm/yr) NJOR BJSA B 0 1 2 Velocity standard deviation (mm/yr) NS component EW component ARMA HNDI KILW A 2 4 6 8 10 12 14 16 18 20 Time span (years) 2 4 6 8 10 12 14 16 18 20 Time span (years) UP component 0 1 2 Residual velocity (mm/yr) Time span (years) NZG2 NJOR EBBE ELDS wrt Nubia wrt Somalia wrt Victoria wrt Rovuma 2 4 6 8 10 12 14 16 Velocity uncertainties from CATREF solution as a function of observation time span after applying 1.5 factor. The uncertainties decrease rapidly after 3 years of observation. A = The EW and NS component, B = Vertical component. Residual velocities with respect to a rigid plate model for the sites used in the defi- nition of Nubia, So- malia, Victoria, and Rovuma. Residual ve- locities range be- tween 0 and 1 mm/yr, consistent with the velocity uncertainties, except for 4 sites on the Victoria plate. Figure shows the Seismotectonic set- ting, data used and Best-fit model of the EAR. Relative mo- tions along plate or block boundaries are shown with dark gray arrows, Open arrow show the pre- dicted velocity from the model. Solid line lines indicate in- ferred plate bound- ary trace. The Rela- tive rotation poles are shown with black stars with 95% confidence error el- lipse. VP = Victoria Plate, RP = Rovuma Plate, LP = Lwandle Plate and AP = Ant- arctic Plate. INTRODUCTION: The African continent, in spite of its large extent and its on-land plate boundaries in northern and eastern Africa, still misses a continent-wide and well-defined reference frame for both surveying and geophysical applications. As a result, our understanding of the kinematic of its major plate boundaries remains limited. Here we analyzed 16 years of GPS and 17 years of DORIS data at continuously operating geodetic sites distributed in Africa to describe the present-day kinematics of the Nubian plate and constrain relative motions across the East African Rift (EAR). The resulting velocity field describes horizontal and vertical motions at more than 120 GPS and 9 DORIS sites. Velocities at sites located on stable Nubia fit a single rigid plate motion model with a weighted root mean squares residual of 0.6 mm/yr. We find no detectable residual motion within Nubia at a 95% confidence level, including in the seismically active southern Africa and Cameroon volcanic line (CVL). We confirm significant motion (~1.5 mm/yr) in Morocco with respect to Nubia, consistent with earlier findings. We propose an updated angular velocity for the diver- gence between Nubia and Somalia, which provides the kinematic boundary conditions to rifting in East Africa. We updated the present-day kine- matics of the Somalian plate and propose, for the first time, a plate motion model for the East African Rift (Victoria and Rovuma microplates) that is based on space geodetic data alone. Vertical velocities range from -2 to +2 mm/yr, close to their uncertainties, with no clear geographic pattern. This study provides the first continent-wide position/velocity solution for Africa, expressed in ITRF2008, a contribution to the upcoming African Reference Frame (AFREF). Velocity solution with respect to Nubia. For only site whose velocity uncertainties is less than 1.6~mm/yr. Stars show the Euler poles and their associated 1-sigma error ellipse for Somalia- Nubia,Victoria-Nubia, and Rovuma-Nubia. The nubian fixed frame is defined with high quality and longterm GPS-DORIS time series located on stable Nubia using 21 GPS sites GOUG, YKRO, RBAY, SUTM, ZAMB, SUTH, NIAM, SHEB, PRE1, RECT, INHB, LLN-GETJI, GMAS, BJCO, ULDI, UMTA, SBOK, TAMP, HNUS and DEAR plus one DORIS site HBKA. The WRMS for horizontal velocity fit is 0.6mm/yr GOUG YKRO ZAMB NIAM SHEB RECT INHB LLNG ETJI GMAS BJCO TAMP SEY1 MAHB REUN REUA REUB MALI MAL2 RCMN ETDD HIMO NKLG 340˚ 60˚ 20˚ 40˚ 0˚ -40˚ -20˚ 0˚ 20˚ 40˚ 5 mm/yr LP Nubian Plate RP Somalian Plate Somalia-Nubia Euler pole VP Arabian Plate Eurasian Plate 340˚ 0˚ 20˚ 40˚ 60˚ -40˚ -20˚ 0˚ 20˚ 40˚ hela asdb AFREF IGS UNAVCO TRIGNET DORIS Individual Investigators or Agencies LP Nubian Plate RP Somalian Plate VP Arabian Plate Eurasian Plate daka liba hbka arma reua mhba djia Distribution of the continuous GPS and DORIS sites used in this study. Sites are color- coded as a function of their availability. AFREF = AFrican REference Frame database; IGS = International GNSS Service data centers; UNAVCO = UNAVCO archive; TRIGNET = South Africa Mapping Agency archive. Sites labeled “indivudual investigators or agen- cies'' are generally not available online. Many other continuous GPS sites operate in Africa whose data are not made public. VP: Victoria Plate, RP: Rovuma Plate, LP: Lwandle Plate A. Distribution of continuous GPS sites in Africa used in this study as a function of obser- vation time span. Many sites are still “young”, with less than 5 years of continuous ob- servations. B. Histogram showing the cumulative number of continuous GPS sites in Africa from 1996 to 2012. Note the rapid increase since 2008. Site distribution (GPS only) as a function of velocity uncertainties (A) and residual velocity (B). Solid line shows category A sites (Stable geodetic monument), dashed line shows other sites. Overall, category A sites perform significantly better than others. 0 10 20 30 40 Number of sites 0 1 2 3 4 5 6 Residual Velocity (mm/yr) Category “A” sites Other categories B 0 20 40 60 Number of sites 0 2 4 6 Velocity uncertainties (mm/yr) Category “A” sites Other categories A 0 5 10 15 20 Number of sites Observation time span (yrs) A 0 20 40 60 80 100 120 Cummulative number of sites Year since instalation B 0 8 4 2 6 10 12 14 16 1996 2000 2004 2008 2012 We are currently using the substantial increases in the geologic, geophysical and geodetic data in Africa to im- prove upon recent kinematic models of the East African Rift (EAR; Calais et al., 2006; Stamps et al., 2008). We use a block modeling approach where observed velocities are described as the contribution of rigid block rotation and strain accumulation on locked faults (Mc Caffrey, 2009). We also use earthquake slip vector directions along the EAR structures (Delvaux et al., 2009), as well as transform fault azimuths and 2Ma average spreading rates along the Southwest Indian ridge (Horner-Johnson et al., 2007, DeMets et al., 2010). We statistically test whether the data is fit significantly better by models that split the EAR into three separate subplates (Victoria, Rovuma, Lwandle. SO RO LW SO-LW SO VI RO LW -20˚ -10˚ 0˚ 10˚ 20˚ 30˚ 40˚ 50˚ 60˚ -40˚ -30˚ -20˚ -10˚ 0˚ 10˚ 20˚ 30˚ Observed Model This work 5 mm/yr Stamps et al., 2008 Rotation poles: GPS velocities w.r.t. Nubia: NUBIA SOMALIA ANTARCTICA LWANDLE ROVUMA VICTORIA VI 5 mm/yr Nubian Plate A B 30 40 50 60 10 0 -10 -20 -30 30 40 50 60 10 0 -10 -20 -30 Nubian Plate Somalian Plate Somalian Plate LP RP VP LP RP VP 5 mm/yr Stamps et al. 2008 Calais et al. 2006 This Study ABPO 10˚ 20˚ 30˚ 40˚ 50˚ -60˚ -50˚ -40˚ -30˚ -20˚ GEODVEL 0.02 0.04 0.06 0.08 0.10 0.12 rate (deg/My) -0.03 0.00 0.03 east-west (deg) 0.02 0.04 0.06 0.08 0.10 0.12 rate (deg/My) -0.03 0.00 0.03 north-east (deg) MORVEL ITRF08 HJ07 ST08 SA12 SO12 NUBIA ANTARCTICA LWANDLE SO, this work LW, this work RO, this work SO, this work LW, this work RO, this work SO, GPS only SO, GPS only GEODVEL ITRF08 ST08 MORVEL HJ07 GEODVEL ITRF08 ST08 MORVEL HJ07 95% confidence ellipses Comparison between Somalia-Nubia angular velocity estimates. Angular velocities and 95% confidence limits in three perpendicular planes are shown: left panel = poles of rotation, bottom right panel = west-east profile, top right panel = south-north profile. HJ07 = Horner-Johnson et al., 2007; ST08 = Stamps et al., 2008; MORVEL = DeMets et al., 2010; GEOD- VEL = Argus et al., 2011; SA12 = Saria et al., submitted; SO12 = this work. Recent estimates agree well with each other, in- cluding the GPS-only GEODVEL estimate and the geological MORVEL estimate. The same holds for our own results except for an angular rate significantly lower than previous estimates, marginally consistent only with GEODVEL. We are currently investigating the cause of this discrepancy. Note that the Rovuma and Lwandle plate estimates (left panels) are signifi- cantly different from Somalia, but indistinguishable from each other. CATREF velocity in mm/yr CATS velocity in mm/yr GLOBK velocity in mm/yr GLOBK velocity in mm/yr CATREF velocity in mm/yr ASMA ULUB A B C 10 0 20 30 40 CATS velocity in mm/yr 10 0 20 30 40 10 0 20 30 40 ASMA ULUB 10 0 20 30 40 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 Spectral indices North-South -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 Spectral indices East-West 0-4 years 4-6 years 6-16 years CATS uncertainty, mm/yr GLOBK uncertainyy, mm/yr GLOBK uncertainty, mm/yr CATREF uncertainty, mm/yr ROBE A B C 2 0 4 6 8 CATS uncertainty, mm/yr 1 0 2 3 4 1 0 2 3 4 NURK MSKU CATREF velocity in mm/yr 2 0 4 6 8 ROBE NURK MSKU 1 0 2 3 4 1 0 2 3 4 Scatter of the spectral indices of the colored noise for horizontal components of the GPS time series. The black star indicates the cen- troid of the scatter. It closely matches a spectral index of 1 for both components, in- dicative of flicker noise. Comparison of GLOBK, CATREF and CATS softwares velocity field. A. CATREF velocities as a function of CATS velocities. B. GLOBK velocities as a function of CATS velocities. C. GLOBK velocities as a function of CATS velocities. Only GPS sites common to both solutions are shown. Gray circles show the NS component, open circles show the EW component. A. Comparison of GLOBK and CATS velocity uncertainties. B. Comparison of CATREF and CATS velocity un- certainties. C. Comparison of CATREF and GLOBK velocity uncertainties. CATREF uncertainties have been scaled by 1.5, as explained in the text. Only GPS sites common to both solutions are shown. 0.0 0.5 1.0 1.5 2.0 2.5 Uncertainties (mm/yr) 0 0.5 1.0 1.5 2.0 2.5 3.0 Vertical velocity (mm/yr) Vertical velocities & uncertainties (mm/yr) Time span (years) 0 1 2 3 4 6 8 10 12 14 16 18 velocity (mm/yr) uncertainty (mm/yr) A B A. Vertical velocities (open circles) and their uncertainties (black circles) as a function of measurement time span. B. Vertical velocities as a function of the corresponding uncertainty. 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