Interferometric synthetic aperture radar (InSAR) provides a practical means of mapping creep along major strike‐slip faults. The small amplitude of the creep signal (<10 mm/yr), combined with its short wavelength, makes it difficult to extract from long time span interferograms, especially in agricultural or heavily vegetated areas. We utilize two approaches to extract the fault creep signal from 37 ERS SAR images along the southern San Andreas Fault. First, amplitude stacking is utilized to identify permanent scatterers, which are then used to weight the interferogram prior to spatial filtering. This weighting improves correlation and also provides a mask for poorly correlated areas. Second, the unwrapped phase is stacked to reduce tropospheric and other short‐wavelength noise. This combined processing enables us to recover the near‐field (∼200 m) slip signal across the fault due to shallow creep. Displacement maps from 60 interferograms reveal a diffuse secular strain buildup, punctuated by localized interseismic creep of 4–6 mm/yr line of sight (LOS, 12–18 mm/yr horizontal). With the exception of Durmid Hill, this entire segment of the southern San Andreas experienced right‐lateral triggered slip of up to 10 cm during the 3.5‐year period spanning the 1992 Landers earthquake. The deformation change following the 1999 Hector Mine earthquake was much smaller (<1 cm) and broader than for the Landers event. Profiles across the fault during the interseismic phase show peak‐to‐trough amplitude ranging from 15 to 25 mm/yr (horizontal component) and the minimum misfit models show a range of creeping/locking depth values that fit the data.

[1] Abstract: Vailulu'u Seamount is identified as an active volcano marking the current location of the Samoan hotspot. This seamount is located 45 km east of Ta'u Island, Samoa, at 169803.5 0 W, 14812.9 0 S. Vailulu'u defines the easternmost edge of the Samoan Swell, rising from the 5000-m ocean floor to a summit depth of 590 m and marked by a 400-m-deep and 2-km-wide summit crater. Its broad western rift and stellate morphology brand it as a juvenile progeny of Ta'u. Seven dredges, ranging from the summit to the SE Rift zone at 4200 m, recovered only alkali basalts and picrites. Isotopically, the volcano is strongly EM2 in character and clearly of Samoan pedigree ( 87 Sr/ Geophysics GeosystemsPublished by AGU and the Geochemical Society AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Geochemistry Geophysics GeosystemsResearch Letter , and a halo of``smog'' several hundred meters thick encircles and extends away from the summit for at least 7 km. The turbid waters are highly enriched in manganese (up to 7.3 nmol/kg), providing further evidence of hydrothermal activity. Vailulu'u is similar to Loihi (Hawaii) in being an active volcanic construct at the eastern end of a hotspot chain; it differs importantly from the Hawaiian model in its total lack of tholeiitic basalt compositions.

Abstract. We characterize the seamount distribution on the Pacific Plate using the gridded vertical gravity gradient (VGG, or geoid curvature) derived from Geosat and ERS-1 satellite altimetry. The VGG amplifies short-wavelength information and suppresses longer wavelength components, making it suitable for seamount detection purposes. Furthermore, the VGG over seamounts has a much more pronounced zero crossing than that of the free-air anomaly (FAA); the distance to the zero crossing can be used as a proxy for seamount radius. After removing a regional field obtained by robust median filtering we identify seamount amplitudes and locations from local maxima in the VGG grid. The radius of a seamount is more difficult to estimate since seamounts tend to cluster and overprint each other's signals. Individual seamounts are modeled as Gaussian, axisymmetric objects loading an elastic lithosphere; the VGG over such features can be approximated by a simple analytical expression which we use to determine the zero-crossing distances for overlapping seamounts. By using the VGG the maximum amplitude and distance to the zero crossing become largely independent of the elastic plate thickness and infill density. We do forward modeling of Gaussian seamounts and their gravimetric response and create a look up table that relates seamount FAA (in milligals), VGG (in E6tv6s), and zero-crossing distance (in kilometers) to actual height and radius (in kilometers). The frequency-size distribution of these predicted seamount heights follows a power law for heights between 2 and 8 km. The seamount density (number of seamounts per area) is greatest in the central Pacific. We confirm earlier results suggesting that the majority of large seamounts are located in the western region of the Pacific Plate, on older crust. As crustal age increases, so does seamount density, peaking on 100-130 m.y. crust, supporting suggestions of high magmatism in the Cretaceous. We demonstrate that there may be an empirical relationship between the seamount VGG amplitude and the age of the lithosphere at the time of seamount formation and invert this relationship to predict seamount ages from VGG amplitudes. These pseudo ages have large uncertainties but, nevertheless, may be used to investigate temporal fluctuations in Pacific intraplate volcanism. Our results indicate that seamount intraplate volcanism attained a maximum level in the mid-Cretaceous to Late Cretaceous, about 70-120 Ma, apparently contemporaneous with the formation of large oceanic plateaus in the Pacific.

Abstract. A three-dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry. This technique is applied to the Louisville Ridge system to study the tectonic history of the region. The inversion is performed as both a firstorder approximation and a nonlinear relationship between gravity and topography based on Parker's [1973] equation. While the higher-order effect on the gravity anomaly is nearly zero for most of the region, the magnitude is significant over the summits of the ridge. Nevertheless, the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region, lowering the value by -1-2 km compared with the linear result. The incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly, resulting in false sidelobe structure of nearly 20 mGal for large seamounts. Our elastic thickness estimates are compared with the contradictory values obtained in previous studies by Cazenave and Dominh [ 1984] and Watts et al. [1988]. We find an increasing elastic thickness along the chain from southeast to northwest, with a discontinuity along the Wishbone scarp. The jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge.