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SUMMARYIn order to better assess earthquake hazards, it is vital to have a better understanding of the spatial and temporal characteristics of fault creep that occur on ruptured faults during the period following major earthquakes. Towards this end, we use new far-field GPS velocities from continuous stations (extending ∼50–70 km from the fault) and updated near-fault GPS survey observations, with high temporal and spatial density, to constrain active deformation along the Mw7.4, 1999 Izmit, Turkey Earthquake fault. We interpret and model deformation as resulting from post-seismic afterslip on the coseismic fault. In the broadest sense, our results demonstrate that logarithmically decaying post-seismic afterslip continues at a significant level 20 yr following 1999 Earthquake. Elastic models indicate substantially shallower apparent locking depths at present than prior to the 1999 Earthquake, consistent with continuing afterslip on the coseismic fault at depth. High-density, near-fault GPS observations indicate shallow creep on the upper 1–2 km of the coseismic fault, with variable rates, the highest and most clearly defined of which reach ∼12 mm yr−1 (10–15 mm yr−1, 95 per cent c.i.) near the epicentre between 2014–2016. This amounts to ∼half the long-term slip deficit rate.

SUMMARY Although the Dead Sea Transform (DST) fault system has been extensively studied in the past, little has been known about the present-day kinematics of its southernmost portion that is offshore in the Gulf of Aqaba. Here, we present a new GPS velocity field based on three surveys conducted between 2015 and 2019 at 30 campaign sites, complemented by 11 permanent stations operating near the gulf coast. Interseismic models of strain accumulation indicate a slip rate of $4.9^{+0.9}_{-0.6}~\mathrm{ mm}\,\mathrm{ yr}^{-1}$ and a locking depth of $6.8^{+3.5}_{-3.1}~\mathrm{ km}$ in the gulf’s northern region. Our results further indicate an apparent reduction of the locking depth from the inland portion of the DST towards its southern junction with the Red Sea rift. Our modelling results reveal a small systematic left-lateral residual motion that we postulate is caused by, at least in part, late post-seismic transient motion from the 1995 MW 7.2 Nuweiba earthquake. Estimates of the moment accumulation rate on the main faults in the gulf, other than the one that ruptured in 1995, suggest that they might be near the end of their current interseismic period, implying elevated seismic hazard in the gulf area.

SUMMARY The present-day tectonics of the southern Red Sea region is complicated by the presence of the overlapping Afar and southern Red Sea rifts as well as the uncertain kinematics and extent of the Danakil block in between. Here we combine up to 16 yr of GPS observations and show that the coherent rotation of the Danakil block is well described by a Danakil-Nubia Euler pole at 16.36°N, 39.96°E with a rotation rate of 2.83 deg Myr–1. The kinematic block modeling also indicates that the Danakil block is significantly smaller than previously suggested, extending only to Hanish-Zukur Islands (∼13.8°N) with the area to the south of the islands being a part of the Arabian Plate. In addition, the GPS velocity field reveals a wide inter-rifting deformation zone across the northern Danakil-Afar rift with ∼5.6 mm yr–1 of east–west opening across Gulf of Zula in Eritrea. Together the results redefine some of the plate boundaries in the region and show how the extension in the southern Red Sea gradually moves over to the Danakil-Afar rift.

Author(s): Konca, A Ozgun; Guvercin, Sezim Ezgi; Ozarpaci, Seda; Ozdemir, Alpay; Funning, Gareth J; Dogan, Ugur; Ergintav, Semih; Floyd, Michael; Karabulut, Hayrullah; Reilinger, Robert | Abstract: SUMMARY The 2017 July 20, Mw6.6 Bodrum–Kos earthquake occurred in the Gulf of Gokova in the SE Aegean, a region characterized by N–S extension in the backarc of the easternmost Hellenic Trench. The dip direction of the fault that ruptured during the earthquake has been a matter of controversy where both north- and south-dipping fault planes were used to model the coseismic slip in previous studies. Here, we use seismic (seismicity, main shock modelling, aftershock relocations and aftershock mechanisms using regional body and surface waves), geodetic (GPS, InSAR) and structural observations to estimate the location, and the dip direction of the fault that ruptured during the 2017 earthquake, and the relationship of this event to regional tectonics. We consider both dip directions and systematically search for the best-fitting locations for the north- and south-dipping fault planes. Comparing the best-fitting planes for both dip directions in terms of their misfit to the geodetic data, proximity to the hypocenter location and Coulomb stress changes at the aftershock locations, we conclude that the 2017 earthquake ruptured a north-dipping fault. We find that the earthquake occurred on a 20–25 km long, ∼E–W striking, 40° north-dipping, pure normal fault with slip primarily confined between 3 and 15 km depth, and the largest slip exceeding 2 m between depths of 4 and 10 km. The coseismic fault, not mapped previously, projects to the surface within the western Gulf, and partly serves both to widen the Gulf and separate Kos Island from the Bodrum Peninsula of SW Anatolia. The coseismic fault may be an extension of a mapped, north-dipping normal fault along the south side of the Gulf of Gokova. While all of the larger aftershocks are consistent with N–S extension, their spatially dispersed pattern attests to the high degree of crustal fracturing within the basin, due to rapid trenchward extension and anticlockwise rotation within the southeastern Aegean.

We present and interpret newly determined site motions derived from GPS observations made from 2008 through 2016 in the Republic of Georgia, which constrain the rate and locus of active shortening in the Lesser–Greater Caucasus continental collision zone. Observation sites are located along two ∼160 km-long profiles crossing the Lesser–Greater Caucasus boundary zone: one crossing the Rioni Basin in western Georgia and the other crossing further east near the longitude of Tbilisi. Convergence across the Rioni Basin Profile occurs along the southern margin of the Greater Caucasus, near the surface trace of the north-dipping Main Caucasus Thrust Fault (MCTF) system, and is consistent with strain accumulation on the fault that generated the 1991 MW6.9 Racha earthquake. In contrast, convergence along the Tbilisi Profile occurs near Tbilisi and the northern boundary of the Lesser Caucasus (near the south-dipping Lesser Caucasus Thrust Fault), approximately 50–70 km south of the MCTF, which is inactive within the resolution of geodetic observations ( ± 0.5 mm/yr ) at the location of the Tbilisi Profile. We suggest that the southward offset of convergence along strike of the range is related to the incipient collision of the Lesser–Greater Caucasus, and closing of the intervening Kura Basin, which is most advanced along this segment of the collision zone. The identification of active shortening near Tbilisi requires a reevaluation of seismic hazards in this area.

A seismic gap along the western segment of the North Anatolian Fault, in the Marmara-Izmit region, was identified before the 1999 M7.6, Izmit and M7.4 Duzce earthquakes, so the region along the coseismic fault has been monitored with geodetic techniques for decades, providing well defined pre-, co- and post-seismic deformations. Here, we report new continuous and survey GPS measurements with near-fault (~2 – 10 km to the fault) and far-fault (~50 – 70 km from the fault) stations, including 7 years (2013 – 2019) of continuous observations, and 5 near-fault campaigns (every six months between 2014 – 2016) to further investigate postseismic deformation. GPS observations were processed with the GAMIT/GLOBK (v10.7) GNSS software. We used these observations to estimate the spatial distribution of current aseismic after-slip, along the 1999 Izmit rupture. We also searched for spatiotemporal changes of shallow creep events along the surface trace. With elastic models and GPS observations, we determined a shallow creep rate that reaches a maximum around the epicenter of the 1999 Izmit earthquake of about 12.7 ± 1.2 mm/yr, consistent with published InSAR results. Creep rates decrease both east and west of the epicentral region. Moreover, we show that broad-scale postseismic effects that diminish logarithmically, continue at present. (This study is supported by TUBITAK 1001 project no: 113Y102 and 117Y278)

Geodetic measurements following the 23 October 2011, Mw = 7.2 Van (eastern Turkey) earthquake reveal that a fault splay on the footwall block of the coseismic thrust fault was reactivated and slipped aseismically for more than 1.5 years following the earthquake. Although long-lasting aseismic slip on coseismic ruptures has been documented following many large earthquakes, long-lasting, triggered slip on neighboring faults that did not rupture during the earthquake has not been reported previously. Elastic dislocation and Coulomb stress modeling indicate that the postseismic deformation can be adequately explained by shallow slip on both the coseismic and splay fault and is likely driven mostly by coseismic stress changes. Thus, the slip deficit on the shallow section of the coseismic fault indicated by interferometric synthetic aperture radar-based models has been partially filled by aseismic slip, suggesting a lower likelihood for a large earthquake on the shallow section of the Van fault than suggested by previous studies.

We use survey and continuous GPS observations during the period 1997-2010 to investigate active deformation in the Isparta Angle region of SW Anatolia, Turkey. This region, bordered by the Fethiye Burdur Fault Zone (FBFZ) in the west and the SE extension of the Aksehir Simav Fault Zone (AKSFZ) in the east, accommodates a part of the active deformation of W Turkey. Our results show that the Isparta Angle region rotates counter-clockwise (CCW) with respect to Anatolia. Both the FBFZ and the AKSFZ are predominantly transtensional boundaries that accommodate southward motion of the Isparta region with respect to Anatolia. The FBFZ has left-lateral strike-slip behaviour along its SW segment that changes to right-lateral strike-slip along its NE extension. This change in the sense of strike-slip motion is accommodated by extension on a NW-SE striking normal fault system that is associated with the Menderes Graben system. Transtensional fault systems along the boundaries of the Isparta Angle with Anatolia are inconsistent with extrusion models for present-day southward motion. An increase in motion rates towards the Hellenic and Cyprus arc subduction systems, and CCW of the Isparta region, supports dynamic models involving active rollback of the subducting African Plate, toroidal mantle flow around the eastern edge of the subducting African slab near the Hellenic arc/Florence Rise junction, and/or gravitational collapse of the overriding Anatolia Plate.

Territory of Azerbaijan is located within the central part of the Mediterranean tectonic belt seismicity of which is caused by intensive geodynamic interaction of the Eurasian and Arabian lithospheric plates (Khain 2000).

The potential for large, shallow earthquakes and their associated seismic hazard in the eastern Caucasus, an area of dense population and sensitive industrial infrastructure, remains speculative based on historical precedent and current geologic and seismologic observations. Here we present updated and expanded results from a GPS network between the northern edge of the Lesser Caucasus and Greater Caucasus, providing geodetic constraints to the problem. A significant strain rate is observed in a profile over a distance of about 150 km across the Kura Basin. We attribute this to inter-seismic strain accumulation on buried fault structures and present simple elastic dislocation models for their plausible geometry and slip rate based on the known geology, seismicity and the GPS velocities. Due to the close proximity of the strain anomaly to Baku, further observations are needed to determine whether observed contraction is due to inter-seismically locked faults and, if so, implications for the seismic hazard in the region.

International audience; We present the results of dynamic modeling of the western Mediterranean that accounts for observed global positioning system (GPS) surface deformation of the Alboran Sea and surrounding Rif and Betic Mountains as the result of the combined effects of relative motion of the Eurasian and Nubian plates, low strength in the Alboran Sea region and sub-lithospheric processes occurring beneath the External Rif domain. Assuming that the lithosphere behaves elastically over the short time period of the GPS observations, an elastic plate model is considered in our study, including an area of weak lithosphere (factor of 10) centered on the Alboran Sea and in which lateral boundary conditions consist of the Nubia-Eurasia oblique convergence. Sub-crustal processes are modeled by application of a horizontal traction on a small area (patch) at the base of the elastic plate. Our modeling studies demonstrate the need for sub-crustal or sub-lithospheric, southwestward-directed forcing to account for observed southwestward motion of the Rif and Betic domains. Based on the location, orientation, and small area of the traction patch, we hypothesize that forcing is associated with delamination and rollback of the subducted African continental lithospheric mantle beneath the External Rif zone, due to the pull of the oceanic part of the Western Mediterranean slab, a dynamic process that may be similar to that where the over-riding plate is driven toward the subduction zone during slab rollback.

SUMMARY New Global Positioning System (GPS) measurements in NW Syria provide the first direct observations of near-field deformation associated with the northern Dead Sea fault system (DSFS) and demonstrate that the kinematics of the northern section of this transform plate boundary between the Arabian and Sinai plates deviate significantly from plate model predictions. Velocity estimates based on GPS survey campaigns in 2000, 2007 and 2008, demonstrate left-lateral shear along the northern DSFS with 1σ uncertainties less than 0.7 mm yr−1. These velocities are consistent with an elastic dislocation model with a slip rate of 1.8–3.3 mm yr−1 and a locking depth of 5–16 km. This geodetically determined slip rate is about half of that reported farther south along the central section (Lebanese restraining bend) and the southern section (Jordan Valley and Wadi Araba) of the transform and consequently requires some deformation to occur away from the transform along other geological structures. The factor of two difference in slip rates along the transform is also consistent with differing estimates of total fault slip that have occurred since the mid Miocene: 20–25 km along the northern DSFS (in NW Syria) versus about 45 km along the southern DSFS segment. Some of the strain deficit may be accommodated by north–south shortening within the southwestern segment of the Palmyride fold belt of central Syria. Additionally, a distinct change in velocity occurs within the Sinai plate itself. These new GPS measurements, when viewed alongside the palaeoseismic record and the modest level of present-day seismicity, suggest that the reported estimates of recurrence time of large earthquakes (M > 7) along the northern section of the DSFS may be underestimated owing to temporal clustering of such large historical earthquakes. Hence, a revised estimate of the earthquake hazard may be needed for NW Syria.

WEGENER is originally the acronym for Working Group of European Geoscientists for the Establishment of Networks for Earth-science Research. It was founded in March 1981 in response to an appeal delivered at the Journées Luxembourgeoises de Geodynamique in December 1980 to respond with a coordinated European proposal to a NASA Announcement of Opportunity inviting participation in the Crustal Dynamics and Earthquake Research Program. WEGENER, during the past 32 years, has always kept a close contact with the Agencies and Institutions responsible for the development and maintenance of the global space geodetic networks with the aim to make them aware of the scientific needs and outcomes of the project which might have an influence on the general science policy trends. WEGENER was serving as Inter-commission Project 3.2, between Commission 1 and Commission 3, of the International Association of Geodesy (IAG) until 2012. Since then, WEGENER project has became the Sub-commission 3.5 of IAG commission 3, namely Tectonics and Earthquake Geodesy. In this study, we briefly review the accomplishments of WEGENER as originally conceived and outline and justify the new focus of the WEGENER consortium. The remarkable and rapid evolution of the present state of global geodetic monitoring in regard to the precision of positioning capabilities (and hence deformation) and global coverage, the development of InSAR for monitoring strain with unprecedented spatial resolution, and continuing and planned data from highly precise satellite gravity and altimetry missions, encourage us to shift principal attention from mainly monitoring capabilities by a combination of space and terrestrial geodetic techniques to applying existing observational methodologies to the critical geophysical phenomena that threaten our planet and society. Our new focus includes developing an improved physical basis to mitigate earthquake, tsunami, and volcanic risks, and the effects of natural and anthropogenic climate change (sea level, ice degradation). In addition, expanded applications of space geodesy to atmospheric studies will remain a major focus with emphasis on ionospheric and tropospheric monitoring to support forecasting extreme events. Towards these ends, we will encourage and foster interdisciplinary, integrated initiatives to develop a range of case studies for these critical problems. Geological studies are needed to extend geodetic deformation studies to geologic time scales, and new modeling approaches will facilitate full exploitation of expanding geodetic databases. In light of this new focus, the WEGENER acronym now represents, “World Earthquake GEodesy Network for Environmental Hazard Research”.

GPS survey sites in the Sinai Peninsula show northerly motion relative to Africa (Nubia) at 1.4F0.8 mm/yr north and 0.4F0.8 mm/yr west. Continuous IGS GPS sites in Israel, west of the Dead Sea fault show a similar northerly sense of motion relative to Nubia (2.4F0.6 mm/yr north and 0.04F0.7 mm/yr east), suggesting that the entire Sinai Block south of Lebanon is characterized by northward translation relative to the Nubian plate. We develop an elastic block model constrained by the GPS results that is consistent with the regional tectonics and allows us to estimate slip rates for Sinai bounding faults, including the Gulf of Aqaba–southern Dead Sea fault system (~4.4F0.3 mm/yr, left lateral), the Gulf of Suez (1.9F0.3 mm/yr left lateral, and 1.5F0.4 mm/yr extension), and the Cyprus Arc (predominantly convergence at 8.9F0.4 mm/yr along the western segment, and ~6.0F0.4 mm/yr left lateral, strike slip along the eastern segment). These observations imply that the Sinai Peninsula and Levant region comprise a separate sub-plate sandwiched between the Arabian and Nubian plates. D 2005 Elsevier B.V. All rights reserved.

We use geodetic, plate tectonic, and geologic observations to quantitatively reconstruct the geologic evolution of the Red Sea and Gulf of Aden since separation of Arabia from Africa in the Late Oligocene. Rifting initiated at 22 ± 3 Ma roughly simultaneously along the full strike of the proto-Red Sea and Gulf of Aden. Rifting began along pre-existing zones of weakness associated with a Pan-African Precambrian collisional suture shortly after the Afro-Arabia Plate was weakened by impingement of the African hot spot (~30 Ma). The initial phase of continental rifting followed a roughly linear trend from the Gulf of Suez in the north, to the Bab-al-Mandab in the south where the Afar Triple Junction (junction of Red Sea, Gulf of Aden, and East African rifts) was located at that time. The initial rate of extension across the rift was roughly half the present-day rate. At 11 ± 2 Ma, the rate of rifting doubled to the present-day rate (24 ± 1 mm/year in the south [~12°N] and 7 ± 1 mm/year in the north [~27°N]) and the configuration of rifting changed in both the northern and southern Red Sea. This time corresponds to the initiation of ocean spreading (i.e., complete severing of the continental lithosphere and intrusion of rift basalts) along the full extent of the Gulf of Aden. The changes in the S Red Sea involved the propagation of the Afar Triple Junction westward to its present location (~11.5°N, 42°E), the transfer of rifting from the S Red Sea (Bab-al-Mandab) to the more N–S-oriented Danakil Depression, and accompanying CCW rotation of the Danakil Block with respect to Africa. In the northern Red Sea, rifting transferred from the Gulf of Suez to the more N–S-oriented Gulf of Aqaba/Dead Sea fault system. The rate of rifting has not changed significantly since that time (i.e., 11 ± 2 Ma). The initiation of rifting at 22 ± 3 Ma corresponds temporally with slowing of Africa–Eurasia convergence by a factor of ~2 and the changes at 11 ± 2 Ma with a second phase of slowing of Africa–Eurasia convergence, while Arabia–Eurasia convergence has remained roughly unchanged since >30 Ma. These observations are consistent with simple models where changes in Africa–Arabia–Eurasia relative plate motions are the fundamental cause of post-Oligocene Middle East and Mediterranean tectonics. Based on the simultaneity between full ocean spreading along the Gulf of Aden and a doubling of the extension rate across the Red Sea, and the change to more N–S-oriented rifting in both the northern and southern Red Sea, we hypothesize that slowing of Africa–Eurasia convergence resulted from a decrease in slab pull on the African Plate across the evolving AR-AF plate boundary.

The 16 October 1999 ( M w 7.1) Hector Mine earthquake was the largest earthquake in California since the 1992 ( M w 7.3) Landers event. The Hector Mine earthquake occurred in the eastern Mojave Desert, where the density of permanent Global Positioning System (GPS) stations is relatively low. Since the earthquake, groups from the United States Geological Survey, University of Southern California, University of California, Los Angeles, University of California, San Diego, and Massachusetts Institute of Technology have made postseismic survey-mode observations to increase the spatial coverage of deformation measurements. A total of 55 sites were surveyed, with markers from a few meters to 100 km from the surface rupture. We present velocity estimates for the 32 sites that had enough repeated observations between 17 October 1999 and 26 March 2000 to provide reliable results; these survey-mode data complement the temporal and spatial coverage provided by newly installed Southern California Integrated Geodetic Network permanent GPS stations and future Interferometric Synthetic Aperture Radar postseismic results. We then use the postseismic velocity estimates to compute a simple afterslip model. Results of inversions show that the observed velocities are consistent with deep afterslip occuring underneath the coseismic rupture area.

We describe the collection and processing of Global Positioning System (GPS) data from 77 locations around the Hector Mine earthquake, which we use to estimate coseismic displacements related to this shock. The existence of pre-event GPS data, some collected to monitor postseismic displacements from the 1992 Landers earthquake and some to establish survey control in the meizoseismal area, provided a relatively dense coverage close to the rupture zone. The data available were collected mostly within the 2 years prior to the 1999 earthquake; we reobserved many points within a few days after the shock, and all within 6 months after. We include corrections for interseismic motion to provide the best value possible for coseismic motion caused by this earthquake. The displacements in general display the pattern expected for a strike-slip fault, though a few show significant vertical motion. The maximum horizontal displacement observed was 2 m; one station between fault ruptures showed little horizontal motion, but significant uplift.

We have developed dynamic finite-element models of Izmit earthquake postseismic deformation to evaluate whether this deformation is better explained by afterslip (via either velocity-strengthening frictional slip or linear viscous creep) or by distributed linear viscoelastic relaxation of the lower crust. We find that velocity-strengthening frictional afterslip driven by coseismic shear stress loading can reproduce time-dependent Global Positioning System data better than either linear viscous creep on a vertical shear zone below the rupture or lower crustal viscoelastic relaxation. Our best frictional afterslip model fits the main features of postseismic slip inversions, in particular, high slip patches at (and below) the hypocenter and on the western Karadere segment, and limited afterslip west of the Hersek Delta (Burgmann et al., 2002). The model requires a weakly velocity-strengthening fault, that is, either low effective normal stress in the slipping regions or a smaller value for the parameter describing rate-dependence of friction ( a - b ) than is indicated by laboratory experiments. Our best afterslip model suggests that the Coulomb stress at the Duzce hypocenter increased by 0.14 MPa (1.4 bars) during the Izmit earthquake (assuming right-lateral slip on a surface dipping 50° to the north), and by another 0.1 MPa during the 87 days between the Izmit and Duzce earthquakes. In the Marmara Sea region (within about 160 km of the Izmit earthquake rupture), this model indicates that the Coulomb stresses increased by 15%-25% of the coseismic amount during the first 300 days after the earthquake. Three hundred days after the earthquake, postseismic contributions to Coulomb stressing rate on the Maramara region faults had fallen to values equal to or less than the inferred secular stress accumulation rate. Our estimates of postseismic Coulomb stress are highly model dependent: in the Marmara region, the linear viscous shear zone and viscoelastic lower crust models predict greater postseismic Coulomb stresses than the frictional afterslip model. Near-field stress and fault-zone rheology estimates are sensitive to the Earth9s elastic structure. When a layered elastic structure is incorporated in our model, it yields a Coulomb stress of 0.24 MPa at the Duzce hypocenter, significantly more than the 0.14 MPa estimated from the uniform elastic model. Because of the higher near-field coseismic stresses, the layered elastic model requires a higher value of velocity-strengthening parameter ( A - B ) ([ a - b ] times effective normal stress r ′) to produce comparable postseismic slip. ( A - B ) is estimated at 0.4 and 0.2 MPa, respectively, for the layered and uniform elastic models. These results highlight the importance of understanding the Earth9s elastic structure and the mechanism for postseismic deformation if we wish to accurately model coseismic and postseismic crustal stresses.

To determine the present day kinematics of the major tectonic elements in Bulgaria, part of the northern extent of the Aegean extensional system, we established and surveyed a regional GPS network in 1996, 1997 and 1998. Based on our estimates of the velocities of 17 stations surveyed at least twice, we identify two areas of significant deformation. Between the central Danubian Plain and a 200-km-wide area adjacent to Black Sea, we find 3±1 mm/year of shortening, consistent with compressional focal mechanisms for the Gorna Oriahovitsa fault zone and a NW-trending flexure between higher topography in northeast Bulgaria and the area to the west. South of the Danubian Plain, there is a broad zone of N-S to NNE-SSW extension encompassing the E-W trending Sub-Balkan graben system and Thracian basin of central Bulgaria and with a total differential motion of 3–5 mm/year. The zone of extension appears to continue into the Sofia graben and through a topographically low part of the Stara Planina mountains. This zone of extension marks the northern boundary of the Aegean extensional region, but its western continuation remains unclear. Velocities of 3–4 mm/year to the south and 3 mm/year to the ESE in south central Bulgaria express a clear transition to the Aegean extensional region farther south.

We use the combined GPS velocity field of the eastern Mediterranean for the period 1988 to 1996 to determine crustal deformation strain rates in a region comprising the Hellenic arc, the Aegean Sea, and western Anatolia. We interpret the velocity field and determine the strain rate tensor by the spatial derivatives of the collocated motion vectors. The region following the line Marmara Sea, North Aegean Trough, northern central Greece, and the central Ionian islands is associated with strong right-lateral shear motion, with maximum shear strain rates of 180 nano-strain/a (180×10−9/a). In the central Aegean Sea, N–S-oriented extensional processes prevail, reaching 100 nano-strain/a. The southern Aegean is characterized by relatively small strain rates. Maximum extensional components of the strain rate tensor, reaching 150 nano-strain/a in a N–S direction, are found in central Greece. The Hellenic arc is associated with moderate arc-parallel extension and strong compression perpendicular to it. Projections of the strain rates parallel to the major fault zones reveal that the northern Aegean is governed by the westward continuation of the North Anatolian Fault Zone which is associated with strong dextral shearing (maximum 220 nano-strain/a), accompanied by numerous large earthquakes in this century.

During the past century, a series of predominantly westward migrating M>7 earthquakes broke an similar to 1000km section of the North Anatolian Fault (NAF). The only major remaining seismic gap along the fault is under the Sea of Marmara (Main Marmara Fault (MMF)). We use 20years of GPS observations to estimate strain accumulation on fault segments in the Marmara Sea seismic gap. We report the first direct observations of strain accumulation on the Princes' Islands segment of the MMF, constraining the slip deficit rate to 10-15mm/yr. In contrast, the central segment of the MMF that was thought to be the most likely location for the anticipated gap-filling earthquakes shows no evidence of strain accumulation, suggesting that fault motion is accommodated by fault creep. We conclude that the Princes' Islands segment is most likely to generate the next M>7 earthquake along the Sea of Marmara segment of the NAF.

We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1988–1994 at 54 sites extending east-west from the Caucasus mountains of southern Russia, Georgia, and Armenia to the Aegean coast of Turkey and north-south from the southern edge of the Eurasian plate (Pontus block) to the northern edge of the Arabian platform. Viewed from a Eurasia-fixed reference frame, sites on the northern Arabian platform move N38±13°W at 20±3 mm/yr, roughly consistent with the velocity implied by NUVEL 1A circuit closure (N23±7°W at 24±2 mm/yr). The motion of Arabia appears to be transferred directly to the region of Turkey north of the suture. However, eastern Turkey is characterized by distributed deformation while central/western Turkey is characterized by coherent plate motion involving westward displacement and counterclockwise rotation of the Anatolian plate. Internal deformation within the central part of the Anatolian plate is less than 2 mm/yr. The Anatolian plate is decoupled from Eurasia along the right-lateral, strike-slip North Anatolian fault (NAF). This different response in eastern and western Turkey to the collision of Arabia may result from the different boundary conditions, the Hellenic arc forming a “free” boundary to the west and the Asian continent and oceanic lithosphere of the Black and Caspian Seas forming a resistant boundary to the north and east. We derive a best fitting Euler vector for Anatolia-Eurasia motion of 29.2±0.8°N, 32.9±0.4°, 1.3±0.1°/m.y. The mapped surface trace of the NAF corresponds well to a small circle about this pole. The new Euler vector implies an upper bound for NAF slip rate of 30±2 mm/yr (i.e., assuming all relative motion is accommodated along the NAF). Using the NUVEL 1A Euler vector for Arabia-Eurasia and the GPS Euler vector for Anatolia-Eurasia, we determine an Arabia-Anatolia Euler vector of 31±2°N, 45±2°E, 0.9±0.1 °/m.y. and an upper bound on the East Anatolian fault slip rate of 15±3 mm/yr. The Aegean Trough region of western Turkey deviates significantly from coherent plate rotation. In addition to rotating with Anatolia, this region shows roughly N-S extension at a rate of 14±5 mm/yr. Taken together with satellite laser ranging results along the Hellenic arc, the contemporary pattern of deformation indicates increasing motions toward the arc, suggesting that the westward displacement and counterclockwise rotation of Anatolia is driven both by “pushing” from the Arabian plate and by “pulling” or basal drag associated with the foundering African plate along the Hellenic subduction zone.

THE Tien Shan—a high, seismically active intracontinental mountain belt, 1,000–2,000 km north of the Himalaya—has grown as a result of India's collision with Asia1. The crustal shortening (~ 200 ± 50 km; refs 2, 3) and thickening that gave rise to the Tien Shan accommodates only a small fraction of India's total penetration into Asia (2,000—3,000km), and the temporal relationship of deformation in this belt to the India–Asia collision remains unclear. Here we report geodetic measurements of the Tien Shan, using the Global Positioning System (GPS), that indicate that the current crustal shortening rate is nearly half of India's convergence rate with Eurasia in this area4. We infer a total shortening rate for the Tien Shan of ~20 mm yr−1, which is approximately twice that inferred previously from the extrapolation of slip rates in the Holocene3 and earthquake-induced displacements during this century5, suggesting that the rate of mountain building in this region has accelerated several-fold since the onset of collision ~50–55 Myr ago6,7. If, as we argue, the current shortening rate can be extrapolated to geological timescales, then our results suggest that most of the Tien Shan has been constructed during the past 10 Myr, perhaps in response to an increased horizontal force following an abrupt rise of the Tibetan plateau8,9.

We use Global Positioning System (GPS) estimates of horizontal site velocity to constrain slip rates on faults comprising the Pacific-North America plate boundary in southern California and northern Mexico. We enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary. We estimate a Pacific-North America relative plate motion rate of 49 ± 3 mm/yr (one standard deviation), consistent with NUVEL-1A estimates. We are able to resolve robust slip rate estimates for the southernmost San Andreas, San Jacinto, and Elsinore faults (26 ± 2, 9 ± 2, and 6 ± 2 mm/yr, respectively) and for the Imperial and Cerro Prieto faults (35 ± 2 and 42 ± 1 mm/yr, respectively), accounting for about 86% of the total plate motion. The remaining 14% appears to be accommodated to the west of these fault systems, probably via slip along the San Clemente fault and/or the San Miguel, Vallecitos, Rose Canyon, and Newport-Inglewood fault systems. These results are highly consistent with paleoseismic estimates for slip rates implying that off-fault strain accumulation within the deforming zone of the plate boundary is largely elastic. We estimate that the seismically quiescent, southernmost San Andreas fault has incurred about 8.2 m of slip deficit over the last few hundred years, presumably to be recovered during a future large earthquake.

International audience; The North Anatolian Fault Zone (NAFZ) is the major transform system that accommodates thewestward movement of the relatively rigid Anatolian block with respect to Eurasia. Mitigating the hazard associated with devastating earthquakes requires understanding how the NAFZ accumulates and releases the potential energy of elastic deformation both in space and in time. In this study, we focus on the central bend of the NAFZ where the strike of the North Anatolian Fault (NAF) changes from N75◦to N105◦within less than 100 km, and where a secondary fault system veers southwards within the interior of Anatolia. We present interseismic velocity fields obtained from a Persistent-Scatterers (PS) Interferometric radar analysis of ERS and Envisat radar archives. Despite the high vegetation cover, the spatial density of measurementsis high (∼10 PS/km2 in average). Interseismic velocities presented below indicate a velocitychange of∼6–8 mm/yr along the satellite line-of-sight (LOS) mainly centred on the NAF surface trace, and are in good agreement with the GPS velocity field published previously. The observed deformation is accommodated within a zone of ∼20 to 30 km width, in this area where no surface creep has been reported, contrary to the Ismetpasa segment located ∼30 km to the west of this study zone. Although less conspicuous, ∼2–3 mm/yr (∼1 mm/yra long the LOS) of the total deformation seems to be localized along the Lacin Fault. Theoverall agreement with horizontal GPS measurements suggests that the vertical component of the ground deformation is minor. This is confirmed, over the western part of our study zone, by the 3-D estimation of the ground deformation from the combination of the GPS-and the PS-mean velocity fields. However, a specific pattern of the PS velocity fields suggests that an area, enclosed between two faults with roughly south–north orientation, experiences uplift. The PS analyses of radar time-series both prior and posterior to the Izmit and Dûzce earthquakes indicate that these events did not induce detectable velocity changes in this central bend. The only temporal changes we identify are due to a moderate local earthquake (Mw5.7,1996 August 14) whose precise location and coseismic deformation are determined here. Finally, we propose a model of slip-rate distribution at depth along the NAF from the joint inversion of the GPS and PS mean velocity fields. This model suggests a long-term slip-rate of ∼20 mm/yr for a rather uniform locking depth in the range of 15–20 km. However, thelocking depth increases to ∼25–30 km in the section comprised between longitudes E34◦20’and E34◦50’. This lateral evolution is in general agreement with the earthquake distributionat depth from three different catalogues.

Coseismic surface deformation associated with the M(sub w) 6.1, April 23, 1992, Joshua Tree earthquake is well represented by estimates of geodetic monument displacements at 20 locations independently derived from Global Positioning System and trilateration measurements. The rms signal to noise ratio for these inferred displacements is 1.8 with near-fault displacement estimates exceeding 40 mm. In order to determine the long-wavelength distribution of slip over the plane of rupture, a Tikhonov regularization operator is applied to these estimates which minimizes stress variability subject to purely right-lateral slip and zero surface slip constraints. The resulting slip distribution yields a geodetic moment estimate of 1.7 x 10(exp 18) N m with corresponding maximum slip around 0.8 m and compares well with independent and complementary information including seismic moment and source time function estimates and main shock and aftershock locations. From empirical Green's functions analyses, a rupture duration of 5 s is obtained which implies a rupture radius of 6-8 km. Most of the inferred slip lies to the north of the hypocenter, consistent with northward rupture propagation. Stress drop estimates are in the range of 2-4 MPa. In addition, predicted Coulomb stress increases correlate remarkably well with the distribution of aftershock hypocenters; most of the aftershocks occur in areas for which the mainshock rupture produced stress increases larger than about 0.1 MPa. In contrast, predicted stress changes are near zero at the hypocenter of the M(sub w) 7.3, June 28, 1992, Landers earthquake which nucleated about 20 km beyond the northernmost edge of the Joshua Tree rupture. Based on aftershock migrations and the predicted static stress field, we speculate that redistribution of Joshua Tree-induced stress perturbations played a role in the spatio-temporal development of the earth sequence culminating in the Landers event.

International audience; Understanding the behavior of active faults in response to tectonic stresses is a critical issue in earthquake physics and seismic hazard assessments (Carpenter et al., 2011). While most active faults are unstable and move abruptly, releasing in seconds to a few minutes the strain accumulated around them for decades to centuries, some faults slip continuously, storing little or no strain and are therefore considered to be unlikely to produce signifi cant earthquakes as long as this behavior persists (Bürgmann et al., 2000). Although aseismic fault creep was fi rst reported over half a century ago along some major plate boundary faults, including the San Andreas fault in California (United States) and the North Anatolian fault in Turkey, the origin and physical processes of fault creep remain subjects of debate (Schleicher et al., 2010). Two primary, complementary, and competing mechanisms have been proposed to explain the mechanics of aseismic fault creep: (1) the presence of weak mineral phases such as clay and phyllosilicates in the bulk composition of fault zones (Carpenter et al., 2011) or the formation of highly localized, phyllosilicate-rich interconnected networks of shear planes or foliations (Collettini et al., 2009) via chemical and mechanical processes, and/ or (2) trapped fl uid overpressures within fault zones (Byerlee, 1990). Although recent observations (Lockner et al., 2011) support the idea that aseismic creep is controlled by the presence of weak rocks in the fault zone rather than by high fl uid pressures, analysis of rock samples from active (Holdsworth et al., 2011) and exhumed (Warr and Cox, 2001) fault zones suggests that transient fl uid overpressures do occur in many fault zones and are most likely generated during coseismic faulting due to shear heating, low per-meability, high water content, and low strength of clay-rich shear zones. Whether due to mineralogy and/or fl uid pressure affects, how or when stable aseismic surface creep initiates on active faults remains uncertain. The fact that creeping segments of the North Anatolian fault at Ismetpasa and the San Andreas fault in the San Francisco Bay area also rupture coseismically suggests to us that fault creep may be initiated as postseismic after-slip (Çakir et al., 2005; Schmidt et al., 2005), a mechanism that could not be confi rmed previously due to the lack of pre-and post-earthquake observations on these fault sections. In this study we use space geodetic data and recent fi eld observations along the A.D. 1999 Izmit earthquake rupture (Fig. 1) to demonstrate continuing afterslip that appears to grade into steady-state creep on a >60-km-long segment of the Izmit coseismic fault, supporting seismic slip as one of the possible mechanisms for initiating steady-state creep on mature strike-slip faults. THE 1999 IZMIT EARTHQUAKE The M w 7.4 Izmit earthquake on 17 August 1999 occurred on the North Anatolian fault

[1] GPS observations along three profiles across the Ethiopian Rift and Afar triple junction record differences in the length scale over which extension is accommodated. In the Afar region, where the mantle lithosphere is nearly or entirely absent, measurable extension occurs over ∼175 km; in the northern Ethiopian Rift, where the mantle lithosphere is anomalously thin and hot, extensional strain occurs over ∼85 km, extending beyond the structural rift valley; in the southern Ethiopian Rift, where the mantle lithosphere approaches standard continental thickness, extensional strain occurs over

International audience; We use velocities from 65 continuous stations and 31 survey-mode GPS sites as well as kinematic modeling to investigate present day deformation along the Africa-Iberia plate boundary zone in the western Mediterranean region. The GPS velocity field shows southwestward motion of the central part of the Rif Mountains in northern Morocco with respect to Africa varying between 3.5 and 4.0 mm/yr, consistent with prior published results. Stations in the southwestern part of the Betic Mountains of southern Spain move west-southwest with respect to Eurasia (similar to 2-3 mm/yr). The western component of Betics motion is consistent with partial transfer of Nubia-Eurasia plate motion into the southern Betics. The southward component of Betics motion with respect to Iberia is kinematically consistent with south to southwest motion of the Rif Mountains with respect to Africa. We use block modeling, constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures. Our preferred plate boundary geometry includes one block between Iberia and Africa including the SW Betics, Alboran Sea, and central Rif. This geometry provides a good fit to the observed motions, suggesting a wide transpressive boundary in the westernmost Mediterranean, with deformation mainly accommodated by the Gloria-Azores fault system to the West and the Rif-Tell lineament to the East. Block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles: (i) extension along the NE-SW trending Trans-Alboran shear zone, (ii) dextral strike-slip in the Betics corresponding to a well defined E-W seismic lineament, and (iii) right lateral strike-slip motion extending West to the Azores and right-lateral motion with compression extending East along the Algerian Tell. We interpret differential motion in the Rif-Alboran-Betic system to be driven both by surface processes related the Africa-Eurasia oblique convergence and sub-crustal dynamic processes associated with the long history of subduction of the Neotethys ocean lithosphere. The dextral slip identified in the Betic Mountains in Southern Spain may be related to the offshore fault that produced the Great 1755 Lisbon Earthquake, and as such may represent a significant seismic hazard for the West Mediterranean region.

[1] Measurements at ∼400 campaign-style GPS points and another 14 continuously recording stations in central Asia define variations in their velocities both along and across the Kyrgyz and neighboring parts of Tien Shan. They show that at the longitude of Kyrgyzstan the Tarim Basin converges with Eurasia at 20 ± 2 mm/yr, nearly two thirds of the total convergence rate between India and Eurasia at this longitude. This high rate suggests that the Tien Shan has grown into a major mountain range only late in the evolution of the India-Eurasia collision. Most of the convergence between Tarim and Eurasia within the upper crust of the Tien Shan presumably occurs by slip on faults on the edges of and within the belt, but 1–3 mm/yr of convergence is absorbed farther north, at the Dzungarian Alatau and at a lower rate with the Kazakh platform to the west. The Tarim Basin is thrust beneath the Tien Shan at ∼4–7 mm/yr. With respect to Eurasia, the Ferghana Valley rotates counterclockwise at ∼0.7° Myr−1 about an axis at the southwest end of the valley. Thus, GPS data place a bound of ∼4 mm/yr on the rate of crustal shortening across the Chatkal and neighboring ranges on the northwest margin of the Ferghana Valley, and they limit the present-day slip rate on the right-lateral Talas-Ferghana fault to less than ∼2 mm/yr. GPS measurements corroborate geologic evidence indicating that the northern margin of the Pamir overthrusts the Alay Valley and require a rate of at least 10 and possibly 15 mm/yr.

GPS measurements adjacent to the southern Red Sea and Afar Triple Junction, indicate that the Red Sea Rift bifurcates south of 17 degrees N latitude with one branch following a continuation of the main Red Sea Rift (similar to 150 degrees Az.) and the other oriented more N-S, traversing the Danakil Depression. These two rift branches account for the full Arabia-Nubia relative motion. The partitioning of extension between rift branches varies approximately linearly along strike; north of similar to 16 degrees N latitude, extension (similar to 15 mm/yr) is all on the main Red Sea Rift while at similar to 13 degrees N, extension (similar to 20 mm/yr) has transferred completely to the Danakil Depression. The Danakil Block separates the two rifts and rotates in a counterclockwise sense with respect to Nubia at a present-day rate of 1.9 +/- 0.1 degrees/Myr around a pole located at 17.0 +/- 0.2 degrees N, 39.7 +/- 0.2 degrees E, accommodating extension along the rifts and developing the roughly triangular geometry of the Danakil Depression. Rotating the Danakil Block back in time to close the Danakil Depression, and assuming that the rotation rate with respect to Nubia has been roughly constant, the present width of the Danakil Depression is consistent with initiation of block rotation at 9.3 +/- 4 Ma, approximately coincident with the initiation of ocean spreading in the Gulf of Aden, and a concomitant similar to 70% increase in the rate of Nubia-Arabia relative motion.

International audience; We present GPS observations in Morocco and adjacent areas of Spain from 15 continuous (CGPS) and 31 survey-mode (SGPS) sites extending from the stable part of the Nubian plate to central Spain. We determine a robust velocity field for the W Mediterranean that we use to constrain models for the Iberia-Nubia plate boundary. South of the High Atlas Mountain system, GPS motions are consistent with Nubia plate motions from prior geodetic studies. We constrain shortening in the Atlas system to

We present evidence that GPS velocity estimates of plate motions and fault slip rates agree to within uncertainties with geologic estimates during the most recent phase of the geologic evolution of the E Mediterranean region (post-Late Miocene). On this basis, we use the GPS differential velocities to estimate the timing of initiation of the principal structures in NW Turkey, the N Aegean Sea, and central Greece, including, the Marmara Sea, the Gulfs of Evia (GoE) and Corinth (GoC). and the Kephalonia Transform fault (KTF). We interpret these ages to indicate that the North Anatolian fault propagated across the N Aegean, opening the GoE and GoC and initiating the KTF, during the past 1-4 Ma. We further suggest that Aegean extension that was earlier more distributed across the Aegean Basin became focused on this new fault system allowing the southern Aegean and Peloponnisos to translate SW with little internal deformation, as observed today with GPS. This change in tectonic configuration may account for the clear geologic evidence for crustal thinning throughout the S Aegean in apparent contradiction with low present-day strain rates. We further show that the low present-day strain rate along the southern edge of the Aegean micro-plate requires substantial aseismic slip along the plate interface below Crete, consistent with the low level of historic, subduction-type earthquakes along this segment of the subduction zone. (C) 2009 Elsevier B.V. All rights reserved.

Five years of continuously recording GPS observations in the Kingdom of Saudi Arabia together with new continuous and survey-mode GPS observations broadly distributed across the Arabian Peninsula provide the basis for substantially improved estimates of present-day motion and internal deformation of the Arabian plate. We derive the following relative, geodetic Euler vectors (latitude (degrees N), longitude (degrees E), rate (degrees/Myr, counterclock-wise)) for Arabia-Nubia (31.7 +/- 0.2, 24.6 +/- 0.3, 0.37 +/- 0.01), Arabia-Somalia (22.0 +/- 0.5, 26.2 +/- 0.5, 0.40 +/- 0.01), Arabia-India (18.0 +/- 3.8, 87.6 +/- 3.3, 0.07 +/- 0.01), Arabia-Sinai (35.7 +/- 0.8, 17.1 +/- 5.0, 0.15 +/- 0.04), and Arabia-Eurasia (27.5 +/- 0.1, 17.6 +/- 0.3, 0.404 +/- 0.004). We use these Euler vectors to estimate present-day stability of the Arabian plate, the rate and direction of extension across the Red Sea and Gulf of Aden, and slip rates along the southern Dead Sea fault south of the Lebanon restraining bend (4.5-4.7 +/- 0.2 mm/yr, left lateral; 0.8-1.1 +/- 0.3 mm/yr extension) and the Owens fracture zone (3.2-2.5 +/- 0.5 mm/yr, right lateral, increasing from north to south; 1-2 mm/yr extension). On a broad scale, the Arabian plate has no resolvable internal deformation (weighted root mean square of residual motions for Arabia equals 0.6 mm/yr), although there is marginally significant evidence for N-S shortening in the Palmyride Mountains, Syria at

We report the results of nearly 7 years of postseismic deformation measurements using continuously recorded and survey mode GPS observations for the 1999 Izmit-Düzce earthquake sequence. Resolvable, time-dependent postseismic changes to the preearthquake interseismic velocity field extend at least as far as the continuous GPS station in Ankara, ∼200 km southeast of the Izmit rupture. Seven years after the earthquake sequence, the relative postseismic velocity across the North Anatolian Fault (NAF) reaches ∼10–12 mm/a, roughly 50% of the steady state interseismic rate, with the highest postseismic velocities within 40 km of the coseismic ruptures. We use a sequence of logarithmic time functions to fit GPS site motions. Up to three logarithmic terms with decay constants of 1, 150, and 3500 days are necessary to fit all the transient motion observed at the continuous GPS stations. The first term is required for the component of site motion parallel to the NAF at near-field sites strongly implicating rapid, shallow afterslip. The intermediate and longer-term postseismic velocity components reflect more broadly distributed strain with a symmetric double-couple pattern suggestive of either localized, deep afterslip or viscoelastic relaxation of the upper mantle and/or lower crust. In two areas (including the Marmara Sea) this pattern is superimposed on north-south extension centered on the NAF. We speculate that this extension may result from aseismic dip slip along coseismically weakened faults, driven by the background tectonic stress., Scientific and Technological Research Council of Turkey (CAYADAG Project 103Y100, EU 6), European Training Foundation (Frame FORESIGHT Project contract 511139), Scientific and Technological Research Council of Turkey (TARAL 1007 Project 105G019), National Science Foundation (U.S.) (grant EAR-0337497), National Science Foundation (U.S.) (grant EAR-0305480), National Science Foundation (U.S.) (grant INT-0001583), Natural Sciences and Engineering Research Council of Canada (Discovery grant RGPIN 261 458-07)

We have modeled postseismic deformation from 1999 to 2003 in the region surrounding the 1999 Izmit and Duzce earthquake ruptures, using a three-dimensional viscoelastic finite element method. Our models confirm earlier findings that surface deformation within the first few months of the Izmit earthquake is principally due to stable frictional afterslip on and below the Izmit earthquake rupture. A second deformation process is required, however, to fit the surface deformation after several months. Viscoelastic relaxation of lower crust and/or upper mantle with a viscosity of the order of 2 to 5 x 10(19) Pa s improves the models' fit to later GPS site velocities. However, for a linear viscous rheology, this range of values is inconsistent with highly localized interseismic deformation around the North Anatolian Fault Zone (NAFZ) that was well observed prior to the earthquake sequence. The simplest solution to this problem is to assume that the effective viscosity of the relaxing material increases with time after large earthquakes, that is, that it has a power law or Burger's body (transient) rheology. A Burger's body rheology with two characteristic viscosities (2 to 5 x 10(19) Pa s and at least 2 x 10(20) Pa s) in the mantle is consistent with deformation around the NAFZ throughout the earthquake cycle.

[1] Deformation measured by regional GPS networks in continental plateaus reflects the geologic and tectonic variability of the plateaus. For two collisional plateaus (Tibet and Anatolia) and one noncollisional (the Altiplano), we analyze the regional strain and rotation rate by inverting GPS velocities to calculate the full two-dimensional velocity gradient tensor. To test the method, we use gridded velocities determined from an elastic block model for the eastern Mediterranean/Middle East region and show that to a first order, the deformation calculated directly from the GPS vectors provides an accurate description of regional deformation patterns. Principal shortening and extension rate axes, vertical axis rotation, and two-dimensional (2-D) volume strain (dilatation) are very consistent with long-term geological features over large areas, indicating that the GPS velocity fields reflect processes responsible for the recent geologic evolution of the plateaus. Differences between geological and GPS descriptions of deformation can be attributed either to GPS networks that are too sparse to capture local interseismic deformation, or to permanent deformation that accrues during strong earthquakes. The Altiplano has higher internal shortening magnitudes than the other two plateaus and negative 2-D dilatation everywhere. Vertical axis rotation changes sign across the topographic symmetry axis and is due to distributed deformation throughout the plateau. In contrast, the collisional plateaus have large regions of quasi-rigid body rotation bounded by strike-slip faults with the opposite rotation sense from the rotating blocks. Tibet and Anatolia are the mirror images of each other; both have regions of positive dilatation on the outboard sides of the rotating blocks. Positive dilatation in the Aegean correlates with a region of crustal thinning, whereas that in eastern Tibet and Yunnan province in China is associated with an area of vertical uplift. Rollback of the Hellenic trench clearly facilitates the rotation of Anatolia; rollback of the Sumatra–Burma trench probably also enables rotation about the eastern syntaxis of Tibet.

[1] The 1991, Ms = 7.0 Racha earthquake is the largest ever recorded in the Caucasus Mountains. Approximately three months after this thrust-faulting earthquake, a GPS network was set up to measure postseismic surface deformation. We present an analysis of these data, which indicate accelerated postseismic motions at several near-field sites. We model this deformation as either afterslip on the rupture surface or viscoelastic relaxation of the lower crust. We find that the postseismic motions are best explained by shallow afterslip on the earthquake rupture plane. The minimum postseismic moment release is estimated at 6.0 × 1018 N m, which is over 200 times the moment released by aftershocks in this same period and about 20% of the coseismic moment. We also show that the effective viscosity of the lower crust in the western Greater Caucasus region exceeds 1018 Pa s.

Approximately 4 yr of campaign and continuous Global Positioning System (GPS) measurements across the Dead Sea fault system (DSFS) in Lebanon provide direct measurements of interseismic strain accumulation along a 200-km-long restraining bend in this continental transform fault. Late Cenozoic transpression within this restraining bend has maintained more than 3000 m of topography in the Mount Lebanon and Anti-Lebanon ranges. The GPS velocity field indicates 4-5 mm yr(-1) of relative plate motion is transferred through the restraining bend to the northern continuation of the DSFS in northwestern Syria. Near-field GPS velocities are generally parallel to the major, left-lateral strike-slip faults, suggesting that much of the expected convergence across the restraining bend is likely accommodated by different structures beyond the aperture of the GPS network (e.g. offshore Lebanon and, possibly, the Palmyride fold belt in SW Syria). Hence, these geodetic results suggest a partitioning of crustal deformation involving strike-slip displacements in the interior of the restraining bend, and crustal shortening in the outer part of the restraining bend. Within the uncertainties, the GPS-based rates of fault slip compare well with Holocene-averaged estimates of slip along the two principal strike-slip faults: the Yammouneh and Serghaya faults. Of these two faults, more slip occurs on the Yammouneh fault, which constitutes the primary plate boundary structure between the Arabia and Sinai plates. Hence, the Yammouneh fault is the structural linkage that transfers slip to the northern part of the transform in northwestern Syria. From the perspective of the regional earthquake hazard, the Yammouneh fault is presently locked and accumulating interseismic strain.

Geophysicists are using the Global Positioning System (GPS), a highly precise satellite navigation system, to monitor the slow movements (cm/yr) of the crustal plates composing the Earth's surface and the deformations where such plates interact. We report the results of repeated GPS measurements in the complex zone of interaction between the Eurasian, Arabian, and African plates in Turkey. The GPS observations made between 1988 and 1992 indicate that western, central, and east central Turkey are decoupled from the Eurasian plate and are moving as a more or less coherent unit about an axis located north of the Sinai peninsula. Other space-based measurements of crustal motion in Greece and along the Hellenic arc [Smith et al., 1994] suggest that this coherent motion includes southern Greece and the south central Aegean Sea. We refer to this region (shaded in Figure 1) as the Anatolian plate.

We model the geodetically observed secular velocity field in north- western Turkey with a block model that accounts for recoverable elastic-strain ac- cumulation. The block model allows us to estimate internally consistent fault slip rates and locking depths. The northern strand of the North Anatolian fault zone (NAFZ) carries approximately four times as much right-lateral motion (24 mm/yr) as does the southern strand. In the Marmara Sea region, the data show strain accu- mulation to be highly localized. We find that a straight fault geometry with a shallow locking depth of 6-7 km fits the observed Global Positioning System velocities better than does a stepped fault geometry that follows the northern and eastern edges of the sea. This shallow locking depth suggests that the moment release associated with an earthquake on these faults should be smaller, by a factor of 2.3, than previously inferred assuming a locking depth of 15 km. Online material: an updated version of velocity-field data.