Simon McClusky

Global Positioning System constraints on plate kinematics and dynamics in the eastern Mediterranean and Caucasus

We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1988–1997 at 189 sites extending east-west from the Caucasus mountains to the Adriatic Sea and north-south from the southern edge of the Eurasian plate to the northern edge of the African plate. Sites on the northern Arabian platform move 18±2 mm/yr at N25°±5°W relative to Eurasia, less than the NUVEL-1A circuit closure rate (25±1 mm/yr at N21°±7°W). Preliminary motion estimates (1994–1997) for stations located in Egypt on the northeastern part of Africa show northward motion at 5–6±2 mm/yr, also slower than NUVEL-IA estimates (10±1 mm/yr at N2°±4°E). Eastern Turkey is characterized by distributed deformation, while central Turkey is characterized by coherent plate motion (internal deformation of <2 mm/yr) involving westward displacement and counterclockwise rotation of the Anatolian plate. The Anatolian plate is de-coupled from Eurasia along the right-lateral, strike-slip North Anatolian fault (NAF). We derive a best fitting Euler vector for Anatolia-Eurasia motion of 30.7°± 0.8°N, 32.6°± 0.4°E, 1.2°±0.1°/Myr. The Euler vector gives an upper bound for NAF slip rate of 24±1 mm/yr. We determine a preliminary GPS Arabia-Anatolia Euler vector of 32.9°±1.2°N, 40.3°±1.1°E, 0.8°±0.2°/Myr and an upper bound on left-lateral slip on the East Anatolian fault (EAF) of 9±1 mm/yr. The central and southern Aegean is characterized by coherent motion (internal deformation of <2 mm/yr) toward the SW at 30±1 mm/yr relative to Eurasia. Stations in the SE Aegean deviate significantly from the overall motion of the southern Aegean, showing increasing velocities toward the trench and reaching 10±1 mm/yr relative to the southern Aegean as a whole.

Global Positioning System measurements of present-day crustal movements in the Arabia-Africa-Eurasia plate collision zone

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.

GPS Meteorology: Direct Estimation of the Absolute Value of Precipitable Water

Abstract A simple approach to estimating vertically integrated atmospheric water vapor, or precipitable water, from Global Positioning System (GPS) radio signals collected by a regional network of ground-based geodetic GPS receiver is illustrated and validated. Standard space geodetic methods are used to estimate the zenith delay caused by the neutral atmosphere, and surface pressure measurements are used to compute the hydrostatic (or “dry”) component of this delay. The zenith hydrostatic delay is subtracted from the zenith neutral delay to determine the zenith wet delay, which is then transformed into an estimate of precipitable water. By incorporating a few remote global tracking stations (and thus long baselines) into the geodetic analysis of a regional GPS network, it is possible to resolve the absolute (not merely the relative) value of the zenith neutral delay at each station in the augmented network. This approach eliminates any need for external comparisons with water vapor radiometer observation...

Estimates of Seismic Potential in the Marmara Sea Region from Block Models of Secular Deformation Constrained by Global Positioning System Measurements

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.

Time-Dependent Distributed Afterslip on and Deep below the İzmit Earthquake Rupture

Surface deformation transients measured with the Global Positioning System during the 87 days between the 17 August 1999 Izmit earthquake and the 12 November 1999 Duzce earthquake indicate rapidly decaying aseismic fault slip on and well below the coseismic rupture. Elastic model inversions for time-dependent distributed fault slip, using a network inversion filter approach, show that afterslip was highest between and below the regions of maximum coseismic slip and propa- gated downward to, or even below, the base of the crust. Maximum afterslip rates decayed from greater than 2 m/yr, immediately after the I zmit earthquake to about 1.2 m/yr just prior to the Duzce earthquake. Maximum afterslip occurred below the eastern Karadere rupture segment and near the I zmit hypocenter. Afterslip in the upper 16 km decayed more rapidly than that below the seismogenic zone. These observations are consistent with a phase of rapid aseismic fault slip concentrated near the base of the seismogenic zone. Continued loading from the rapid deep afterslip along the eastern rupture zone is a plausible mechanism that helped trigger the nearby, Mw 7.2, 12 November Duzce earthquake.

Present day kinematics of the Eastern California Shear Zone from a geodetically constrained block model

We use Global Positioning System (GPS) data from 1993–2000 to determine horizontal velocities of 65 stations in eastern California and western Nevada between 35° and 37° N. We relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km. The velocity of the Sierra Nevada block with respect to Nevada is 11.1±0.3 mm/yr, with slip partitioned across the Death Valley, (2.8±0.5 mm/yr), Panamint Valley (2.5±0.8 mm/yr), and Airport Lake/Owens Valley (5.3±0.7/4.6±0.5 mm/yr) faults. The western Mojave block rotates at 2.1±0.8°/My clockwise, with 3.7±0.7 mm/yr of left lateral motion across the western Garlock Fault. We infer 11±2 mm/yr of right lateral motion across the Mojave region of the Eastern California Shear Zone.

Estimation of current plate motions in Papua New Guinea from Global Positioning System observations

Plate tectonic motions have been estimated in Papua New Guinea from a 20 station network of Global Positioning System sites that has been observed over five campaigns from 1990 to 1996. The present velocities of the sites are consistent with geological models in which the South Bismarck, Woodlark, and Solomon Sea Plates form the principal tectonic elements between the Pacific and Australian Plates in this region. Active spreading is observed on the Woodlark Basin Spreading Centre but at a rate that is about half the rate determined from magnetic reversals. The other major motions observed are subduction on the New Britain Trench, seafloor spreading across the Bismarck Sea Seismic Lineation, convergence across the Ramu-Markham Fault and left-lateral strike slip across the Papuan Peninsula. These motions are consistent with a 8.2° Myr -1 clockwise rotation of the South Bismarck Plate about a pole in the Huon Gulf and a rotation of the Woodlark Plate away from the Australian Plate. Second order deformation may also be occurring; in particular, Manus Island and northern New Ireland may be moving northward relative to the Pacific Plate at ∼5-8 mm yr -1 (significant at the 95% but not at the 99% confidence level) which may suggest the existence of a North Bismarck Plate.

The strain rate field in the eastern Mediterranean region, estimated by repeated GPS measurements

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.

Active tectonics of the western Mediterranean: Geodetic evidence for rollback of a delaminated subcontinental lithospheric slab beneath the Rif Mountains, Morocco

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

Kinematics of the southern Red Sea-Afar Triple Junction and implications for plate dynamics

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.