Robert W. King

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.

A geological and geophysical context for the Wenchuan earthquake of 12 May 2008, Sichuan, People's Republic of China

On 12 May 2008, a magnitude 7.9 earthquake ruptured the Longmen Shan margin of the eastern Tibetan plateau. This event occurred within the context of long-term uplift and eastward enlargement of the plateau. The area has numerous geological features not typical of active convergent mountain belts, including the presence of a steep mountain front (>4 km relief) but an absence of large-magnitude low-angle thrust faults; young high topography (post ca. 15 Ma) and thickened crust but low global positioning system (GPS) shortening rates (<3 mm/yr); and no coeval foreland subsidence. In our interpretation, crustal thickening beneath the eastern Tibetan plateau occurred without large-scale shortening of the upper crust but instead is caused by ductile thickening of the deep crust in a weak (lowviscosity) layer. Late Cenozoic shortening across the Longmen Shan could be as little as 10–20 km, with folding and faulting mainly accommodating differential surface uplift between the plateau and the Sichuan Basin. The earthquake of 12 May probably reflects long-term uplift, with slow convergence and right-slip, of the eastern plateau relative to the Sichuan Basin. GPS-determined rates in the vicinity of the 12 May event suggest an average recurrence interval of ~2,000–10,000 yr.

Global Positioning System measurements from eastern Tibet and their implications for India/Eurasia intercontinental deformation

We present and interpret Global Positioning System (GPS) measurements of crustal motions for the period 1991–1998 for a network encompassing the eastern part of the Tibetan Plateau and its foreland. Relative to a Eurasian frame defined by minimizing the velocities of 16 GPS stations in Europe, central Asia, and Siberia, stations within all parts of the plateau foreland in south China move 6–10 mm/yr east-southeast, indicating that the eastward movement within the plateau is part of a broader eastward movement that involves the plateau and its eastern and northern foreland. North of the plateau, foreland stations move northeastward at ∼10 mm/yr, indicating that the northern boundary of the deformation zone lies north of the plateau. With this realization of a Eurasian frame, the velocity of the GPS station at Bangalore in southern India implies that the northward motion of India is 5–12 mm/yr slower than that predicted from the NUVEL-1A plate reconstruction. Viewed relative to the South China Block, stations of the northeast plateau, bounded on the north by the Qilian Shan and the Altyn Tagh fault, move NNE to NE with velocities ranging from 19 mm/yr within the plateau to 5–11 mm/yr in its foreland. The Altyn Tagh fault shows left-lateral slip of ∼10 mm/yr at 95°E and shortening across the fault of <5 mm/yr. Stations south and west of the Xianshuihe/Xiaojiang fault system define a crustal fragment rotating clockwise at ∼10 mm/yr relative to the South China Block around the eastern Himalayan syntaxis. The GPS measurements indicate no significant shortening (< 3 mm/yr) within the Longmen Shan of the central eastern plateau and its adjacent foreland, although the Longmen Shan rise over 6 km in <100 km horizontal distance. Geological studies indicate that the deformational field was established diachronously in late Miocene to Pliocene time, was characterized by no east-west shortening of Tibetan crust, and has an inhomogeneous style of deformation resulting from a balance of different tectonic processes.

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...

Space geodetic measurement of crustal deformation in central and southern California, 1984-1992

A laboratory type of analyzer for quantitatively determining the percent third element content of a hydrocarbon sample. A unique rhodium/americium radioactive source is disclosed.

Geodetic measurement of crustal motion in southwest China

Global Positioning System measurements performed over a 2–4 yr period confirm the Xianshuihe-Xiaojiang fault system as one of the primary active structures in southwest China. Stations southwest of these faults show southerly motions of 5–15 mm/yr relative to the western Sichuan basin, and stations in a 200 km geodetic network located northwest of the basin move at only 0–5 mm/yr. These results are consistent with clockwise rotation of southwestern Sichuan and western Yunnan about the eastern Tibetan syntaxis, accommodated by left-lateral slip of 12 ± 4 mm/yr on the Xianshuihe-Xiaojiang fault system. The results imply that if eastward extrusion of crustal material from the plateau occurs at present, it is not accommodated by east-west shortening along the margin of the plateau and must involve wholesale eastward motion of low-lying regions to the east.

Geodynamics of the southeastern Tibetan Plateau from seismic anisotropy and geodesy

Ongoing plate convergence between India and Eurasia provides a natural laboratory for studying the dynamics of continental collision, a fi rst-order process in the evolution of continents, regional climate, and natural hazards. In southeastern Tibet, the fast directions of seismic anisotropy determined using shear-wave splitting analysis correlate with the surfi cial geology including major sutures and shear zones and with the surface strain derived from the global positioning system velocity fi eld. These observations are consistent with a clockwise rotation of material around the eastern Himalayan syntaxis and suggest coherent distributed lithospheric deformation beneath much of southeastern Tibet. At the southeastern edge of the Tibetan Plateau we observe a sharp transition in mantle anisotropy with a change in fast directions to a consistent E-W direction and a clockwise rotation of the surface velocity, surface strain fi eld, and fault network toward Burma. Around the eastern Himalayan syntaxis, the coincidence between structural crustal features, surface strain, and mantle anisotropy suggests that the deformation in the lithosphere is mechanically coupled across the crust-mantle interface and that the lower crust is suffi ciently strong to transmit stress. At the southeastern margin of the plateau in Yunnan province, a change in orientation between mantle anisotropy and surface strain suggests a change in the relationship between crustal and mantle deformation. Lateral variations in boundary conditions and rheological properties of the lithosphere play an important role in the geodynamic evolution of the Himalayan orogen and Tibetan Plateau and require the development of three-dimensional models that incorporate lateral heterogeneity.

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.

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.