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Earthquakes pose a major threat to the people of Haiti, as tragically shown by the catastrophic 2010 Mw 7.0 earthquake and more recently by the 2021 Mw 7.2 earthquake. Both events occurred within the transpressional Enriquillo–Plantain Garden fault zone (EPGFZ), which runs through the southern peninsula of Haiti and is a major source of seismic hazard for the region. Satellite-based Interferometric Synthetic Aperture Radar (InSAR) data are used to illuminate the ground deformation patterns associated with the 2021 event. The analysis of Sentinel-1 and Advanced Land Observation Satellite (ALOS)-2 InSAR data shows (1) the broad coseismic deformation field; (2) detailed secondary fault structures as far as 12 km from the main Enriquillo–Plantain Garden fault (EPGF), which are active during and after the earthquake; and (3) postseismic shallow slip, which migrates along an ∼40 km unruptured section of the EPGF for approximately two weeks following the earthquake. The involvement of secondary faults in this rupture requires adjustments to the representation of hazard that assumes a simple segmented strike-slip EPGF. This work presents the first successful use of phase gradient techniques to map postseismic deformation in a vegetated region, which opens the door to future studies of a larger number of events in a wider variety of climates.

SUMMARY Seamounts are volcanic constructs that litter the seafloor. They are important for understanding numerous aspects of marine science, such as plate tectonics, the volcanic melt budget, oceanic circulation, tsunami wave diffraction, tidal energy dissipation and mass wasting. Geometrically, seamounts come in many sizes and shapes, and for the purpose of modelling them for morphological, gravimetric or isostatic studies it is convenient to have simple analytical models whose properties are well known. Here, we present a family of seamount models that may be used in such studies, covering both the initial construction phase and later mass-wasting by sectoral collapses. We also derive realistic axisymmetric density variations that are compatible with observed first-order structure from seismic tomography studies.

Rapid melting of ice sheets and glaciers drives a unique geometry, or fingerprint, of sea-level change, including a sea-level fall in the vicinity of the ice sheet that is an order of magnitude greater than the associated global mean sea-level rise of the melt event. The detection of individual fingerprints has been challenging due to sparse sea surface height measurements at high latitudes and the difficulty of disentangling ocean dynamic variability from the signal. Efforts to date have analyzed sea level records outside the zone of major sea-level fall, where the gradients and amplitudes of the fingerprint signal are significantly lower. We predict the fingerprint of Greenland Ice Sheet (GrIS) melt using new ice mass loss estimates from radar altimetry data and model reconstructions of nearby glaciers, and compare this prediction to an independent, altimetry-derived sea surface height trend corrected for ocean dynamic variability in the region adjacent to the ice sheet. The two fields show consistent gradients across the region, with the expected strong drawdown of the sea surface toward GrIS. A statistically significant correlation between the two fields (p < 0.001) provides the first unambiguous observational detection of the near-field sea level fingerprint of recent GrIS melting in our warming world. This detection provides a robust map of the impact of ice mass flux on global oceans since the early 1990s, and validates theoretical and numerical developments in the sea level modelling community.

Satellite radar altimetry collected during a number of geodetic missions has provided a new understanding of the topography and tectonics of the deep oceans. As altimeter performance and coverage improves, smaller structures are revealed. Here we investigate the contribution of six altimeter missions that have been placed into geodetic mapping phases for more than one year. Two types of evaluations are performed. We first compare the composite (all six altimeters) grids of east and north vertical deflection to matching grids where one altimeter has been omitted evaluate their contribution versus latitude. We then estimate the noise in each altimeter by computing the median absolute deviation of the profiles with the best composite grid. Both analyses show that SARAL/AltiKa provides the greatest contribution and ERS-1 no longer provides any significant improvement. The major limitation for recovering small scale gravity features is the sea surface roughness from ocean waves. There have been steady improvements in instrumentation and processing methods that will continue into the future with higher frequency radars and interferometric swath altimeters planned for future missions.

Sea surface slope (SSS) responds to oceanic processes and other environmental parameters. This study aims to identify the parameters that influence SSS variability. We use SSS calculated from multiyear satellite altimeter observations and focus on small resolvable scales in the 30–100-km wavelength band. First, we revisit the correlation of mesoscale ocean variability with seafloor roughness as a function of depth, as proposed by Gille et al. Our results confirm that in shallow water there is statistically significant positive correlation between rough bathymetry and surface variability, whereas the opposite is true in the deep ocean. In the next step, we assemble 27 features as input variables to fit the SSS with a linear regression model and a boosted trees regression model, and then we make predictions. Model performance metrics for the linear regression model are R2 = 0.381 and mean square error = 0.010 μrad2. For the boosted trees model, R2 = 0.563 and mean square error = 0.007 μrad2. Using the hold-out data, we identify the most important influencing factors to be the distance to the nearest thermocline boundary, significant wave height, mean dynamic topography gradient, and M2 tidal speed. However, there are individual regions, that is, the Amazon outflow, that cannot be predicted by our model, suggesting that these regions are governed by processes that are not represented in our input features. The results highlight both the value of machine learning and its shortcomings in identifying mechanisms governing oceanic phenomena.

Mapping out backward motion Most deformation associated with an earthquake is, not surprisingly, in the same direction as the fault rupture. Xu et al. used satellite imaging to find areas of deformation associated with the 2019 Ridgecrest earthquake sequence that moved in the opposite direction. These regions moved in this direction because of inelastic deformation, which helped to accommodate the overall fault rupture for the sequence. The observations were possible because of improved radar imaging and are likely more common than previously believed. Science , this issue p. 605

The Mw 6.4 and Mw 7.1 Ridgecrest earthquake sequence occurred on 4 and 5 July 2019 within the eastern California shear zone of southern California. Both events produced extensive surface faulting and ground deformation within Indian Wells Valley and Searles Valley. In the weeks following the earthquakes, more than six dozen scientists from government, academia, and the private sector carefully documented the surface faulting and ground-deformation features. As of December 2019, we have compiled a total of more than 6000 ground observations; approximately 1500 of these simply note the presence or absence of fault rupture or ground failure, but the remainder include detailed descriptions and other documentation, including tens of thousands of photographs. More than 1100 of these observations also include quantitative field measurements of displacement sense and magnitude. These field observations were supplemented by mapping of fault rupture and ground-deformation features directly in the field as well as by interpreting the location and extent of surface faulting and ground deformation from optical imagery and geodetic image products. We identified greater than 68 km of fault rupture produced by both earthquakes as well as numerous sites of ground deformation resulting from liquefaction or slope failure. These observations comprise a dataset that is fundamental to understanding the processes that controlled this earthquake sequence and for improving earthquake hazard estimates in the region. This article documents the types of data collected during postearthquake field investigations, the compilation effort, and the digital data products resulting from these efforts.

Interferometric Synthetic Aperture Radar is an important tool for imaging surface deformation from large continental earthquakes. Here, we present maps of coseismic displacement and strain from the 2019 Ridgecrest earthquakes using multiple Sentinel-1 images. We provide three types of interferometric products. (1) Standard interferograms from two look directions provide an overview of the deformation and can be used for modeling coseismic slip. (2) Phase gradient maps from stacks of coseismic interferograms provide high-resolution (∼30 m) images of strain concentration and surface fracturing that can be used to guide field surveys. (3) High-pass filtered, stacked, unwrapped phase is decomposed into east–west and up–down, south–north components and is used to determine the sense of fault slip. The resulting phase gradient maps reveal over 300 surface fractures, including triggered slip on the Garlock fault. The east–west component of high-pass filtered phase reveals the polarity of the strike-slip offset (right lateral or left lateral) for many of the fractures. We find a small number of fractures that have slip polarity that is retrograde to the background tectonic stress. This is similar to observations of retrograde slip observed near the 1999 Mw 7.1 Hector Mine rupture, but the Ridgecrest observations are more completely imaged by the frequent and high-quality acquisitions from the twin Sentinel-1 spacecrafts. Determining whether the retrograde features are triggered slip on existing faults, or compliant fault deformation in response to stress changes from the Ridgecrest earthquakes, or new Coulomb-style failures, will require additional field work, modeling, and analysis.

Radar interferograms provide a map of the phase difference between the reference and repeat acquisitions modulo ${2\pi }$ . Under ideal conditions, the phase can be unwrapped to provide an absolute phase connection across the map, although there is always an unknown integer phase ambiguity (i.e., ${N2\pi }$ ) for the entire map. Here, we demonstrate a practical time series method to solve for these integer ambiguities in order to recover the absolute phase change between the first and last SAR images. An important first step is to correct the phase of each SAR image for the well-known solid earth tide, which typically produces a line of sight offset ±150 mm, as well as, trends along and across each image of ~20 mm. This tide correction significantly reduces the noise in the InSAR time series, especially at the L-band. These tidally corrected interferograms are then unwrapped and used to solve for a set of integer ambiguities that achieves phase closure when summing around loops in the stack. There is an infinite number of ambiguity combinations that achieve loop closure; thus, regularization is required. In contrast to previous studies that use a least-squares approach to find the ambiguities, we adopt an ${L}_{1}$ -norm approach to find the minimum number of ambiguity corrections needed to achieve loop closure. We note that the split-spectrum ionospheric correction can introduce ${N\pi }$ ambiguities and suggest two approaches for correcting both ${N2\pi }$ and ${N\pi }$ ambiguities.

We calibrate a technique to use repeated multibeam sidescan surveys in the deep ocean to recover seafloor displacements greater than a few meters. Displacement measurements from seafloor patches (3...

Author(s): Yu, Yao; Sandwell, David T; Gille, Sarah T; Boas, Ana Beatriz Villas | Abstract: SUMMARY The Ice, Cloud and land Elevation Satellite 2 (ICESat-2) laser altimetry mission, launched in September 2018, uses six parallel lidar tracks with very fine along-track resolution (15nm) to measure the topography of ice, land and ocean surfaces. Here we assess the ability of ICESat-2 ocean data to recover oceanographic signals ranging from surface gravity waves to the marine geoid. We focus on a region in the tropical Pacific and study photon height data in both the wavenumber and space domain. Results show that an ICESat-2 single track can recover the marine geoid at wavelengths g20nkm which is similar to the best radar altimeter data. The wavelength and propagation direction of surface gravity waves are sometimes well resolved by using a combination of the strong and weak beams, which are separated by 90nm. We find higher than expected power in the 3–20nkm wavelength band where geoid and ocean signals should be small. This artificial power is caused by the projection of 2-D surface waves with ∼300nm wavelengths into longer wavelengths (5–10nkm) because of the 1-D sampling along the narrow ICESat-2 profile. Thus ICESat-2 will not provide major improvements to the geoid recovery in most of the ocean.

There are two dominating approaches of modeling the marine gravity field based on satellite altimetry observations. In this study, the marine gravity field is determined in four selected areas (Northwestern Atlantic, Hawaii ocean area, Mariana Trench area, and Aegean Sea) by using exact same input datasets but different methods which are based on sea surface height (SSH) and sea surface slope (SSS), respectively. The impact of the methodology is evaluated by conducting validations with shipborne gravity observation. The CryoSat-2, Jason-1/2, and SARAL/Altika geodetic mission data (similarly 3-year-long time series) are firstly retracked by the two-pass retracker. After that, the obtained SSHs are used for the derivation of geoid undulations and vertical deflections, and then for the resulting marine gravity field separately. The validation results indicate that the SSH-based method has advantages in robustly estimating marine gravity anomalies near the coastal zone. The SSS-based method has advantages over regions with intermedium ocean depths (2000–4000 m) where seamounts and ridges are found, but obvious disadvantages when the ocean currents flow along the north–south direction (e.g., western boundary currents) or the topography features north–south directional trenches. In the deep ocean where the seafloor topography is plain and smooth, the two methods have similar accuracy.

Seafloor geodetic studies such as Global Positioning System (GPS)-Acoustic experiments often require the measurement platform on the sea surface to be positioned accurately to within a few centimetres. In this paper, we test the utility of Precise Point Positioning (PPP) for this application with two experiments. The first fixed platform experiment is a comparison between three independent processing software packages: Positioning and Navigation Data Analyst (PANDA), Global Navigation Satellite System-Inferred Positioning System and Orbit Analysis Simulation Software (GIPSY-OASIS), and the Canadian Spatial Reference System (CSRS)) and a more accurate solution based on conventional differential processing of a remote GPS station in the Aleutian Islands. The second moving platform experiment is a comparison among the three PPP software packages using 40 hours of ship navigation data collected during theRoger RevelleRR1605 cruise 170 nautical miles southwest of Palau in May 2016. We found the PPP solutions were repeatable to 5·49 cm in the horizontal components and 12·4 cm in the vertical component. This demonstrates not only that PPP is a useful tool for positioning marine platforms in remote locations, but also that modern ship navigation instruments such as the Kongsberg Seapath 330 + are suitable for seafloor geodetic application.

We have developed a semi-analytic approach (and computational code) for rapidly calculating 3D time-dependent deformation and stress caused by screw dislocations imbedded within an elastic layer overlying a Maxwell viscoelastic half-space. The maxwell model is developed in the Fourier domain to exploit the computational advantages of the convolution theorem, hence substantially reducing the computational burden associated with an arbitrarily complex distribution of force couples necessary for fault modeling. The new aspect of this development is the ability to model lateral variations in shear modulus. Ten benchmark examples are provided for testing and verification of the algorithms and code. One final example simulates interseismic deformation along the San Andreas Fault System where lateral variations in shear modulus are included to simulate lateral variations in lithospheric structure.

The Cerro Prieto geothermal field (CPGF) lies at the step-over between the imperial and the Cerro Prieto faults in northern Baja California, Mexico. While tectonically this is the most active section of the southern San Andreas Fault system, the spatial and temporal deformation in the area is poorly resolved by the sparse global positioning system (GPS) network coverage. Moreover, interferograms from satellite observations spanning more than a few months are decorrelated due to the extensive agricultural activity in this region. Here we investigate the use of frequent, short temporal baseline interferograms offered by the new Sentinel-1A satellite to recover two components of deformation time series across these faults. Following previous studies, we developed a purely geometric approach for image alignment that achieves better than 1/200 pixel alignment needed for accurate phase recovery. We construct interferometric synthetic aperture radar time series using a coherence-based small baseline subset method with atmospheric corrections by means of common-point stacking. We did not apply enhanced spectral diversity because the burst discontinuities are generally small (

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′ × 1′) 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′ × 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.

Our understanding of plate boundary deformation has been enhanced by transient signals observed against the backdrop of time‐independent secular motions. We make use of a new analysis of displacement time series from about 1,000 continuous Global Positioning System (GPS) stations in California from 1999 to 2018 to distinguish tectonic and nontectonic transients from secular motion. A primary objective is to define a high‐resolution three‐dimensional reference frame (datum) for California that can be rapidly maintained with geodetic data to accommodate both secular and time‐dependent motions. To this end, we compare the displacements to those predicted by a horizontal secular fault slip model for the region and construct displacement and strain rate fields. Over the past 19 years, California has experienced 19 geodetically detectable earthquakes and widespread postseismic deformation. We observe postseismic strain rate variations as large as 1,000 nstrain/year with moment releases equivalent up to an Mw6.8 earthquake. We find significant secular differences up to 10 mm/year with the fault slip model, from the Mendocino Triple Junction to the southern Cascadia subduction zone, the northern Basin and Range, and the Santa Barbara channel. Secular vertical uplift is observed across the Transverse Ranges, Coastal Ranges, Sierra Nevada, as well as large‐scale postseismic uplift after the 1999 Mw7.1 Hector Mine and 2010 Mw7.2 El Mayor‐Cucapah earthquakes. We also identify areas of vertical land motions due to anthropogenic, natural, and magmatic processes. Finally, we demonstrate the utility of the kinematic datum by improving the accuracy of high‐spatial‐resolution 12‐day repeat‐cycle Sentinel‐1 Interferometric Synthetic Aperture Radar displacement and velocity maps., Key Points Displacement fields from 19 years of GPS data in California are preferable to velocity fields in characterizing 3‐D transient motionsMapping of secular and transient motions at high temporal and spatial resolution is required for maintaining a 3‐D kinematic reference frameDisplacement fields derived from continuous GPS and InSAR provide 200‐m resolution with millimeter‐level velocity uncertainties

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 achiev...

We present a method for interpolation of sparse two-dimensional vector data. The method is based on the Green's functions of an elastic body subjected to in-plane forces. This approach ensures elastic coupling between the two components of the interpolation. Users may adjust the coupling by varying Poisson's ratio. Smoothing can be achieved by ignoring the smallest eigenvalues in the matrix solution for the strengths of the unknown body forces. We demonstrate the method using irregularly distributed GPS velocities from southern California. Our technique has been implemented in both GMT and MATLAB®.

Accurate seafloor geodetic methods are critical to the study of marine natural hazards such as megathrust earthquakes, landslides, and volcanoes. We propose digital image correlation of repeated shipboard sidescan sonar surveys as a measurement of seafloor deformation. We test this method using multibeam surveys collected in two locales: 2500 m deep lightly sedimented seafloor on the flank of a spreading ridge and 4300 m deep heavily sedimented seafloor far from any plate boundary. Correlation of these surveys are able to recover synthetic displacements in the across-track (range) direction accurate to within 1 m and in the along-track (azimuth) direction accurate to within 1–10 m. We attribute these accuracies to the inherent resolution of sidescan data being better in the range dimension than the azimuth dimension. These measurements are primarily limited by the accuracy of the ship navigation. Dual-frequency GPS units are accurate to ∼10 cm, but single-frequency GPS units drift on the order of 1 m/h and are insufficient for geodetic application.

We describe the uncertainties of altimetry-predicted bathymetry models and then quantify the impact of this uncertainty in tsunami hazard assessments. The study consists of three stages. First, we study the statistics of errors of altimetry-predicted bathymetry models. Second, we employ these statistics to propose a random field model for errors anywhere. Third, we use bathymetry samples to conduct a Monte Carlo simulation and describe the tsunami response uncertainty. We found that bathymetry uncertainties have a greater impact in shallow areas. We also noted that tsunami leading waves are less affected by uncertainties.Recorded Presentation from the vICCE (YouTube Link): https://youtu.be/zzL_XWWAQ7o

Author(s): Funning, Gareth J; Brooks, Benjamin; Fialko, Yuri; Floyd, Michael; Haase, Jennifer; Hammond, William; Sandwell, David T; Svarc, Jerry; Xu, Xiaohua

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. Published version

The aim of this study is to assess the impact of sea level changes on both the stress field and the potential of fault reactivation in west Iberia. The analysis is applied to a set of five active faults distributed across Portugal, selected for representing predominant fault directions and for being seismically active. The results show that the rise of sea level since the Last Glacial Maximum has produced flexural effects with distinct impacts on different faults. The Coulomb stress changes induced by the sea level rise along the S. Marcos-Quarteira (south Portugal) and the Horseshoe (offshore SW Iberia) faults are found to be extremely small, independently of the elastic plate thickness. These faults are thus unaffected by flexural effects related to ocean loading, and are unlikely to possess any paleoseismic record of this phenomenon. In contrast, the eustatic sea level rise during the late Pleistocene could have raised the Coulomb stress by 0.5–1 MPa along the Manteigas–Vilarica–Braganca (north Portugal) and Lower Tagus Valley (Lisbon area) fault systems. Such stress perturbations are probably sufficient to impact the seismic cycle of the Manteigas–Vilarica–Braganca fault, bringing it closer to failure and possibly triggering the earthquake clusters that have been observed in previous paleoseismologic studies.

Interferometric synthetic aperture radar (InSAR) is a key tool for the analysis of displacement and stress changes caused by large crustal earthquakes, particularly in remote areas. A challenge for traditional InSAR has been its limited spatial and temporal coverage especially for very large events, whose dimensions exceed the typical swath width of 70–100 km. This problem is addressed by the ALOS-2 satellite, whose PALSAR-2 instrument operates in ScanSAR mode, enabling a repeat time of 2 weeks and a swath width of 350 km. Here we present InSAR line-of-sight displacement data from ALOS-2/PALSAR-2 observations covering the Mw 7.8 Gorkha, Nepal earthquake and its Mw 7.3 aftershock that were acquired within 1 week of each event. The data are made freely available and we encourage their use in models of the fault slip and associated stress changes. The Mw 7.3 aftershock not only extended the rupture area of the main shock toward the east but also left a 20 km gap where the fault has little or no coseismic slip. We estimate this unslipped fault patch has the potential to generate a Mw 6.9 event.

S U M M A R Y Thin plate flexure theory provides an accurate model for the response of the lithosphere to vertical loads on horizontal length scales ranging from tens to hundreds of kilometres. Examples include flexure at seamounts, fracture zones, sedimentary basins and subduction zones. When applying this theory to real world situations, most studies assume a locally uniform plate thickness to enable simple Fourier transform solutions. However, in cases where the amplitude of the flexure is prominent, such as subduction zones, or there are rapid variations in seafloor age, such as fracture zones, these models are inadequate. Here we present a computationally efficient algorithm for solving the thin plate flexure equation for non-uniform plate thickness and arbitrary vertical load. The iterative scheme takes advantage of the 2-D fast Fourier transform to perform calculations in both the spatial and spectral domains, resulting in an accurate and computationally efficient solution. We illustrate the accuracy of the method through comparisons with known analytic solutions. Finally, we present results from three simple models demonstrating the differences in trench outer rise flexure when 2-D variations in plate rigidity and applied bending moment are taken into account. Although we focus our analysis on ocean trench flexure, the method is applicable to other 2-D flexure problems having spatial rigidity variations such as seamount loading of a thermally eroded lithosphere or flexure across the continental–oceanic crust boundary.

The Mw 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 15 km 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.

We investigate the spatial pattern of surface creep and off-fault deformation along the southern segment of the San Andreas Fault using a combination of multiple interferometric synthetic aperture radar viewing geometries and survey-mode GPS occupations of a dense array crossing the fault. Radar observations from Envisat during the period 2003-2010 were used to separate the pattern of horizontal and vertical motion, providing a high-resolution image of uplift and shallow creep along the fault trace. The data reveal pervasive shallow creep along the southernmost 50 km of the fault. Creep is localized on a well-defined fault trace only in the Mecca Hills and Durmid Hill areas, while elsewhere creep appears to be distributed over a 1-2 km wide zone surrounding the fault. The degree of strain localization is correlated with variations in the local fault strike. Using a two-dimensional boundary element model, we show that stresses resulting from slip on a curved fault can promote or inhibit inelastic failure within the fault zone in a pattern matching the observations. The occurrence of shallow, localized interseismic fault creep within mature fault zones may thus be partly controlled by the local fault geometry and normal stress, with implications for models of fault zone evolution, shallow coseismic slip deficit, and geologic estimates of long-term slip rates.

Suban field in southern Sumatra, Indonesia, is a fractured carbonate/crystalline wet-gas reservoir in a tectonically active island-arc setting. Reservoir-scale right-oblique reverse faults and folds that have trapped the hydrocarbons have been related previously to deformation in the back-arc setting of Sumatra associated with oblique subduction of the Indo-Australian Plate at the Sunda trench. Increased well productivity in some parts of the field was inferred to correlate with completing wells in the damage zones of critically stressed faults. Satellite interferometry acquired in 2008 through 2011 involving specially stacked and filtered interferograms, following prior applications to heavy-oil fields and Arctic sea ice, reveals active deformation in Suban field. Several areas of localized subsidence potentially exceeding ∼5 mm/yr have been identified in the field. Horizontal movements of comparable magnitude were resolved above the major right-oblique, critically stressed fault zone in the southwestern part of the field, corroborating wellbore-based inferences from the reservoir. Block-tectonic models constrained by GPS measurements across the entire Sumatra contractional orogen predict comparable magnitudes and directions for horizontal motions observed locally at Suban. The combination of InSAR and GPS-based plate-tectonic models provides a robust tool for monitoring the deformation of oil and gas fields in tectonically active areas.

Interseismic motion along complex strike-slip fault systems such as the San Andreas Fault System (SAFS) can produce vertical velocities that are ~10 times smaller than horizontal velocities, caused by along-strike variations in fault orientation and locking depth. Tide gauge stations provide a long (50–100 year) recording history of sea level change due to several oceanographic and geologic processes, including vertical earthquake cycle deformation. Here we compare relative sea level displacements with predictions from a 3-D elastic/viscoelastic earthquake cycle model of the SAFS. We find that models with lithospheric structure reflecting a thick elastic plate (>50 km) and moderate viscosities produce vertical motions in surprisingly good agreement with the relative tide gauge uplift rates. These results suggest that sea level variations along the California coastline contain a small but identifiable tectonic signal reflecting the flexure of the elastic plate caused by bending moments applied at the ends of locked faults.

We develop a slope correction model to improve the accuracy of mean sea surface topography models as well as marine gravity models. The correction is greatest above ocean trenches and large seamounts where the slope of the geoid exceeds 100 $$\upmu $$ rad. In extreme cases, the correction to the mean sea surface height is 40 mm and the correction to the along-track altimeter slope is 1–2 $$\upmu $$ rad which maps into a 1–2 mGal gravity error. Both corrections are easily applied using existing grids of sea surface slope from satellite altimetry.

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 1019Pa 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.

Author(s): Gonzalez-Ortega, A; Fialko, Y; Sandwell, D; Nava-Pichardo, FA; Fletcher, J; Gonzalez-Garcia, J; Lipovsky, B; Floyd, M; Funning, G | Abstract: El Mayor-Cucapah earthquake occurred on 4 April 2010 in northeastern Baja California just south of the U.S.-Mexico border. The earthquake ruptured several previously mapped faults, as well as some unidentified ones, including the Pescadores, Borrego, Paso Inferior and Paso Superior faults in the Sierra Cucapah, and the Indiviso fault in the Mexicali Valley and Colorado River Delta.We conducted several Global Positioning System (GPS) campaign surveys of preexisting and newly established benchmarks within 30 km of the earthquake rupture. Most of the benchmarks were occupied within days after the earthquake, allowing us to capture the very early postseismic transient motions. The GPS data show postseismic displacements in the same direction as the coseismic displacements; time series indicate a gradual decay in postseismic velocities with characteristic time scales of 66 ± 9 days and 20 ± 3 days, assuming exponential and logarithmic decay, respectively. We also analyzed interferometric synthetic aperture radar (InSAR) data from the Envisat and ALOS satellites. The main deformation features seen in the line-of-sight displacement maps indicate subsidence concentrated in the southern and northern parts of the main rupture, in particular at the Indiviso fault, at the Laguna Salada basin, and at the Paso Superior fault. We show that the near-field GPS and InSAR observations over a time period of 5 months after the earthquake can be explained by a combination of afterslip, fault zone contraction, and a possible minor contribution of poroelastic rebound. Far-field data require an additional mechanism, most likely viscoelastic relaxation in the ductile substrate. ©2014. American Geophysical Union.