GPS Probes the Kinematics of the Vrancea Seismogenic Zone

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1 Eos,Vol. 85, No. 19,11 May 2004 VOLUME 85 NUMBER MAY 2004 PAGES EOS, TRANSACTIONS, AMERICAN GEOPHYSICAL UNION GPS Probes the Kinematics of the Vrancea Seismogenic Zone PAGES 1 8 5, In late 2001, the Surface Behavior and Dynamical Units of the Southeast Carpathian Tectonics (SUBDUCT) program was initiated by the Netherlands Research Center for Integrated Solid Earth S c i e n c e s (ISES), together with the faculty of geology and geophysics at the Uni versity of Bucharest, and the ( R o m a n i a n ) National Institute for Earth Physics. The aim of this program is to monitor, analyze, and interpret the surface motions occurring in response to active crust-lithosphere dynamics of the southeast Carpathians in Romania (Figure 1), using the Global Positioning System (GPS). For this region, observations of surface kinematics constitute a new and independent data source. In combination with other infor mation, mainly obtained by geologic and geo physical studies, surface motions may help to unravel the intriguing tectonics of the region. Particularly SUBDUCT focuses on the dynamic processes of the Vrancea high-seismicity region. The present-day tectonic activity in the Vrancea region is best characterized by a small zone of intense shallow- to intermediate-depth seismicity that is often interpreted as reflecting the late stage of intra-continental collision. Other regions with a similar intra-continental setting are Bucaramanga (Colombia-Bolivia) and HinduKush (Afghanistan).Although few in number, these peculiar regions may hold important clues to the late-stage development of active ocean-continent or continent-continent inter action in regions of relatively small spatial extent, when compared to the scale of global plate tectonics. Plate interactions on this scale may have occurred many times in the geologic past and elsewhere on Earth, but are perhaps not recognized as such in the geologic record. These tectonically complicated areas are not yet well understood. To shed light on the active geodynamic processes of the southeast Carpathians, the region is the subject of many complementary geological and geophysical studies.these range from classical potential-field investigations B Y ANDRE VAN DER HOEVEN, GUNTER SCHMITT, GEORG DINTER,VICTOR MOCANU, AND WIM SPAKMAN (for example, gravity air magnetics, and magnetotellurics),to deep seismic reflection/ refraction studies to image the crustal and upper lithosphere structure; seismic tomography experiments for deep lithosphere and mantle imaging; seismic anisotropy and attenuation measurements aiming to identify mantle flow and the type of lithosphere in which the Vrancea earthquakes are being generated; and a range of integrated geological studies (see Cloetingh et al [ ] ). None of these studies could, however, lead to estimates of the present-day tectonic surface motions that may or may not b e related to the sometimeshazardous Vrancea earthquake activity SUBDUCT aims to fill this gap in knowledge for o n e of the most puzzling regions of intra-continental deformation. Vrancea: Slab Break-off, Delamination? Lithosphere The Vrancea Zone is part of the Carpathian arc, which represents the suture zone between the east European and Moesian platforms (to the north and east, respectively) and southern European continental units (to the west/south west). These units amalgamated during the past 20 Ma after the subduction of what is thought to b e the eastern part of the Alpine Tethys, an elongated o c e a n i c basin of Triassic- 190 ometers Fig. 1. In this overview of the Romanian GPS Network, squares indicate IGS permanent stations, diamonds indicate the ISES permanent stations, circles indicate campaign points, while the num bers represent the year of first measurements at the point. EC = East Carpathians, SC = South Carpathians, EEP = East-European Platform, MP - Moesian Platform, PB = Pannonian Basin, TB = Transylvanian Basin, AM = Apuseni Mountains, SP = Scythian Platform, IF = Intramoesian Fault, TF = Trotus Fault, PCF = Paceneaga-Camena Fault, and COF = Capidava-Ovidiu Fault. Original color image appears at back of this volume.

Eos, Vol. 85, No. 19,11 May 2004 Jurassic age that stretched along the entire southern margin of the Iberian-European continent [e.g.,stampfli et al, 2 0 0 2 ].

2 Eos, Vol. 85, No. 19,11 May 2004 Jurassic age that stretched along the entire southern margin of the Iberian-European continent [e.g.,stampfli et al, ]. Mantle tomography images have led to the identification of remnants of subducted lithosphere that has accumulated at the bottom of the upper mantle beneath the Pannonian Basin (Figure 2, Wortel and Spakman [ ] ). It is conceivable that the late stage of o c e a n i c subduction (late M i o c e n e ) may b e followed by a recent phase of continental collision in the process of suturing of south European continental frag ments with the old European margins, partic ularly in the southeastern Carpathians. Here, the very active Vrancea seismic zone, the recent unusual volcanism of the inner Carpathians (2.25 Ma in the Persani Mountains, Ma in the Harghita Mountains),and vertical motions and folding in the Carpathian foreland basin east of Vrancea attest to the ongoing lithosphere activity. The nature of the geodynamic processes involved in the recent evolution of the Vrancea region is still unknown. One possible tectonic scenario invokes subduction of a normal o c e a n i c lithosphere that is presumed to have been detached from the continental lithosphere of eastern Europe (beneath the northeast Carpathians), and may b e in the process of breaking off from the Moesian platform below Vrancea [Wortel and Spakman, 2000].A s e c o n d model holds that oceanic lithosphere subduction ended s o m e time in the late Miocene, and that since then, a portion of the continental east European and/or Moesian platform lithosphere has b e e n delaminating along a horizontal interface, and is now dipping steeply down into the upper mantle (Gvirtzman [2002]).The predicted spatial patterns of the high-velocity "slab" and low-velocity asthenosphere in the upper mantle beneath the Carpathians Bending Zone differ between models, and could b e seismically determined by a combination of seismic tomography measurements of the seismic attenuation, and S-wave splitting, all of which are subjects of ongoing research. The determination of horizontal and vertical motions, fault slip, and surface deformation of the continental blocks through the SUBDUCT program adds new and independent informa tion, which may prove to b e discriminating between the different geodynamic models. The latter will require an assessment of the surface motions predicted from the distinct models proposed, which involves an integration of available seismological and geological information. Luckily the Carpathian-Pannonian system is completely above sea level, which provides the opportunity to investigate this late stage of o c e a n i c closure/continental collision by incorporating both the downgoing and the overriding plate in GPS and other field studies. GPS Research in Romania of Crustal Deformation GPS is among the most powerful space-geo detic tools to measure present-day, threedimensional surface deformation. It is used for permanent networks to provide accurate geodetic reference frames, and for continuous % MW\ I I I +1-5 % Fig. 2. This tomographic image shows P-wave velocity anomalies for the Carpathian region from the model oabijwaard and Spakman [2000]. Colors indicate seismic wave speed anomalies as percentage deviations from average mantle velocities given by the one-dimensional reference model ak!35.a vertical slice computed along the great-circle segment (red line in map) is shown; above the slice, the map provides geographical orientation. The white arrow of the compass needle points north. The horizontal axis is in degrees along the great-circle segment defining the slice. The vertical axis shows depth with tics at 100-km intervals. White dots indicate earthquakes. The dashed lines in the tomographic section indicate the 410 and 660 km discontinuities. The fast (blue) wave speed anomalies in the mantle transition zone ( km) are interpreted as subducted lithosphere remnants. The steeply dipping fast anomaly below the tic-mark of 12 is the image of the Vrancea lithosphere slab, which may extend to a depth of 350 km. In the upper 200 km, the anomaly coincides with the Vrancea seismicity zone (see Wortel and Spakman [2000] for further discussion). Figure reproducedvmodified by permission ofaaas, Washington, D.C; copyright Original color image appears at back of this volume. monitoring of long-term changes in surface motion, as well as for field campaigns using for example dense networks in local crustal motion studies. GPS observations lead to relative positions of well-defined points on the Earths surface with very high precision of the order of 2-3 mm horizontally and 6-7 mm vertically By repeating these measurements over spans of several years, time series of relative displacement can b e constructed, from which the relative motion between the points is estimated. The starting point for crustal deformation research in Romania based on s p a c e geodetic observation was made by the Central Europe Regional Geodynamic Project (CERGOP) embedded into the Central Europe Initiative (CEI). One of the major CERGOP tasks is to establish a Central European GPS Geodynamic Reference Network (CEGRN) that covers the main tectonic features of the central-eastern part of the continent.this network has b e e n installed to serve as a regional geodetic refer e n c e frame for local geodynamic research projects. Since 1995, Romania has participated in the CERGOP project with eight GPS sites evenly distributed over the entire country (Figure l).to date, most of these Romanian sites were observed by CERGOP in four cam paigns in 1995,1996,1997, and 1999, and a few of them also in the 2001 and 2003 campaigns. Within the CEGRN-campaigns, each station was usually observed for six days with 24-hr measurements and a satellite cutoff angle of 15.The status of the CERGOP project and the results of the CEGRN campaigns were regularly published in the journal series Reports on Geo desy, published by Warsaw University of Tech nology, (see, for example, nos. 3 9, 4 0, 4 5, 5 7, and 61.) In 1996, the German Research Foundation (DFG) funded a new Collaborative Research Center (CRC) 461,"Strong Earthquakes: A Chal lenge for Geosciences and Civil Engineering," at the University of Karlsruhe. It is a German contribution to the United Nations initiative "International D e c a d e of Natural Disaster Reduction (IDNDR)." The Geodetic Institute of the University of Karlsruhe (GIK) takes part in CRC 461 with a GPS monitoring project titled "Three-dimensional Plate Kinematics in Romania," for which a GPS network for campaign measure ments was installed in two stages.the CRC network currently consists of 28 sites across an area of 250 x 380 km in the eastern part of

3 Eos,Vol. 85, No. 19,11 May 2004 Romania, centered in the Vrancea region. In close cooperation with the Technical University of Civil Engineering in Bucharest and the (Romanian) National Institute for Earth's Physics (NIEP),the CRC network has been measured three times in 1997,1998, and 2000 by GIK. Each site was observed for at least 3 days, except in 1997, using an observation time of 8 hours and a cutoff angle of 5. Additional information about the CRC 461 geodetic project has been published; e.g., Dinter and Schmitt [2001], Dinter et al. [2001], and Dinter and Schmitt [ 1999]. In 1999 and 2001, the University of Savoie, France, in cooperation with the University of Bucharest and NIETf with funding of NATO program ENVIR.LG /1998, performed two measurement campaigns.twelve GPS points were measured during these campaigns. Some were newly installed, and the others were part of the networks mentioned earlier. In 1999, a new geodynamic research program, The Netherlands Research Center for Integrated Solid Earth Sciences (ISES), was set up by a consortium of Dutch Earth science groups from the Free University of Amsterdam, the University of Utrecht, and Delft University of Technology One ISESlheme focuses on the Pannonian Basin-Carpathian system and involves geophysics, geology, and geodesy approaches.the latter work constitutes our SUBDUCT project, which began in the fall of In close collaboration with the University of Bucharest, four permanent receivers were installed in the Vrancea Bending Zone, specifically to monitor vertical motions. A specially developed position monitoring system equipped with GPS receivers, choke ring antennas, and an industrial PC system were developed to work almost autonomously, while recording the GPS data. Intensive cooperation was further established in 2002 with GIK and NIEP to measure and extend the existing, non-permanent GPS network in Romania.The first SUBDUCT campaign took place in July 2002, when over 50 GPS points were measured. Each site was observed for at least 3 days for a full 24 hours using a 10 cut-off angle. Another campaign was held in August 2003, and at least one more is planned for During the 2003 campaign, a further extension to the North Dobrogean Orogen (eastern Romania, Black Sea coast) was established, and another permanent GPS station was installed in central Dobrogea (eastern Moesian Platform). The data generated during the campaigns mentioned will be processed independently by both the German and Dutch groups. In Delft, the data are processed using the Jet Propulsion Laboratory's (JPL) GIPSY-OASIS software package with a version of JPL's precise point positioning, which is able to solve for ambiguities in a network-based processing iteration. In Karlsruhe, the data are processed using the Bernese GPS software package, which is based on baseline processing. Both techniques are significantly different, and thus provide an independent assessment of the processing/data quality Fig. 3. Preliminary results of the GPS processing show a southeast-oriented motion in the Moesian Platform area and a west-oriented motion in the East European Platform. All motions shown are relative to stable Eurasia. Preliminary results of the processing of currently available data on 28 points measured at least four times show a consistent pattern of horizontal velocities, suggesting active/reactivated fault systems in the region (Figure 3).The vertical motions in campaign points need longer time series, in combination with the results of the permanent GPS stations in the area, to provide stable solutions. Integration of the GPS-derived motions with results from seismic tomography, refraction and reflection seismics, paleo-magnetics, numerical modeling of geodynamic processes, etc., will be conducted within the ISES program in the coming years. Acknowledgments Very important input to this paper and a key role in the scientific program was provided by Boudewijn Ambrosius, Michael Nutto, Laurentiu Munteanu,and Constantin Marcu.The Dutch part of this research has been entirely funded by ISES, in the form of equipment, field campaigns, and personnel.the German contribution has been entirely funded by the German Research Foundation, supported by CRC-461. We would like to thank the Best Western Hotel Balvanyos (Romania),Moldocim Bicaz S.A. (Bicaz, Romania), the Romanian Direction of Radio Telecommunications, and the Meteorological Institute of Romania for providing us the opportunity to put permanent GPS stations on their properties.the work of the University of Bucharest and NIEP was partially supported through MENER grant 215/2002 and CERES grant 1/2002 of the Romanian Ministry of Education, Research, and Youth. Furthermore, we

Eos,Vol. 85, No. 19,11 May 2004 are grateful for the tremendous efforts of students and colleagues who participated in the field campaigns; without them, this work would not b e possible.

4 Eos,Vol. 85, No. 19,11 May 2004 are grateful for the tremendous efforts of students and colleagues who participated in the field campaigns; without them, this work would not b e possible. References Bijwaard, H.,and W S p a k m a n (2000), Nonlinear global P-wave tomography by iterated linearised inversion, Geophys. J. Int., 141, Cloetingh,S. et al. (2003), Probing tectonic topography in the aftermath of continental c o n v e r g e n c e in central Europe, Eos, Trans. AGU, 84,89,93. Dinter, G. and G. Schmitt (1999),Three-dimensional plate kinematics in R o m a n i a, P r o c e e d i n g s of the EGS Symposium G4 "Geodetic and G e o d y n a m i c Programmes of the CEI",The Hague,The Nether lands, April 1999, Reports on Geodesy,Warsaw University of Technology, No. 4 ( 4 5 ). Dinter, G., and G. Schmitt (2001),Three-dimensional plate kinematics in R o m a n i a, Natural Hazards, 23, Dinter, G., M. Nutto, G. Schmitt, U. Schmidt, D. Ghitau and C.Marcu (2001),Three-dimensional deformation analysis with respect to plate kinematics in Romania, Proceedings of the EGS Symposium G9 "Geodetic and G e o d y n a m i c Programmes of the CEI" Nice, France, March , Reports on Geodesy, Warsaw University of Technology, No. 2 ( 5 7 ). Gvirtzman.Z. ( ), Partial detachment of a lithospheric root under the southeast Carpathians:Towards a better definition of the d e t a c h m e n t c o n c e p t, Geology 30, Stampfli, G. M., G. D. Borel, R. Marchant, and J. Mosar (2002),Western Alps geological constraints on western Tethyan reconstructions, in Reconstruction of the Evolution of the Alpine-Himalayan Orogen, edited by G. R o s e n b a u m and G. S. Lister, J.Virtual Explorer, vol. 8, pp Argo Profiling Floats Bring New Era of In Situ Ocean Observations PAGES 1 7 9, The Argo profiling float project will enable, for the first time, continuous global observations of the temperature, salinity, and velocity of the upper o c e a n in near-real time.this new capa bility will improve our understanding of the ocean's role in climate, as well as spawn an enormous range of valuable ocean applications. B e c a u s e over 90% of the observed increase in heat content of the air/land/sea climate system over the past 50 years occurred in the ocean [Leuitus et al., 2001 ], Argo will effectively monitor the pulse of the global heat b a l a n c e. T h e end of 2003 was marked by two significant events for Argo. In mid-november 2003, over 200 sci entists from 22 countries met at Argo's first sci ence workshop to discuss early results from the floats. Two weeks later, Argo had 1000 pro filing floats one-third of the target total delivering data. As of 7 May that total was The Argo returns to its original density and sinks to drift until the cycle is repeated. Floats are designed to make about 150 such cycles. Profiling floats were developed during the World Ocean Circulation Experiment, but despite their extensive earlier use, the technological challenges of building and maintaining the 3000-float Argo array should not b e underestimated. Over 800 deployments will b e required each year, and each float must deliver high-quality data while cycling over 200 atmospheres and through a temperature range that may approach 30 C. Argo continues to improve float performance and reliability through close collaboration between float operators and manufacturers. Wortel, M. J. R., and W S p a k m a n ( ), S u b d u c t i o n and Slab D e t a c h m e n t in the MediterraneanCarpathian Region, Science, Author 290, Information Andre van der Hoeven, Delft University of Technol ogy, T h e Netherlands; Giinter Schmitt and Georg Dinter, G e o d e t i c Institute of the University of Karl sruhe, Germany; Victor Mocanu, University of Bucharest, R o m a n i a ; and Wim Spakman, University of Utrecht, T h e Netherlands For additional information, contact Andre van der Hoeven, Faculty of Aerospace Engineering, Kluyverweg 1,2629 HS Delft,The Netherlands; a.g.a.vanderhoeven@lr.tudelft.nl Building the Array and Delivering Data The array is made up of 18 different countries' contributions that range from a single float, to the U.S. contribution, which is 50% of the global array. Argo is quite new: the first floats were launched in 1999, and in most countries, funding is still identified as supporting the pilot phase of the program.that phase needs to cover the completion of the global array and its opera tion and evaluation over a multi-year period of the order of 10 years. Funding mechanisms differ widely between counties and involve over 50 research and operational agencies. Each national program has its own priorities, but all nations subscribe to the goal of building the global array and to Argo's open data policy. Almost all Argo observations are available with gross errors corrected or flagged to anyone wanting to use them from Global Data Assembly Centers (GDACS) in Brest, France, and Monterey, California.The target is for data to b e available within approximately 24 hours of its transmission from the float.the data reach Project Argo is an international effort collecting high-quality temperature and salinity profiles from the upper 2000 m of the ice-free global o c e a n and currents from intermediate depths. The data come from battery-powered autonomous floats (Figure 1) that drift mostly at depth, where they are stabilized at a constant pressure level by being less compressible than sea water. At typically 10-day intervals, the floats pump fluid into an external bladder and rise to the surface over about 6 hours while measuring temperature and salinity. On surfacing, satellites position the floats, and receive the transmitted data. The bladder then deflates and the float B Y J. GOULD, D. ROEMMICH, S. WIJFFELS, H. FREELAND, M. IGNASZEWSKY, X. JIANPING, S. POULIQUEN,Y DESAUBIES, U. SEND, K. RADHAKRISHNAN, K.TAKEUCHI, K. KIM, M. DANCHENKOV, P SUTTON, B. KING, B. OWENS, AND S. RISER Fig. 1. An Argo float is launched from a research vessel. (Photo courtesy oflnstitut furmeereskunde/ GEOMAR, Kiel, Germany.) Original color image appears at back of this volume.

5 Eos,Vol. 85, No. 19,11 May 2004 Fig. I. In this overview of the Romanian GPS Network, squares indicate IGS permanent stations, diamonds indicate the ISES permanent stations, circles indicate campaign points, while the num bers represent the year of first measurements at the point. EC = East Carpathians, SC = South Carpathians, EEP = East-European Platform, MP = Moesian Platform, PB = Pannonian Basin, TB = Transylvanian Basin, AM = Apuseni Mountains, SP = Scythian Platform, IF = Intramoesian Fault, TF = Trotus Fault, PCF = Paceneaga-Camena Fault, and COF = Capidava-Ovidiu Fault.

Eos,Vol. 85, No. 19,11 May 2004 Fig. 2. This tomographic image shows P-wave velocity anomalies for the Carpathian region from the model o/bijwaard and Spakman [2000].

6 Eos,Vol. 85, No. 19,11 May 2004 Fig. 2. This tomographic image shows P-wave velocity anomalies for the Carpathian region from the model o/bijwaard and Spakman [2000]. Colors indicate seismic wave speed anomalies as percentage deviations from average mantle velocities given by the one-dimensional reference model akl35.a vertical slice computed along the great-circle segment (red line in map) is shown; above the slice, the map provides geographical orientation. The white arrow of the compass needle points north. The horizontal axis is in degrees along the great-circle segment defining the slice. The vertical axis shows depth with tics at 100-km intervals. White dots indicate earthquakes. The dashed lines in the tomographic section indicate the 410 and 660 km discontinuities. The fast (blue) wave speed anomalies in the mantle transition zone ( km) are interpreted as subducted lithosphere remnants. The steeply dipping fast anomaly below the tic-mark of 12 is the image of the Vrancea lithosphere slab, which may extend to a depth of 350 km. In the upper 200 km, the anomaly coincides with the Vrancea seismicity zone (see Wortel and Spakman [2000] for further discussion). Figure reproduced/modified by permission ofaaas, Washington, D.C; copyright 2000.

DEEP SEISMIC SOUNDING ACROSS THE VRANCEA REGION

International Symposium on Strong Vrancea Earthquakes and Risk Mitigation Oct. 4-6, 2007, Bucharest, Romania DEEP SEISMIC SOUNDING ACROSS THE VRANCEA REGION V. Raileanu 1, F. Hauser 2, 4, A. Bala 1, W.