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Zhang, SJ, Sandwell DT.  2017.  Retracking of SARAL/AltiKa Radar Altimetry Waveforms for Optimal Gravity Field Recovery. Marine Geodesy. 40:40-56.   10.1080/01490419.2016.1265032   AbstractWebsite

The accuracy of the marine gravity field derived from satellite altimetry depends on dense track spacing as well as high range precision. Here, we investigate the range precision that can be achieved using a new shorter wavelength Ka-band altimeter AltiKa aboard the SARAL spacecraft. We agree with a previous study that found that the range precision given in the SARAL/AltiKa Geophysical Data Records is more precise than that of Ku-band altimeter by a factor of two. Moreover, we show that two-pass retracking can further improve the range precision by a factor of 1.7 with respect to the 40 Hz-retracked data (item of range_40 hz) provided in the Geophysical Data Records. The important conclusion is that a dedicated Ka-band altimeter-mapping mission could substantially improve the global accuracy of the marine gravity field with complete coverage and a track spacing of <6 km achievable in similar to 1.3 years. This would reveal thousands of uncharted seamounts on the ocean floor as well as important tectonic features such as microplates and abyssal hill fabric.

Zhang, SJ, Sandwell DT, Jin TY, Li DW.  2017.  Inversion of marine gravity anomalies over southeastern China seas from multi-satellite altimeter vertical deflections. Journal of Applied Geophysics. 137:128-137.   10.1016/j.jappgeo.2016.12.014   AbstractWebsite

The accuracy and resolution of marine gravity field derived from satellite altimetry mainly depends on the range precision and dense spatial distribution. This paper aims at modeling a regional marine gravity field with improved accuracy and higher resolution (1' x V') over Southeastern China Seas using additional data from CryoSat-2 as well as new data from AltiKa. Three approaches are used to enhance the precision level of satellite-derived gravity anomalies. Firstly we evaluate a suite of published retracking algorithms and find the two-step retracker is optimal for open ocean waveforms. Secondly, we evaluate the filtering and resampling procedure used to reduce the full 20 or 40 Hz data to a lower rate having lower noise. We adopt a uniform low-pass filter for all altimeter missions and resample at 5 Hz and then perform a second editing based on sea surface slope estimates from previous models. Thirdly, we selected WHU12 model to update the corrections provided in geophysical data record. We finally calculated the 1' x 1' marine gravity field model by using EGM2008 model as reference field during the remove/restore procedure. The root mean squares of the discrepancies between the new result and DTU10, DTU13, V23.1, EGM2008 are within the range of 1.8-3.9 mGal, while the verification with respect to shipboard gravity data shows that the accuracy of the new result reached a comparable level with DTU13 and was slightly superior to V23.1, DTU10 and EGM2008 models. Moreover, the new result has a 2 mGal better accuracy over open seas than coastal areas with shallow water depth. (C) 2016 Elsevier B.V. All rights reserved.

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Yale, MM, Sandwell DT.  1999.  Stacked global satellite gravity profiles. Geophysics. 64:1748-1755.   10.1190/1.1444680   AbstractWebsite

Gravity field recovery from satellite altimetry provides global marine coverage but lacks the accuracy and resolution needed for many exploration geophysics studies. The repeating ground tracks of the ERS-1/2, Geosat, and Topex/Poseidon altimeters offer the possibility of improving the accuracy and resolution of gravity anomalies along widely spaced (similar to 40-km spacing) tracks. However, complete ocean coverage is usually needed to convert the sea-surface height (br along-track slope) measurements into gravity anomalies. Here we develop and test a method for constructing stacked gravity profiles by using a published global gravity grid (Sandwell and Smith, 1997), V7.2, as a reference model for the slope-to-gravity anomaly conversion. The method is applied to stacks (averages) of Geosat/ERM (up to 62 cycles), ERS-1/2 (up to 43 cycles), and Topex (up to 142 cycles) satellite altimeter profiles. We assess the accuracies of the ERS-1/2 profiles through a comparison with a gravity model of the northern Gulf of Mexico (profiles provided by EDCON Inc.). The 40 ERS profiles evaluated have a mean rms difference of 3.77 mGal and full wavelength resolution (0.5 coherence) of 24 km. Our processing retains wavelengths as short as 10 km so smaller, large-amplitude features can be resolved, especially in shallow ocean areas (<1000 m deep). We provide an example of combining these higher resolution profiles with lower resolution gravity data in the Caspian Sea.

Yale, MM, Sandwell DT, Smith WHF.  1995.  Comparison of Along-Track Resolution of Stacked Geosat, Ers-1, and Topex Satellite Altimeters. Journal of Geophysical Research-Solid Earth. 100:15117-15127.   10.1029/95jb01308   AbstractWebsite

Cross-spectral analysis of repeat satellite altimeter profiles was performed to compare the along-track resolution capabilities of Geosat, ERS 1 and TOPEX data. Geophysical Data Records were edited, differentiated, low-pass-filtered, and resampled at 5 Hz. All available data were then loaded into three-dimensional files where repeat cycles were aligned along-track (62 cycles of Geosat/Exact Repeat Mission; 16 cycles of ERS 1, 35-day orbit; 73 cycles of TOPEX). The coherence versus wave number between pairs of repeat profiles was used to estimate along-track resolution for individual cycles, eight-cycle-average profiles, and 31-cycle-average profiles (Geosat and TOPEX only). Coherence, which depends on signal to noise ratio, reflects factors such as seafloor gravity amplitude, regional seafloor depth, instrument noise, oceanographic noise, and the number of cycles available for stacking (averaging). Detailed resolution analyses are presented for two areas: the equatorial Atlantic, a region with high tectonic signal and low oceanographic noise; and the South Pacific, a region with low tectonic signal and high oceanographic variability. For all three altimeters, along-track resolution is better in the equatorial Atlantic than in the South Pacific. Global maps of along-track resolution show considerable geographic variation. On average globally, the along-track resolution (0.5 coherence) of eight-cycle stacks are approximately the same, 28, 29, and 30 km for TOPEX, Geosat, and ERS 1, respectively. TOPEX 31-cycle stacks (22 km) resolve slightly shorter wavelengths than Geosat 31-cycle stacks (24 km). The stacked data, which are publicly available, will be used in future global gravity grids, and for detailed studies of mid-ocean ridge axes, fracture zones, sea mounts, and seafloor roughness.

Yale, M, Sandwell D, Herring A.  1998.  What are the limitations of satellite altimetry? The Leading Edge. 17:73-76.: Society of Exploration Geophysicists   10.1190/1.1437832   AbstractWebsite

Radar altimeter measurements of the marine geoid collected during the Seasat altimeter mission gave geophysicists hope of uncovering the gravity field over all the ocean basins. However, because of insufficient track density, it has taken 16 years for the full potential of the satellite altimeter to be realized. The high‐density coverage obtained by ERS-1 during its geodetic mapping phase (April 1994–March 1995) prompted the U.S. Navy to declassify all of the Geosat altimeter data in June 1995. The combination of these two high‐density data sets provided the first global view of all the ocean basins at a wavelength resolution of 20–30 km.

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Xu, XH, Ward LA, Jiang JL, Smith-Konter B, Tymofyeyeva E, Lindsey EO, Sylvester AG, Sandwell DT.  2018.  Surface creep rate of the southern San Andreas Fault modulated by stress perturbations from nearby large events. Geophysical Research Letters. 45:10259-10268.   10.1029/2018gl080137   AbstractWebsite

A major challenge for understanding the physics of shallow fault creep has been to observe and model the long-term effect of stress changes on creep rate. Here we investigate the surface creep along the southern San Andreas fault (SSAF) using data from interferometric synthetic aperture radar spanning over 25 years (ERS 1992-1999, ENVISAT 2003-2010, and Sentinel-1 2014-present). The main result of this analysis is that the average surface creep rate increased after the Landers event and then decreased by a factor of 2-7 over the past few decades. We consider quasi-static and dynamic Coulomb stress changes on the SSAF due to these three major events. From our analysis, the elevated creep rates after the Landers can only be explained by static stress changes, indicating that even in the presence of dynamically triggered creep, static stress changes may have a long-lasting effect on SSAF creep rates. Plain Language Summary There are two significant conclusions from this study. First, we analyzed 25 years of InSAR measurements over the Southern San Andreas Fault system to document a major increase in the average creep rate following the 1992 Mw 7.3 Landers Earthquake which is then followed by creep rate reductions after the 1999 Mw 7.1 Hector Mine Earthquake and the 2010 Mw 7.2 El Major Cucapah Earthquake. Second, we attribute all these creep rate changes to the Coulomb stress variations from these three major Earthquakes. The dynamic Coulomb stress changes are similar for all three events, contributing to triggered creep on the SSAF. In contrast, the static Coulomb stress changes on the SSAF are positive after the Landers and negative after the Hector Mine and El Major Cucapah, coinciding with the higher average creep rate after the Landers and lower rates after the other two events. An implication of this study is that small but steady Coulomb stress changes have a larger impact on shallow creep than the larger dynamic stress changes associated with passing seismic waves. These results illuminate the significance of time scale-dependent complexity of shallow fault creep and how these behaviors are communicated by stress perturbations from regional earthquakes.

Xu, XH, Sandwell DT, Bassett D.  2018.  A spectral expansion approach for geodetic slip inversion: implications for the downdip rupture limits of oceanic and continental megathrust earthquakes. Geophysical Journal International. 212:400-411.   10.1093/gji/ggx408   AbstractWebsite

We have developed a data-driven spectral expansion inversion method to place bounds on the downdip rupture depth of large megathrust earthquakes having good InSAR and GPS coverage. This inverse theory approach is used to establish the set of models that are consistent with the observations. In addition, the inverse theory method demonstrates that the spatial resolution of the slip models depends on two factors, the spatial coverage and accuracy of the surface deformation measurements, and the slip depth. Application of this method to the 2010 M-w 8.8 Maule Earthquake shows a slip maximum at 19 km depth tapering to zero at similar to 40 km depth. In contrast, the continent-continent megathrust earthquakes of the Himalayas, for example 2015 M-w 7.8 Gorkha Earthquake, shows a slip maximum at 9 km depth tapering to zero at similar to 18 km depth. The main question is why is the maximum slip depth of the continental megathrust earthquake only 50 per cent of that observed in oceanic megathrust earthquakes. To understand this difference, we have developed a simple 1-D heat conduction model that includes the effects of uplift and surface erosion. The relatively low erosion rates above the ocean megathrust results in a geotherm where the 450-600 degrees C transition is centred at similar to 40 km depth. In contrast, the relatively high average erosion rates in the Himalayas of similar to 1 mm yr-1 results in a geotherm where the 450-600 degrees C transition is centred at similar to 20 km. Based on these new observations and models, we suggest that the effect of erosion rate on temperature explains the difference in the maximum depth of the seismogenic zone between Chile and the Himalayas.

Xu, X, Tong X, Sandwell DT, Milliner CWD, Dolan JF, Hollingsworth J, Leprince S, Ayoub F.  2016.  Refining the shallow slip deficit. Geophysical Journal International. 204:1867-1886.   10.1093/gji/ggv563   Abstract

Geodetic slip inversions for three major (Mw > 7) strike-slip earthquakes (1992 Landers, 1999 Hector Mine and 2010 El Mayor–Cucapah) show a 15–60 per cent reduction in slip near the surface (depth < 2 km) relative to the slip at deeper depths (4–6 km). This significant difference between surface coseismic slip and slip at depth has been termed the shallow slip deficit (SSD). The large magnitude of this deficit has been an enigma since it cannot be explained by shallow creep during the interseismic period or by triggered slip from nearby earthquakes. One potential explanation for the SSD is that the previous geodetic inversions lack data coverage close to surface rupture such that the shallow portions of the slip models are poorly resolved and generally underestimated. In this study, we improve the static coseismic slip inversion for these three earthquakes, especially at shallow depths, by: (1) including data capturing the near-fault deformation from optical imagery and SAR azimuth offsets; (2) refining the interferometric synthetic aperture radar processing with non-boxcar phase filtering, model-dependent range corrections, more complete phase unwrapping by SNAPHU (Statistical Non-linear Approach for Phase Unwrapping) assuming a maximum discontinuity and an on-fault correlation mask; (3) using more detailed, geologically constrained fault geometries and (4) incorporating additional campaign global positioning system (GPS) data. The refined slip models result in much smaller SSDs of 3–19 per cent. We suspect that the remaining minor SSD for these earthquakes likely reflects a combination of our elastic model's inability to fully account for near-surface deformation, which will render our estimates of shallow slip minima, and potentially small amounts of interseismic fault creep or triggered slip, which could ‘make up’ a small percentages of the coseismic SSD during the interseismic period. Our results indicate that it is imperative that slip inversions include accurate measurements of near-fault surface deformation to reliably constrain spatial patterns of slip during major strike-slip earthquakes.

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Winterer, EL, Sandwell DT.  1987.  Evidence from EN-Echelon Cross-Grain Ridges for Tensional Cracks in the Pacific Plate. Nature. 329:534-537.   10.1038/329534a0   AbstractWebsite

Sea-floor topography in the Pacific is mainly aligned with original spreading directions1, but is overprinted by alignments created by mid-plate processes. Spreading produces abyssal hills and fracture zones, and mid-plate volcanism generates seamounts, isolated or in chains. A different category of topography, the 'Cross-grain', discovered in geoid-height data collected by the Seasat radar altimeter2, comprises linear troughs and swells spaced ~200 km apart, oblique to fracture zones and abyssal hills but parallel to the Hawaiian chain. Three models have been proposed for the Cross-grain: small-scale convection, organized into longitudinal rolls by the shear of the Pacific Plate2; compressive buckling3; and lithospheric boudinage resulting from plate-wide tensile stresses4,5. None of the previously available data ruled out any of these models. Here we report multi-beam bathymetric data revealing long, narrow en-echelon ridges along the Cross-grain, interpreted as evidence of tension cracks in the Pacific plate.

Wessel, P, Sandwell DT, Kim SS.  2010.  The Global Seamount Census. Oceanography. 23:24-33. AbstractWebsite

Seamounts are active or extinct undersea volcanoes with heights exceeding similar to 100 m. They represent a small but significant fraction of the volcanic extrusive budget for oceanic seafloor and their distribution gives information about spatial and temporal variations in intraplate volcanic activity. In addition, they sustain important ecological communities, determine habitats for fish, and act as obstacles to Currents, thus enhancing tidal energy dissipation and ocean mixing. Mapping the complete global distribution will help constrain models of seamount formation as well as aid in understanding marine habitats and deep ocean circulation. Two approaches have been used to map the global seamount distribution. Depth soundings from single- and multibeam echosounders can provide the most detailed maps with up to 200-m horizontal resolution. However, soundings from the > 5000 publicly available cruises sample only a small fraction of the ocean floor. Satellite altimetry can detect seamounts taller than similar to 1.5 km, and. studies using altimetry have produced seamount catalogues holding almost 13,000 seamounts. Based on the size-frequency relationship for larger seamounts, we predict over 100,000 seamounts > 1 km in height remain uncharted, and speculatively 25 million > 100 m in height. Future altimetry missions could improve on resolution and significantly decrease noise levels, allowing for an even larger number of intermediate (1-1.5-km height) seamounts to be detected. Recent retracking of the radar altimeter waveforms to improve the accuracy of the gravity field has resulted in a twofold increase in resolution. Thus, improved analyses of existing altimetry with better calibration from multibeam bathymetry could also increase census estimates.

Wei, M, Sandwell D, Fialko Y, Bilham R.  2011.  Slip on faults in the Imperial Valley triggered by the 4 April 2010 Mw 7.2 El Mayor-Cucapah earthquake revealed by InSAR. Geophysical Research Letters. 38   10.1029/2010gl045235   AbstractWebsite

Radar interferometry (InSAR), field measurements and creepmeters reveal surface slip on multiple faults in the Imperial Valley triggered by the main shock of the 4 April 2010 El Mayor-Cucapah M(w) 7.2 earthquake. Co-seismic offsets occurred on the San Andreas, Superstition Hills, Imperial, Elmore Ranch, Wienert, Coyote Creek, Elsinore, Yuha, and several minor faults near the town of Ocotillo at the northern end of the mainshock rupture. We documented right-lateral slip (<40 mm) on northwest-striking faults and left-lateral slip (<40 mm) on southwest-striking faults. Slip occurred on 15-km- and 20-km-long segments of the San Andreas Fault in the Mecca Hills (<= 50 mm) and Durmid Hill (<= 10 mm) respectively, and on 25 km of the Superstition Hills Fault (<= 37 mm). Field measurements of slip on the Superstition Hills Fault agree with InSAR and creepmeter measurements to within a few millimeters. Dislocation models of the InSAR data from the Superstition Hills Fault confirm that creep in this sequence, as in previous slip events, is confined to shallow depths (<3 km). Citation: Wei, M., D. Sandwell, Y. Fialko, and R. Bilham (2011), Slip on faults in the Imperial Valley triggered by the 4 April 2010 Mw 7.2 El Mayor-Cucapah earthquake revealed by InSAR, Geophys. Res. Lett., 38, L01308, doi:10.1029/2010GL045235.

Wei, M, Sandwell D.  2006.  Estimates of heat flow from Cenozoic seafloor using global depth and age data. Tectonophysics. 417:325-335.   10.1016/j.tecto.2006.02.004   AbstractWebsite

The total heat output of the Earth constrains models of mantle and core dynamics. Previously published estimates (42-44 TW) have recently been questioned because the measured conductive heat flow on young oceanic lithosphere is about a factor of 2 less than the expected heat flow based on half-space cooling models. Taking the conductive ocean heat flow values at face value reduces the global heat flow from 44 to 31 TW, which has major implications for geodynamics and Earth history. To help resolve this issue, we develop a new method of estimating total oceanic heat flow from depth and age data. The overall elevation of the global ridge system, relative to the deep ocean basins, provides an independent estimate of the total heat content of the lithosphere. Heat flow is proportional to the measured subsidence rate times the heat capacity divided by the thermal expansion coefficient. The largest uncertainty in this method is due to uncertainties in the thermal expansion coefficient and heat capacity. Scalar subsidence rate is computed from gradients of depth and age grids. The method cannot be applied over very young seafloor (< 3 Ma) where age gradient is discontinuous and the assumption of isostasy is invalid. Between 3 and 66 Ma, the new estimates are in agreement with half-space cooling model. Our rnodel-independent estimate of the total heat output of Cenozoic seafloor is 18.6 to 20.5 TW, which leads to a global output of 42 to 44 TW in agreement with previous studies. (c) 2006 Elsevier B.V. All rights reserved.

Wei, M, Sandwell D, Fialko Y.  2009.  A silent M-w 4.7 slip event of October 2006 on the Superstition Hills fault, southern California. Journal of Geophysical Research-Solid Earth. 114   10.1029/2008jb006135   AbstractWebsite

During October 2006, the 20-km-long Superstition Hills fault (SHF) in the Salton Trough, southern California, slipped aseismically, producing a maximum offset of 27 mm, as recorded by a creepmeter. We investigate this creep event as well as the spatial and temporal variations in slip history since 1992 using ERS-1/2 and Envisat satellite data. During a 15-year period, steady creep is punctuated by at least three events. The first two events were dynamically triggered by the 1992 Landers and 1999 Hector Mine earthquakes. In contrast, there is no obvious triggering mechanism for the October 2006 event. Field measurements of fault offset after the 1999 and 2006 events are in good agreement with the interferometric synthetic aperture radar data indicating that creep occurred along the 20-km-long fault above 4 km depth, with most of the slip occurring at the surface. The moment released during this event is equivalent to a M-w 4.7 earthquake. This event produced no detectable aftershocks and was not recorded by the continuous GPS stations that were 9 km away. Modeling of the long-term creep from 1992 to 2007 creep using stacked ERS-1/2 interferograms also shows a maximum creep depth of 2-4 km, with slip tapering with depth. Considering that the sediment thickness varies between 3 km and 5 km along the SHF, our results are consistent with previous studies suggesting that shallow creep is controlled by sediment depth, perhaps due to high pore pressures in the unconsolidated sediments.

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.

Wei, M, Sandwell DT.  2010.  Decorrelation of L-Band and C-Band Interferometry Over Vegetated Areas in California. IEEE Transactions on Geoscience and Remote Sensing. 48:2942-2952.   10.1109/tgrs.2010.2043442   AbstractWebsite

Temporal decorrelation is one of the main limitations for recovering interseismic deformation along the San Andreas Fault system using interferometric synthetic aperture radar. To assess the improved correlation properties of L-band with respect to C-band, we analyzed L-band Advanced Land Observation Satellite (ALOS) interferograms with a range of temporal and spatial baselines over three vegetated areas in California and compared them with corresponding C-band European Remote Sensing Satellite (ERS) interferograms. Over the highly vegetated Northern California forests in the Coast Range area, ALOS remains remarkably well correlated over a 2-year period, whereas an ERS interferogram with a similar temporal and spatial baseline lost correlation. In Central California near Parkfield, we found a similar pattern in decorrelation behavior, which enabled the recovery of a fault creep and a local uplifting signal at L-band that was not apparent at C-band. In the Imperial Valley in Southern California, both ALOS and ERS have low correlation over farmlands. ALOS has lower correlation over some sandy surfaces than ERS, probably due to low signal-to-noise ratio. In general, L-band interferograms with similar seasonal acquisitions have higher correlation than those with dissimilar season. For both L-and C-band, correlation over vegetated areas decreases with time for intervals less than 1 year and then remains relatively constant at longer time intervals. The decorrelation time for L-band is more than 2 years in the forest in California whereas that for C-band is less than 6 months. Overall, these results suggest that L-band interferograms will reveal near-fault interseismic deformation once sufficient data become available.

Wdowinski, S, Smith-Konter B, Bock Y, Sandwell D.  2007.  Diffuse interseismic deformation across the Pacific-North America plate boundary. Geology. 35:311-314.   10.1130/g22938a.1   AbstractWebsite

Crustal movements and deformation within the diffuse Pacific-North America (Pa-NA) plate boundary are dominated by the right-lateral motion between the two plates. By using the Pa-NA pole of rotation (PoR) spherical coordinate system, we decompose observed crustal movements into parallel and normal components to the Pa-NA plate motion. We transformed the 840 velocity vectors of the Southern California Earthquake Center (SCEC) 3.0 velocity field into the Pa-NA PoR system in order to characterize the interseismic velocity across the plate boundary. Our results show that despite the very different deformation styles occurring across the San Andreas fault, the fault trace follows the half plate motion contour. Deviation occurs in the southern section, where the half motion contour correlates with the San Jacinto and Imperial fault segments. Our analysis yields interesting asymmetric patterns in both parallel and normal components. The parallel component shows asymmetrical velocity gradients across the San Andreas fault, and the normal component indicates compression southwest of the Big Bend, but not northeastward. The observations are compared with viscoelastic modeling results, which show a similar velocity field. The main disagreements between the observations and the model are in a narrow band along the San Andreas fault and in the Mojave block, suggesting that crustal heterogeneities and additional unmodeled fault segments should be considered in future models.

Watts, AB, Sandwell DT, Smith WHF, Wessel P.  2006.  Global gravity, bathymetry, and the distribution of submarine volcanism through space and time. Journal of Geophysical Research-Solid Earth. 111   10.1029/2005jb004083   AbstractWebsite

[ 1] The seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin. Relatively few of these features (< similar to 0.1%), however, have been dated, and so little is known about their tectonic setting. One parameter that is sensitive to whether a seamount formed on, near, or far from a mid-ocean ridge is the elastic thickness, T(e), which is a proxy for the long-term strength of the lithosphere. Most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of T(e) and then comparing the calculated and observed gravity anomaly. The problem with such an approach is that bathymetry data are usually limited to single-beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape. We therefore use the satellite-derived gravity anomaly to predict the bathymetry for different values of T(e). By comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the Wessel global seamount database, we have obtained 9758 T(e) estimates from a wide range of submarine volcanic features in the Pacific, Indian, and Atlantic oceans. Comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate T(e) and its upper and lower bounds. T(e) at sites where there is both a sample and crustal age show considerable scatter, however, and there is no simple relationship between T(e) and age. Nevertheless, we are able to tentatively assign a tectonic setting to each T(e) estimate. The most striking results are in the Pacific Ocean where a broad swath of "on-ridge'' volcanism extends from the Foundation seamounts and Ducie Island/Easter Island ridge in the southeast, across the equator, to the Shatsky and Hess rises in the northwest. Interspersed among the on-ridge volcanism are "flank ridge'' and "off-ridge'' features. The Indian and Atlantic oceans also show a mix of tectonic settings. Off-ridge volcanism dominates in the eastern North Atlantic and northeast Indian oceans, while flank ridge volcanism dominates the northeastern Indian and western south Atlantic oceans. We have been unable to assign the flank ridge and off-ridge estimates an age, but the on-ridge estimates generally reflect, we believe, the age of the underlying oceanic crust. We estimate the volume of on-ridge volcanism to be similar to 1.1 x 10(6) km(3) which implies a mean seamount addition rate of similar to 0.007 km(3) yr(-1). Rates appear to have varied through geological time, reaching their peak during the Late/Early Cretaceous and then declining to the present-day.

Watson, KM, Bock Y, Sandwell DT.  2002.  Satellite interferometric observations of displacements associated with seasonal groundwater in the Los Angeles basin. Journal of Geophysical Research-Solid Earth. 107   10.1029/2001jb000470   AbstractWebsite

[1] The Newport-Inglewood fault zone (NIFZ) displays interferometric synthetic aperture radar (SAR) phase features along most of its length having amplitudes of up to 60 mm. However, interpretation in terms of right-lateral, shallow slip along the fault fails to match the range of geologic estimates of slip. Recently, Bawden et al. [2001] proposed that these phase features, as well as a broader deformation pattern in the Los Angeles basin, are due to vertical motion related to annual variations in the elevation of the water table. We confirm this hypothesis through the analysis of a longer span of data consisting of 26 SAR images collected by the ERS-1 and ERS-2 spacecraft between June 1992 and June 2000. Moreover, we use continuous GPS measurements from 1995 to the present to establish the amplitude and phase of the vertical deformation. The Los Angeles basin becomes most inflated one quarter of the way through the year, which is consistent with water table measurements as well as with the end of the rainy season when the aquifer should be at a maximum. The spatial pattern of the amplitude of the annual signal derived from continuous GPS measurements is consistent with the shape of the interferometric fringes. GPS sites both near the NIFZ and in a 20 by 40 km zone within the basin also show significant N-S annual variations that may be related to the differential expansion across the fault. Since these horizontal signals have peak-to-trough amplitudes of 6 mm, they mask the smaller tectonic signals and need to be taken into account when interpreting GPS time series of site position. Moreover, since the groundwater signal appears to have a long-term vertical trend which varies in sign depending on location, it will be difficult to distinguish interseismic tectonic slip along the NIFZ and within the affected areas in the basin.

Wagner, CA, Sandwell DT.  1984.  The Gravsat Signal over Tectonic Features. Journal of Geophysical Research. 89:4419-4426.   10.1029/JB089iB06p04419   AbstractWebsite

The range rate between two close gravitational satellites (GRAVSAT) in low earth orbit has been evaluated over model tectonic features such as mountains and ranges, fracture zones, and trenches. Models are locally compensated and consist of both point mass dipoles and sheet mass dipoles. Masses and depths of compensation are chosen to approximate known gravity signatures. The results show that for two satellites at 160 km altitude with 3° separation, significant signal power (>1 μm/s) remains for most extended features at wavelengths less than 200 km. Furthermore, there is strong sensitivity in the signal from these features to lateral and vertical changes of the order of 1 km and less. In addition, the signal of hidden geologic structures such as dikes, salt domes, and ore bodies may also stand above 1 μm/s for this low orbiting pair. Thus, it may prove to be efficient to model the high-frequency GRAVSAT signal directly in terms of the parameters of tectonic-topographic features and their compensation.

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Trugman, DT, Borsa AA, Sandwell DT.  2014.  Did stresses from the Cerro Prieto Geothermal Field influence the El Mayor-Cucapah rupture sequence? Geophysical Research Letters. 41:8767-8774.   10.1002/2014gl061959   AbstractWebsite

The M-w 7.2 El Mayor-Cucapah (EMC) earthquake ruptured a complex fault system in northern Baja California that was previously considered inactive. The Cerro Prieto Geothermal Field (CPGF), site of the world's second largest geothermal power plant, is located approximately 15km to the northeast of the EMC hypocenter. We investigate whether anthropogenic fluid extraction at the CPGF caused a significant perturbation to the stress field in the EMC rupture zone. We use Advanced Land Observing Satellite interferometric synthetic aperture radar data to develop a laterally heterogeneous model of fluid extraction at the CPGF and estimate that this extraction generates positive Coulomb stressing rates of order 15 kPa/yr near the EMC hypocenter, a value which exceeds the local tectonic stressing rate. Although we cannot definitively conclude that production at the CPGF triggered the EMC earthquake, its influence on the local stress field is substantial and should not be neglected in local seismic hazard assessments.

Tong, XP, Sandwell D, Luttrell K, Brooks B, Bevis M, Shimada M, Foster J, Smalley R, Parra H, Soto JCB, Blanco M, Kendrick E, Genrich J, Caccamise DJ.  2010.  The 2010 Maule, Chile earthquake: Downdip rupture limit revealed by space geodesy. Geophysical Research Letters. 37   10.1029/2010gl045805   AbstractWebsite

Radar interferometry from the ALOS satellite captured the coseismic ground deformation associated with the 2010 Mw 8.8 Maule, Chile earthquake. The ALOS interferograms reveal a sharp transition in fringe pattern at similar to 150 km from the trench axis that is diagnostic of the downdip rupture limit of the Maule earthquake. An elastic dislocation model based on ascending and descending ALOS interferograms and 13 near-field 3-component GPS measurements reveals that the coseismic slip decreases more or less linearly from a maximum of 17 m (along-strike average of 6.5 m) at 18 km depth to near zero at 43-48 km depth, quantitatively indicating the downdip limit of the seismogenic zone. The depth at which slip drops to near zero appears to be at the intersection of the subducting plate with the continental Moho. Our model also suggests that the depth where coseismic slip vanishes is nearly uniform along the strike direction for a rupture length of similar to 600 km. The average coseismic slip vector and the interseismic velocity vector are not parallel, which can be interpreted as a deficit in strike-slip moment release. Citation: Tong, X., et al. (2010), The 2010 Maule, Chile earthquake: Downdip rupture limit revealed by space geodesy, Geophys. Res. Lett., 37, L24311, doi:10.1029/2010GL045805.

Tong, XP, Sandwell DT, Fialko Y.  2010.  Coseismic slip model of the 2008 Wenchuan earthquake derived from joint inversion of interferometric synthetic aperture radar, GPS, and field data. Journal of Geophysical Research-Solid Earth. 115   10.1029/2009jb006625   AbstractWebsite

We derived a coseismic slip model for the M(w) 7.9 2008 Wenchuan earthquake on the basis of radar line-of-sight displacements from ALOS interferograms, GPS vectors, and geological field data. Available interferometric synthetic aperture radar (InSAR) data provided a nearly complete coverage of the surface deformation along both ascending (fine beam mode) and descending orbits (ScanSAR to ScanSAR mode). The earthquake was modeled using four subfaults with variable geometry and dip to capture the simultaneous rupture of both the Beichuan fault and the Pengguan fault. Our model misfits show that the InSAR and GPS data are highly compatible; the combined inversion yields a 93% variance reduction. The best fit model has fault planes that rotate from shallow dip in the south (35 degrees) to nearly vertical dip toward the north (70 degrees). Our rupture model is complex with variations in both depth and rake along two major fault strands. In the southern segment of the Beichuan fault, the slip is mostly thrust (<13 m) and occurred principally in the upper 10 km of the crust; the rupture progressively transformed to right-lateral strike slip as it propagated northeast (with maximum offsets of 7 m). Our model suggests that most of the moment release was limited to the shallow part of the crust (depth less than 10 km). We did not find any "shallow slip deficit" in the slip depth distribution of this mixed mechanism earthquake. Aftershocks were primarily distributed below the section of the fault that ruptured coseismically.

Tong, XP, Sandwell DT, Smith-Konter B.  2015.  An integral method to estimate the moment accumulation rate on the Creeping Section of the San Andreas Fault. Geophysical Journal International. 203:48-62.   10.1093/gji/ggv269   AbstractWebsite

Moment accumulation rate (also referred to as moment deficit rate) is a fundamental quantity for evaluating seismic hazard. The conventional approach for evaluating moment accumulation rate of creeping faults is to invert for the slip distribution from geodetic measurements, although even with perfect data these slip-rate inversions are non-unique. In this study, we show that the slip-rate versus depth inversion is not needed because moment accumulation rate can be estimated directly from surface geodetic data. We propose an integral approach that uses dense geodetic observations from Interferometric Synthetic Aperture Radar (InSAR) and the Global Positioning System (GPS) to constrain the moment accumulation rate. The moment accumulation rate is related to the integral of the product of the along-strike velocity and the distance from the fault. We demonstrate our methods by studying the Creeping Section of the San Andreas fault observed by GPS and radar interferometry onboard the ERS and ALOS satellites. Along-strike variation of the moment accumulation rate is derived in order to investigate the degree of partial locking of the Creeping Section. The central Creeping Segment has a moment accumulation rate of 0.25-3.1 x 10(15) Nm yr(-1) km(-1). The upper and lower bounds of the moment accumulation rates are derived based on the statistics of the noise. Our best-fitting model indicates that the central portion of the Creeping Section is accumulating seismic moment at rates that are about 5 per cent to 23 per cent of the fully locked Carrizo segment that will eventually be released seismically. A cumulative moment budget calculation with the historical earthquake catalogue (M > 5.5) since 1857 shows that the net moment deficit at present is equivalent to a M-w 6.3 earthquake.

Tong, XP, Smith-Konter B, Sandwell DT.  2014.  Is there a discrepancy between geological and geodetic slip rates along the San Andreas Fault System? Journal of Geophysical Research-Solid Earth. 119:2518-2538.   10.1002/2013jb010765   AbstractWebsite

Previous inversions for slip rate along the San Andreas Fault System (SAFS), based on elastic half-space models, show a discrepancy between the geologic and geodetic slip rates along a few major fault segments. In this study, we use an earthquake cycle model representing an elastic plate over a viscoelastic half-space to demonstrate that there is no significant discrepancy between long-term geologic and geodetic slip rates. The California statewide model includes 41 major fault segments having steady slip from the base of the locked zone to the base of the elastic plate and episodic shallow slip based on known historical ruptures and geologic recurrence intervals. The slip rates are constrained by 1981 secular velocity measurements from GPS and L-band intereferometric synthetic aperture radar. A model with a thick elastic layer (60 km) and half-space viscosity of 10(19)Pa s is preferred because it produces the smallest misfit to both the geologic and the geodetic data. We find that the geodetic slip rates from the thick plate model agrees to within the bounds of the geologic slip rates, while the rates from the elastic half-space model disagree on specific important fault segments such as the Mojave and the North Coast segment of the San Andreas Fault. The viscoelastic earthquake cycle models have generally higher slip rates than the half-space model because most of the faults along the SAFS are late in the earthquake cycle, so today they are moving slower than the long-term cycle-averaged velocity as governed by the viscoelastic relaxation process.