Shimon Wdowinski

Southern California permanent GPS geodetic array: Spatial filtering of daily positions for estimating coseismic and postseismic displacements induced by the 1992 Landers earthquake

The June 28, 1992 (M w =7.3) Landers, California, earthquake was the first earthquake to be surveyed by a continuously operating Global Positioning System (GPS) array. The coordinate time series of seven sites are evaluated for station displacements during an interval of 100 days centered on the day of the earthquake. We employ a new spatial filtering technique that removes common-mode errors from the coordinate time series. This approach provides precise estimates of site-specific displacements compared to the cumbersome method of analyzing baselines between pairs of stations. All sites indicate significant coseismic horizontal displacements of 5-65 mm with uncertainties of 1-2 mm. Horizontal displacements are in general agreement with elastic dislocation models, in particular for sites closer to the epicenter. Vertical displacements range from -13 to +7 mm with uncertainties of 2-4 mm. The observed vertical displacements in all cases show 5-10 mm more subsidence than expected from geodetic and seismic/geologic models. Significant postseismic horizontal displacements totaling 6±2 mm (10-20% of the coseismic displacement) are detected at the three sites closest to the epicenter. These displacements are modeled as a short-term exponential relaxation with a decay time of 22±10 days superimposed on a longer-term linear interseismic trend. Scaling the observed coseismic and postseismic displacements at one of the sites with the distance to the epicenter provides a measure of site strain, which agrees well with the direction and magnitude determined from more precise laser strain meter data. The time series do not show any detectable preseismic displacements.

Space geodesy: Subsidence and flooding in New Orleans

It has long been recognized that New Orleans is subsiding and is therefore susceptible to catastrophic flooding. Here we present a new subsidence map for the city, generated from space-based synthetic-aperture radar measurements, which reveals that parts of New Orleans underwent rapid subsidence in the three years before Hurricane Katrina struck in August 2005. One such area is next to the Mississippi River–Gulf Outlet (MRGO) canal, where levees failed during the peak storm surge: the map indicates that this weakness could be explained by subsidence of a metre or more since their construction.

Mexico City subsidence observed with persistent scatterer InSAR

We analyzed 23 satellite SAR (synthetic aperture radar) scenes using Persistent Scatter Interferometry (PSI) to study subsidence in Mexico City associated with groundwater withdrawal. The data were acquired by the Envisat ASAR system between January 2004 and July 2006. The spatial pattern of subsidence and the maximum subsidence rate (300 mm/year) are similar to earlier studies. Comparison to independent GPS data indicates RMS agreement between the two techniques of 6.9 mm/year, about the level expected based on joint data uncertainty. Significant annual variation in the GPS vertical data is not observed, suggesting minimal aquifer recharge during the rainy season, and justifying a simple linear model of phase variation through time for the PSI analysis.

Southern California Permanent GPS Geodetic Array: Continuous measurements of regional crustal deformation between the 1992 Landers and 1994 Northridge earthquakes

The southern California Permanent GPS Geodetic Array (PGGA) was established in 1990 across the Pacific-North America plate boundary to continuously monitor crustal deformation. We describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19-month period between the June 28, 1992 (Mw=7.3), Landers and January 17, 1994 (Mw=6.7), Northridge earthquakes. We compare displacement rates at four site locations with those reported by Feigl et al. [1993], which were derived from an independent set of Global Positioning System (GPS) and very long baseline interferometry (VLBI) measurements collected over nearly a decade prior to the Landers earthquake. The velocity differences for three sites 65–100 km from the earthquakes epicenter are of order of 3–5 mm/yr and are systematically coupled with the corresponding directions of coseismic displacement. The fourth site, 300 km from the epicenter, shows no significant velocity difference. These observations suggest large-scale postseismic deformation with a relaxation time of at least 800 days. The statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets; two possible noise models fit the PGGA data equally well as described in the companion paper by Zhang et al. [this issue]; the pre-Landers data are too sparse and heterogeneous to derive a reliable noise model. Under a fractal white noise model for the PGGA data we find that the velocity differences for all three sites are statistically different at the 99% significance level. A white noise plus flicker noise model results in significance levels of only 94%, 43%, and 88%. Additional investigations of the pre-Landers data, and analysis of longer spans of PGGA data, could have an important effect on the significance of these results and will be addressed in future work.

Distribution of slip at the northern Sumatran fault system

We model spatial variations in horizontal displacements of 117 geodetic sites measured during annual surveys in 1989-1996 with the Global Positioning System (GPS) as elastic strain across a locked strike-slip fault to infer the contemporary slip rate, locking depth, and location of the Sumatran fault (SF) in northern Sumatra (1 S-3 N). GPS-derived slip rate estimates increase slightly northward from 23 plus or minus 3 mm/yr at 0.8 deg S to 26 plus or minus 2mm/yr at 2.7 N. They agree with geologic estimates north of the Equator, but at 0.5 S they are about 10 mm/yr higher. Strain appears to be distributed asymmetrically about the fault. South of 2 N, about 5 mm/yr of shear is required within the offshore forearc, west of the fault, to achieve a closer agreement of fault locations inferred from GPS velocities with geologically identified traces of the SF. Locking depth estimates are on the order of 10-20 km. The western branch of the major fault bifurcation near 1 N slips at a rate five times higher than the eastern branch. The two main strands of the fault at the northwestern tip of Sumatra (5.5 N) appear to be nearly free of horizontal strain; significant slip must occur away from the two strands, probably further east at two other geologically active branches. The Banda Aceh embayment is extruded to the northwest at a rate of 5 plus or minus 2 mm/yr. Within the estimated velocity uncertainties of several mm/yr, fault-normal deformation along the SF is insignificant. Almost strain free, the northern part of the back-arc basin is part of a rigid Sunda shelf, while the northern forearc is subjected to 8 plus or minus 5 x 10 (exp -8)/yr of extension nearly parallel to the arc.

The lowest place on Earth is subsiding—An InSAR (interferometric synthetic aperture radar) perspective

Since the early 1990s, sinkholes and wide, shallow subsidence features (WSSFs) have become major problems along the Dead Sea shores in Israel and Jordan. Sinkholes are readily observed in the field, but their locations and timing are unpredictable. WSSFs are often difficult to observe in the field. However, once identified, they delineate zones of instability and increasing hazard. In this study we identify, characterize, and measure rates of subsidence along the Dead Sea shores by the interferometric synthetic aperture radar (InSAR) technique. We analyze 16 SAR scenes acquired during the years 1992 to 1999 by the European Remote Sensing ERS-1 and ERS- 2 satellites. The interferograms span periods of between 2 and 71 months. WSSFs are observed in the Lisan Peninsula and along the Dead Sea shores, in a variety of appearances, including circular and elongate coastal depressions (a few hundred meters to a few kilometers in length), depressions in ancient alluvial fans, and depressions along salt-diapir margins. Phase differences measured in our interferograms correspond to subsidence rates generally in the range of 0–20 mm/yr within the studied period, with exceptional high rates that exceed 60 mm/yr in two specific regions. During the study period, the level of the Dead Sea and of the associated ground water has dropped by ∼6 m. This water-level drop within an aquifer overlying fine-grained, marly layers, would be expected to have caused aquifer-system consolidation, resulting in gradual subsidence. Comparison of our InSAR observations with calculations of the expected consolidation shows that in areas where marl layers are known to compose part of the upper 30 m of the profile, estimated consolidation settlements are of the order of the measured subsidence. Our observations also show that in certain locations, subsidence appears to be structurally controlled by faults, seaward landslides, and salt domes. Gradual subsidence is unlikely to be directly related to the sinkholes, excluding the use of the WSSFs features as predictable precursors to sinkhole formation.

Isostatic rebound due to tectonic denudation: A viscous flow model of a layered lithosphere

A four-layer model of the upper 150 km of the Earth is used to calculate the viscous response of continental crust and the underlying mantle to tectonic denudation. The model comprises a strong upper crustal layer, a weak lower crustal layer, a very strong mantle lithosphere layer, and a weak mantle asthenosphere layer, which is in accord with experimental constraints on strength-depth profiles for continental lithosphere. The strength of each layer is represented by its effective viscosity. Flow in the crust and mantle is driven by buoyancy forces, which arise from the unloading of an allochthon along a detachment fault by a series of instantaneous displacements (earthquakes or rapid creep events). Numerical solutions, obtained by using a finite element technique, predict footwall uplift, Moho deflection, and surface topography that are consistent with observations from the Basin and Range province of the western United States. The calculated curvature of the footwall uplift is also similar to that observed and is sensitive to the geometry of the detachment fault. Such bending need not be elastically controlled; hence the curvatures of footwall domes do not clearly place limits on the effective elastic thickness of the extending crust. The upward deflection of the Moho and the surface topography are sensitive to the viscosity structure and enable us to bound the range of the various viscosities. By matching observations from the Basin and Range province, which indicate no Moho deflection and low magnitude of surface topography (≤3–5 km), we estimate the upper crustal, lower crustal, and mantle lithospheric viscosities in the ranges 1021–1023 Pa s, 1019–1021 Pa s, and 1021–1023 Pa s, respectively.

GPS measurements of crustal deformation within the Pacific-Australia plate boundary zone in Irian Jaya, Indonesia

Abstract Global Positioning System (GPS) measurements made in 1991, 1992 and 1993 provide preliminary estimates of slip distribution between the Australian and Pacific plates in Irian Jaya, Indonesia. We interpret the GPS results with constraints from earthquake mechanisms and slip vectors, recent marine surveys, and geology. Three GPS sites in southeastern Irian Jaya show motions that are within 10 mm/yr of the expected motion of Australia. A coast-to-coast N-S baseline along 140.5°E crosses all known onland regions of active deformation but reveals no more than 15 mm/yr of shortening and 20 mm/yr of left-lateral shear in the 27-month period. The remaining 40 mm/yr of expected convergence between the Pacific and Australian plates probably occurs at the New Guinea trough. GPS sites on the island of Biak, at 136°E, and at Sorong, near the western tip of Birds Head (at 131°E), both move 90–100 mm/yr in a WSW direction relative to Irian Jaya, but less than 15 mm/yr relative to each other. These sites are on either side of the Sorong fault and demonstrate that it is not presently the major boundary between the Australian and Pacific plates. Instead the plate boundary is now south of the Sorong and Biak sites. Earthquakes suggest possible structures that accomodate motion between Birds Head and Australia but the relative importance of them remains uncertain.

Systematic analyses of the large-scale topography and structure across the Dead Sea Rift

The Dead Sea Rift (DSR) is one of the deepest continental depressions on the Earths surface and is the best example of a continental rift lying along a transform plate boundary (the Dead Sea Transform). We systematically analyze the large-scale topography, structure, and morphology across the central part of the DSR between Lake Kinneret and the Gulf of Elat and show a distinct asymmetrical topographic pattern across the rift axis. The topography analysis uses a Digital Terrain Model (DTM) of Israel and adjacent areas to plot a series of 64 profiles perpendicular to the rift axis. The profiles show that the eastern side is topographically higher than the western side and that its overall shape resembles an uplifted shoulder; the lower western side resembles an arch. This analysis also reveals along-strike variations in the topography that allow us to subdivide the central DSR into five segments of similar topography. The large-scale structure across the DSR is investigated by a series of 10 geological cross sections drawn perpendicular to the rift axis along the five segments. On the basis of the stratigraphic record and the geological history of the region, we identify a regional marker (Top Eocene Sequence) to trace the rift-related structure. This marker shows that the structure parallels the topographic asymmetry across the rift axis: the rifts eastern margin is uplifted toward the axis, whereas the rifts western margin is downflexed toward the axis and defines a wide asymmetrical monocline. Our analyses indicate that (1) the large-scale asymmetry across the DSR reflects a wide half-graben structure (30–60 km wide), (2) the rifts eastern margin reflects broad regional uplift along the rift, and (3) the western side arching is a subsidary structure that follows the main rift structure.

Evaluation of TerraSAR-X Observations for Wetland InSAR Application

This paper assesses the potential of using spaceborne X-band synthetic aperture radar (SAR) data for monitoring water-level changes over wetlands. Our analysis is based on three sets of TerraSAR-X (TSX) observations acquired over South Floridas Everglades wetlands during an eight-month period in 2008. The first set was acquired in single HH polarization stripmap mode over our northern study area, consisting of managed wetlands and urban environments. The second set was acquired in dual-polarization stripmap mode over the western half of the same area, consisting mostly of managed wetlands. The third set was also acquired with dual-polarization stripmap mode over our southern study area, consisting of natural flow freshand salt-water wetlands in the southern Everglades. The first data set was used for a proof-of-concept study to verify that X-band data can generate coherent interferograms in wetland areas. Interferometric processing of this data set shows a high level of coherence (> 0.35) over both wetland and urban regions, maintaining interferometric phase in all three interferograms spanning 11 days. Surprisingly, phase is maintained over some of the wetlands even for interferograms spanning 33 days. The other two data sets were used to evaluate interferometric coherence of all four polarization modes and to determine dominant scattering mechanism in each wetland environment. Our results show high coherence values (> 0.4) in all polarization modes, with highest values in HH, then VV, and lowest in HV or VH. Interferograms calculated from multipolarization data show very similar fringe patterns regardless of the polarization type, suggesting that the phase information in all polarization data reflects water-level changes in wetlands and that volume scattering may be less important than commonly believed. We also used the two multipolarization data sets to conduct the Pauli decomposition, finding a strong dependence of scattering mechanism on vegetation type. The high interferometric coherence level of all polarization data suggests that a significant part of the X-band scattered signal interacts with lower sections of the vegetation (trunks and branches), because scattering from wind-affected canopies cannot support such a high coherence level. The high spatial resolution of TSX, combined with its 11-day repeat orbit, makes this X-band sensor surprisingly suitable for wetland interferometric SAR applications.