Danan Dong

Anatomy of apparent seasonal variations from GPS‐derived site position time series

[1] Apparent seasonal site position variations are derived from 4.5 years of global continuous GPS time series and are explored through the ‘‘peering’’ approach. Peering is a way to depict the contributions of the comparatively well-known seasonal sources to garner insight into the relatively poorly known contributors. Contributions from pole tide effects, ocean tide loading, atmospheric loading, nontidal oceanic mass, and groundwater loading are evaluated. Our results show that � 40% of the power of the observed annual vertical variations in site positions can be explained by the joint contribution of these seasonal surface mass redistributions. After removing these seasonal effects from the observations the potential contributions from unmodeled wet troposphere effects, bedrock thermal expansion, errors in phase center variation models, and errors in orbital modeling are also investigated. A scaled sensitivity matrix analysis is proposed to assess the contributions from highly correlated parameters. The effects of employing different analysis strategies are investigated by comparing the solutions from different GPS data analysis centers. Comparison results indicate that current solutions of several analysis centers are able to detect the seasonal signals but that the differences among these solutions are the main cause for residual seasonal effects. Potential implications for modeling seasonal variations in global site positions are explored, in particular, as a way to improve the stability of the terrestrial reference frame on seasonal timescales. INDEX TERMS: 1223 Geodesy and Gravity: Ocean/Earth/atmosphere interactions (3339); 1247 Geodesy and Gravity: Terrestrial reference systems; KEYWORDS: seasonal variation, GPS, time series

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.

Contemporary crustal deformation in east Asia constrained by Global Positioning System measurements

Global Positioning System (GPS) measurements collected since the early 90s allow us to derive geodetic velocities at 16 permanent stations in east Asia and 68 campaign mode sites in north China. The resulting velocity field shows the following: (1) Contrary to the early inferences that the Shanxi Rift has accommodated significant right-slip motion, our results suggest that the rift system, at least in its northern part in north China, is under ESE-WNW extension at a rate of 4±2 mm/yr. The velocity field also suggests that an ESE-WNW trending left-lateral shear zone deforming at a rate of 2±1 mm/yr may exist along the north rim of north China at the latitude of ∼40°N, separating actively extending north China in the south from relatively stable Mongolia in the north. (2) Central and east China move at a rate of 8–11 mm/yr east-southeast with respect to Siberia, implying that the overall east-southeastward motion is the dominant mode of deformation in east China. (3) The India plate moves at a rate of 6±1 mm/yr slower than the NUVEL-1A model prediction relative to the Eurasia plate. (4) Significant eastward motion (20±2 mm/yr) with respect to Siberia occurs in southeastern Tibet. About half of this eastward motion (∼11 mm/yr) is absorbed by structures along the eastern boundary of the Tibetan Plateau.

Crustal deformation along the Altyn Tagh fault system, western China, from GPS

We collected GPS data from the southern Tarim basin, the Qaidam basin, and the western Kunlun Shan region between 1993 and 1998 to determine crustal deformation along the Altyn Tagh fault system at the northern margin of the Tibetan plateau. We conclude from these data that the Altyn Tagh is a left-lateral strike slip fault with a current slip rate of ∼9 mm/yr, in sharp contrast with geological estimates of 20-30 mm/yr. This contrast poses an enigma: because the GPS data cover a wider region than the geologic data, they might be expected to reveal somewhat more slip. We also find that the Tarim and Qaidam basins behave as rigid blocks within the uncertainty of our measurements, rotating clockwise at a rate of ∼11 and ∼4.5 nrad/yr, respectively, with respect to the Eurasia plate. The rotation of the Tarim basin causes convergence across the Tian Shan, increasing progressively westward from ∼6 mm/yr at 87°E to ∼18 mm/yr at 77°E. Our data and other GPS data suggest that the Indo-Asia collision is mainly accommodated by crustal shortening along the main Himalayan thrust system (∼53%) and the Tian Shan contractional belt (∼19%). Eastward extrusion of the Tibetan plateau along the Altyn Tagh and Kunlun faults accommodates only ∼23% of the Indo-Asia convergence.

Geocenter variations caused by atmosphere, ocean and surface ground water

The geocenter variations reflect the global scale mass redistribution and the interaction between the solid Earth and the mass load. Determination of the geocenter variations due to surface mass load from various geophysical sources places constraints on the variations of the origin of terrestrial reference frame, and provides the expected range of variations for space-geodesy. Our results suggest that on the time scale from 30 days to 10 years the primary variability of geocenter variations from atmosphere, ocean and surface ground water occurs on the annual and semiannual scales. The lumped sum of these surface mass load induced geocenter variations is within 1 cm level.

Measurement of diurnal and semidiurnal rotational variations and tidal parameters of Earth

We discuss the determination of diurnal and semidiurnal variations in the rotation rate and the direction of rotation axis of Earth from the analysis of 8 years of very long baseline interferometry (VLBI) data. This analysis clearly show that these variations are largely periodic and tidally driven; that is, the periods of the variations correspond to the periods of the largest lunar and solar tides. For rotation rate variations, expressed in terms of changes in universal time (UT), the tidal lines with the largest observed signals are O1 (amplitude 23.5 microseconds in time (μs), period 25.82 solar hours); K1 (18.9 μs, 23.93 hours); M2 (17.9 μs, 12.54 hours); and S2 (8.6 μs, 12.00 hours). For variations in the direction of the rotation axis (polar motion), significant signals exist in the retrograde semidiurnal band at the M2 and S2 tides (amplitudes 265 and 119 microarc seconds (μas), respectively); the prograde diurnal band at the O1, K1, and P1 tides (amplitudes 199,152, and 60 μas, respectively); and the prograde semidiurnal band at the M2 and K2 tides (amplitudes 58 and 39 μas, respectively). Variations in the retrograde diurnal band are represented by corrections to the nutations of Earths body axes, and our estimates here are consistent with previous estimates except that a previously noted discrepancy in the 13.66-day nutation (corresponding to the O1 tide) is largely removed in this new analysis. We estimate that the standard deviations of these estimates are 1.0 μs for the UTl variations and 14–16 μas for the polar motion terms. These uncertainties correspond to surface displacements of ∼0.5 mm. From the analysis of atmospheric angular momentum data we conclude that variations in UTl excited by the atmosphere with subdaily periods are small (∼1 μs). We find that the average radial tidal displacements of the VLBI sites in the diurnal band are largely consistent with known deficiencies in current tidal models, i.e., deficiencies of up to 0.9 mm in the treatment of the free core nutation resonance. In the semidiurnal band, our analysis yields estimates of the second-degree harmonic radial Love number h2 at the M2 tide of 0.604+i0.005±0.002. The most likely explanation for the rotational variations are the effects of ocean tides, but there may also be some contributions from atmospheric tides, the effects of triaxiality of Earth, and the equatorial second-degree-harmonic components of the core-mantle boundary.

Shortening and thickening of metropolitan Los Angeles measured and inferred by using geodesy

Geodetic observations using the Global Positioning System (GPS) and other techniques record a high rate of north-south shortening in an east-southeast–trending, 5–40-km-wide belt in northern metropolitan Los Angeles, California. Downtown Los Angeles is observed to be converging upon the southern San Gabriel Mountains at 6 mm/yr. Aside from the elastic strain that will be released during earthquakes rupturing the San Andreas and San Jacinto faults, east-west lengthening across northern metropolitan Los Angeles is minor, <2.5 mm/yr. Therefore north-south shortening is accommodated mainly by vertical crustal thickening.

Reconciling rapid strain accumulation with deep seismogenic fault planes in the Ventura Basin, California

Global Positioning System measurements across the east central Ventura basin, Transverse Ranges, southern California, before the nearby 1994 Northridge earthquake show high strain rates. Interpreting this rapid strain accumulation using the usual model of deep slip on a dislocation in a uniform elastic half-space requires slip to extend to within 5 km of the surface. Such shallow slip is difficult to reconcile with the substantial coseismic displacement at depths from 7 to 19 km during the Northridge earthquake. Here we model the displacement and velocity fields throughout the earthquake cycle using a two-dimensional finite element model with a viscoelastic rheology. Displacements are driven by far-field and basal velocity boundary conditions and by imposed periodic earthquakes on the thrust faults bounding the basin. The thrust faults rupture through an elastic upper crust to a depth of 15 km. After a transient stage, during which stresses and strains build up to quasi-equilibrium values, the behavior of the model becomes periodic. The sum of the coseismic displacement divided by the repeat interval, plus the average interseismic velocity, is equal to the geologic velocity. The temporal variation in surface velocity depends mainly on the Elsasser relaxation time (proportional to the product of the Maxwell time of the lower crust and the ratio of the thicknesses of the entire crust and viscoelastic lower crust). We are able to match the observed high strain rate only if we include the observed variations in elastic modulus associated with the deep basin sediments. The model reconciles geologic, geodetic, and seismological observations of deformation. There are trade-offs among the far-field convergence rate, the Elsasser time, the earthquake repeat time, and the time into the earthquake cycle. Acceptable convergence rates range from 8 mm/yr, for a relaxation time of the lower crust of 300 years, to 12 mm/yr, for a 30-year relaxation time.

Seasonal variations of the Earth's gravitational field: An analysis of atmospheric pressure, ocean tidal, and surface water excitation

Monthly mean gravitational field parameters (denoted here as Ceven) that represent linear combinations of the primarily even-degree zonal spherical harmonic coefficients of the Earths gravitational field have been recovered using LAGEOS I data and are compared with those derived from gridded global surface pressure data of the National Meteorological Center (NMC) spanning 1984–1992. The effect of equilibrium ocean tides and surface water variations are also considered. Atmospheric pressure and surface water fluctuations are shown to be the dominant causes of the observed annual Ceven variations. Closure with observations is seen at the 1σ level when atmospheric pressure, ocean tide and surface water effects are included. Equilibrium ocean tides are shown to be the main source of excitation at the semiannual period with closure at the 1o level seen when both atmospheric pressure and ocean tide effects are included. The inverted barometer (IB) case is shown to give the best agreement with the observation series. The potential of the observed Ceven variations for monitoring mass variations in the polar regions of the Earth and the effect of the land-ocean mask in the IB calculation are discussed.

Crustal deformation measured in Southern California

Studies at the Southern California Earthquake Center (SCEC) are suggesting that postseismic deformation is significant and long lasting. This seems the case, at least, in a region whose dimension is comparable to the fault rupture length. Researchers at SCEC found strong spatial correlation between the high strain rates and the past large earthquakes at the epicentral areas of the 1952 Kern County, 1971 Imperial, and 1992 Landers earthquakes. Southern California spans a plate boundary composed of hundreds of faults, major and minor, over a region hundreds of km wide. Measuring the crustal deformation field across this broad and complex plate boundary poses a great challenge. The Crustal Deformation Working Group of the SCEC orchestrated a major effort to provide, for the first time, a unified geodetic crustal deformation field covering southern California.