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Garcia, ESM, Sandwell DT, Bassett D.  2019.  Outer trench slope flexure and faulting at Pacific basin subduction zones. Geophysical Journal International. 218:708-728.   10.1093/gji/ggz155   AbstractWebsite

Flexure and fracturing of the seafloor on the outer trench wall of subduction zones reflect bending of the lithosphere beyond its elastic limit. To investigate these inelastic processes, we have developed a full nonlinear inversion approach for estimating the bending moment, curvature and outer trench wall fracturing using shipboard bathymetry and satellite altimetry-derived gravity data as constraints. Bending moments and downward forces are imposed along curved trench axes and an iterative method is used to calculate the nonlinear response for 26 sites in the circum-Pacific region having seafloor age ranging from 15 to 148Ma. We use standard thermal and yield strength envelope models to develop the nonlinear moment versus curvature relationship. Two coefficients of friction of 0.6 and 0.3 are considered and we find that the lower value provides a better overall fit to the data. The main result is that the lithosphere is nearly moment saturated at the trench axis. The effective elastic thickness of the plate on the outer trench slope is at least three times smaller than the elastic thickness of the plate before bending at the outer rise in agreement with previous studies. The average seafloor depth of the unbent plate in these 26 sites matches the Parsons & Sclater depth versus age model beyond 120Ma. We also use the model to predict the offsets of normal faults on the outer trench walls and compare this with the horst and graben structures observed by multibeam surveys. The model with the lower coefficient of friction fits the fault offset data close to the trench axis. However, the model predicts significant fracturing of the lithosphere between 75 and 150km away from the trench axis where no fracturing is observed. To reconcile these observations, we impose a thermoelastic pre-stress in the lithosphere prior to subduction. This pre-stress delays the onset of fracturing in better agreement with the data.

Wei, M, Sandwell D, Smith-Konter B.  2010.  Optimal combination of InSAR and GPS for measuring interseismic crustal deformation. Advances in Space Research. 46:236-249.   10.1016/j.asr.2010.03.013   AbstractWebsite

High spatial resolution measurements of interseismic deformation along major faults are critical for understanding the earthquake cycle and for assessing earthquake hazard. We propose a new remove/filter/restore technique to optimally combine GPS and InSAR data to measure interseismic crustal deformation, considering the spacing of GPS stations in California and the characteristics of interseismic signal and noise using InSAR. To constrain the longer wavelengths (>40 km) we use GPS measurements, combined with a dislocation model, and for the shorter wavelength information we rely on InSAR measurements. Expanding the standard techniques, which use a planar ramp to remove long wavelength error, we use a Gaussian filter technique. Our method has the advantage of increasing the signal-to-noise ratio, controlling the variance of atmosphere error, and being isotropic. Our theoretical analysis indicates this technique can improve the signal-to-noise ratio by up to 20%. We test this method along three segments of the San Andreas Fault (Southern section near Salton Sea, Creeping section near Parkfield and Mojave/Big Bend section near Los Angeles), and find improvements of 26%, 11% and 8% in these areas, respectively. Our data shows a zone of uplift to the west of the Creeping section of the San Andreas Fault and an area of subsidence near the city of Lancaster. This work suggests that after only 5 years of data collection, ALOS interferograms will provide a major improvement in measuring details of interseismic deformation. (C) 2010 COSPAR. Published by Elsevier Ltd. All rights reserved.

McAdoo, DC, Sandwell DT.  1989.  On the Source of Cross-Grain Lineations in the Central Pacific Gravity-Field. Journal of Geophysical Research-Solid Earth and Planets. 94:9341-9352.   10.1029/JB094iB07p09341   AbstractWebsite

Subtle lineations in the marine gravity field of the central Pacific derived from Seasat altimeter data were observed by Haxby and Weissel (1986). They suggested that these “cross-grain” lineations were evidence of small-scale convection beneath the Pacific plate. We have examined these features by comparing multiple, collinear gravity and bathymetry profiles in the Fourier transform domain. Our nine gravity profiles were each obtained by stacking (averaging) three or more individual, repeat Geosat/ERM altimeter passes. Prior to stacking, the individual Geosat passes were fit to a cubic spline and then differentiated along track to produce along-track deflections of the vertical (or horizontal gravity). Corresponding bathymetric profiles were produced by projecting, onto Geosat ground tracks, bathymetric observations from six R/V Thomas Washington legs and three R/V Conrad legs that virtually coincide with these Geosat tracks. After Fourier transforming the resulting gravity and bathymetry profiles, we estimate admittances of gravity to bathymetry. These admittances are generally low; they also tend to be negative at very short wavelengths (λ<50 km). They are consistent with models of flexural isostatic compensation by a very thin lithosphere (approximately 2 km). They are not consistent with models of dynamic compensation. We suggest, therefore, that either (1) these cross-grain lineations began to form very near the East Pacific Rise or (2) they formed on older, anomalously weak lithosphere. We also suggest that the gravity lineations result primarily from loads beneath the seafloor in combination with, secondarily, loads on the seafloor. Depths of these subseafloor loads appear not to exceed significantly typical Moho depths.

Matthews, KJ, Mullner RD, Sandwell DT.  2016.  Oceanic microplate formation records the onset of India-Eurasia collision. Earth and Planetary Science Letters. 433:204-214.   10.1016/j.epsl.2015.10.040   AbstractWebsite

Mapping of seafloor tectonic fabric in the Indian Ocean, using high-resolution satellite-derived vertical gravity gradient data, reveals an extinct Pacific-style oceanic microplate ('Mammerickx Microplate') west of the Ninetyeast Ridge. It is one of the first Pacific-style microplates to be mapped outside the Pacific basin, suggesting that geophysical conditions during formation probably resembled those that have dominated at eastern Pacific ridges. The microplate formed at the Indian-Antarctic ridge and is bordered by an extinct ridge in the north and pseudofault in the south, whose conjugate is located north of the Kerguelen Plateau. Independent microplate rotation is indicated by asymmetric pseudofaults and rotated abyssal hill fabric, also seen in multibeam data. Magnetic anomaly picks and age estimates calculated from published spreading rates suggest formation during chron 21o (similar to 47.3 Ma). Plate reorganizations can trigger ridge propagation and microplate development, and we propose that Mammerickx Microplate formation is linked with the India-Eurasia collision (initial 'soft' collision). The collision altered the stress regime at the Indian-Antarctic ridge, leading to a change in segmentation and ridge propagation from an establishing transform. Fast Indian-Antarctic spreading that preceded microplate formation, and Kerguelen Plume activity, may have facilitated ridge propagation via the production of thin and weak lithosphere; however both factors had been present for tens of millions of years and are therefore unlikely to have triggered the event. Prior to the collision, the combination of fast spreading and plume activity was responsible for the production of a wide region of undulate seafloor to the north of the extinct ridge and 'W' shaped lineations that record back and forth ridge propagation. Microplate formation provides a precise means of dating the onset of the India-Eurasia collision, and is completely independent of and complementary to timing constraints derived from continental geology or convergence histories. (C) 2015 Elsevier B.V. All rights reserved.

Luttrell, K, Sandwell D.  2010.  Ocean loading effects on stress at near shore plate boundary fault systems. Journal of Geophysical Research-Solid Earth. 115   10.1029/2009jb006541   AbstractWebsite

Changes in eustatic sea level since the Last Glacial Maximum create a differential load across coastlines globally. The resulting plate bending in response to this load alters the state of stress within the lithosphere within a half flexural wavelength of the coast. We calculate the perturbation to the total stress tensor due to ocean loading in coastal regions. Our stress calculation is fully 3-D and makes use of a semianalytic model to efficiently calculate stresses within a thick elastic plate overlying a viscoelastic or fluid half-space. The 3-D stress perturbation is resolved into normal and shear stresses on plate boundary fault planes of known orientation so that Coulomb stress perturbations can be calculated. In the absence of complete paleoseismic indicators that span the time since the Last Glacial Maximum, we investigate the possibility that the seismic cycle of coastal plate boundary faults was affected by stress perturbations due to the change in sea level. Coulomb stress on onshore transform faults, such as the San Andreas and Alpine faults, is increased by up to 1-1.5 MPa, respectively, promoting failure primarily through a reduction in normal stress. These stress perturbations may perceptibly alter the seismic cycle of major plate boundary faults, but such effects are more likely to be observed on nearby secondary faults with a lower tectonic stress accumulation rate. In the specific instance of rapid sea level rise at the Black Sea, the seismic cycle of the nearby North Anatolian fault was likely significantly advanced.

Sandwell, D.  2007.  Ocean Bathymetry and Plate Tectonics. Our changing planet : the view from space. ( King MD, Parkinson CL, Partington KC, Williams RG, Eds.).:149-152., Cambridge ; New York: Cambridge University Press Abstract

Examines what orbital imagery tells us about the atmosphere, land, ocean, and polar ice caps of our planet and the ways that it changes naturally, and in response to human activity.

Douglas, BC, Agreen RW, Sandwell DT.  1984.  Observing Global Ocean Circulation with Seasat Altimeter Data. Marine Geodesy. 8:67-83. AbstractWebsite
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