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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. 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 mid‐ocean ridge system comprises a series of spreading ridges, transform faults, propagating ridges, and other non‐transform offsets. Transform faults remain stable for millions of years leaving long linear scars, or fracture zones, on older seafloor. Propagating ridges migrate in the ridge parallel direction leaving V‐shaped or W‐shaped scars on older seafloor. Vertical gravity gradient maps can now resolve the details of the ridge segmentation. For slow‐ and intermediate‐spreading ridges, there appears to be an offset length threshold above which adjacent ridges do not propagate so remain as stable transform faults. We propose this threshold is due to the yield strength of the lithosphere, and we develop a model framework based on a force balance wherein forces driving propagation must exceed the integrated shear strength of the offset zone. We apply this model framework to 5 major propagating ridges, 55 seesaw propagating ridges, and 69 transform faults. The model correctly predicts the migration of 4 out of 5 major propagating ridges and the stability of transform faults, but the results for seesaw propagators are less accurate. Model predictions for direction of ridge propagation are mixed as well. This model framework simplifies deformation in the shear zone, but can possibly explain why non‐transform deformation is preferred at short offsets. Mid‐ocean ridges are constructive plate boundaries where new crust is created. In map view, the system resembles a stair‐step configuration of alternating spreading ridges and ridge offsets. Some ridges and offsets, typically large ones, remain fixed and maintain their plan‐view shape over many millions of years, while other ridges, usually those bound by shorter offsets, may slowly grow and shrink–such behavior is revealed in maps of the seafloor. The different behavior is possibly due to the material strength of the oceanic crust and upper mantle which, if great, will inhibit ridge growth. To test our hypothesis, we estimate the total material strength at identified ridge offsets and compare this to an estimate of forces contributing to ridge growth. Our estimates can explain why large offsets maintain their shape, and may explain why short offsets do not and allow some segments to grow and shrink. Non‐transform offsets at slower‐spreading ridges rarely propagate when offsets exceed 30 km, possibly a result of lithospheric strengthWe develop a model framework that balances material strength at ridge offsets and forces driving ridge propagationGreater strength of the lithosphere as ridge offset increases may limit ridge propagation Non‐transform offsets at slower‐spreading ridges rarely propagate when offsets exceed 30 km, possibly a result of lithospheric strength We develop a model framework that balances material strength at ridge offsets and forces driving ridge propagation Greater strength of the lithosphere as ridge offset increases may limit ridge propagation

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 the Generic Mapping Tools and MATLAB®. Interpolation of vector dataGPS griddingGreen's functions for thin elastic sheet

The March 2011 Tohoku-oki earthquake was only the second giant (moment magnitude Mw≥ 9.0) earthquake to occur in the last 50 years and is the most recent to be recorded using modern geophysical techniques. Available data place high-resolution constraints on the kinematics of earthquake rupture, which have challenged prior knowledge about how much a fault can slip in a single earthquake and the seismic potential of a partially coupled megathrust interface. But it is not clear what physical or structural characteristics controlled either the rupture extent or the amplitude of slip in this earthquake. Here we use residual topography and gravity anomalies to constrain the geological structure of the overthrusting (upper) plate offshore northeast Japan. These data reveal an abrupt southwest–northeast-striking boundary in upper-plate structure, across which gravity modelling indicates a south-to-north increase in the density of rocks overlying the megathrust of 150–200 kilograms per cubic metre. We suggest that this boundary represents the offshore continuation of the Median Tectonic Line, which onshore juxtaposes geological terranes composed of granite batholiths (in the north) and accretionary complexes (in the south). The megathrust north of the Median Tectonic Line is interseismically locked, has a history of large earthquakes (18 with Mw> 7 since 1896) and produced peak slip exceeding 40 metres in the Tohoku-oki earthquake. In contrast, the megathrust south of this boundary has higher rates of interseismic creep, has not generated an earthquake with MJ> 7 (local magnitude estimated by the Japan Meteorological Agency) since 1923, and experienced relatively minor (if any) co-seismic slip in 2011. We propose that the structure and frictional properties of the overthrusting plate control megathrust coupling and seismogenic behaviour in northeast Japan.

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 Mw7.8 Gorkha, Nepal earthquake and its Mw7.3 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 Mw7.3 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 Mw6.9 event. Observations of the Mw7.8 Gorkha, Nepal earthquake and Mw7.3 aftershock are presentedALOS‐2 provides burst‐aligned ScanSAR interferometry with 350 km swath widthData from coseismic and postseismic interferograms are available online for use in modeling studies

The 2019 Ridgecrest conjugate Mw6.4 and Mw7.1 events resulted in several meters of strike‐slip and dip‐slip along an intricate rupture, extending from the surface down to 15 km. Now with >2 years of post‐rupture observations, we utilize these results to better understand vertical postseismic deformation from the Ridgecrest sequence and illuminate the emerging significance of vertical earthquake cycle deformation data. We determine the cumulative vertical displacement observed by the continuous GNSS network since Ridgecrest, which requires additional time series analyses to adequately resolve vertical deformation compared to the horizontal. Using a Maxwell‐type viscoelastic relaxation model, with a best fit time‐averaged asthenosphere viscosity of 4e17 Pa·s and a laterally heterogeneous lithosphere, we find that viscoelastic relaxation accounts for a majority of the cumulative vertical deformation at Ridgecrest and strongly controls far‐field observations in all north‐east‐up components. The viscoelastic model alone generally underpredicts deformation from GNSS and the remaining nonviscoelastic displacement is most prominent in the horizontal near‐field (−16 to 19 mm), revealing a deformation pattern matching the coseismic observations. This suggests that multiple deformation mechanisms are contributing to Ridgecrest's postseismic displacement, where afterslip likely dominates the near‐field while viscoelastic relaxation controls the far‐field. Similar deformation at individual GNSS stations has been observed for past earthquakes and additionally reveals long‐term transient viscosity over several years. Moreover, the greater temporal and spatial resolution of the GNSS array for Ridgecrest will help resolve the evolution of deformation for the entire network of observations as regional postseismic deformation persists for the next several years. The 2019 Ridgecrest earthquake sequence is one the most well observed seismic events in California's history. We take advantage of the unprecedented amount of satellite observations and previous modeling efforts to better understand the vertical post‐earthquake signal resulting from Ridgecrest. We compare cumulative surface displacement since the earthquakes, derived from Global Navigation Satellite System (GNSS) point observations to a viscoelastic model, which allows us to quantify postseismic deformation due to viscoelastic (i.e., viscoelastic relaxation) and nonviscoelastic (i.e., afterslip and poroelastic rebound) contributions. We find that viscoelastic relaxation accounted for a majority of the currently observed vertical deformation since Ridgecrest and strongly controlled far‐field observations in all north‐east‐up components, emphasizing the importance of utilizing both horizontal and vertical observations when developing earthquake cycle models. The remaining observed deformation insinuates that multiple postseismic deformation mechanisms, and thus nonviscoelastic contributions, are also needed to reproduce vertical postseismic displacement, where afterslip is the most likely mechanism here. We additionally see similarities between Ridgecrest and past large regional earthquakes and emphasize the ability of our deformation model, derived from 3D surface observations, to provide important insight on crustal parameters and the characteristics of the seismic region for past, current, and future events. Viscoelastic relaxation accounts for the majority of vertical postseismic displacement >1 year after Ridgecrest and controls the far‐fieldVertical observations provide important constraints on time‐averaged and transient upper mantle viscosityAdditional time series analysis techniques are needed to produce coherent vertical deformation patterns compared to the horizontal Viscoelastic relaxation accounts for the majority of vertical postseismic displacement >1 year after Ridgecrest and controls the far‐field Vertical observations provide important constraints on time‐averaged and transient upper mantle viscosity Additional time series analysis techniques are needed to produce coherent vertical deformation patterns compared to the horizontal

Ocean islands, seamounts and volcanic ridges are thought to form above mantle plumes. Yet, this mechanism cannot explain many volcanic features on the Pacific Ocean floor and some might instead be caused by cracks in the oceanic crust linked to the reorganization of plate motions. A distinctive bend in the Hawaiian–Emperor volcanic chain has been linked to changes in the direction of motion of the Pacific Plate, movement of the Hawaiian plume, or a combination of both. However, these links are uncertain because there is no independent record that precisely dates tectonic events that affected the Pacific Plate. Here we analyse the geochemical characteristics of lava samples collected from the Musicians Ridges, lines of volcanic seamounts formed close to the Hawaiian–Emperor bend. We find that the geochemical signature of these lavas is unlike typical ocean island basalts and instead resembles mid-ocean ridge basalts. We infer that the seamounts are unrelated to mantle plume activity and instead formed in an extensional setting, due to deformation of the Pacific Plate. 40Ar/39Ar dating reveals that the Musicians Ridges formed during two time windows that bracket the time of formation of the Hawaiian–Emperor bend, 53–52 and 48–47 million years ago. We conclude that the Hawaiian–Emperor bend was formed by plate–mantle reorganization, potentially triggered by a series of subduction events at the Pacific Plate margins.

The Mw7.2 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. Geothermal energy production causes surface subsidence and crustal stressingProduction at CPGF generates positive Coulomb stresses in EMC rupture zoneAnthropogenic stresses exceed the tectonic loading rate at EMC hypocenter

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. Shallow creep is pervasive along the southernmost 50 km of the San Andreas FaultCreep is localized only along transpressional fault segmentsIn transtensional areas, creep is distributed over a 1–2 km wide fault zone

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. Sea level and earthquake cycle model vertical displacements are compatibleThick plates/moderate viscosities reproduce tide gauge ratesLateral variations in sea level may be due to fault locking depth changes

Measuring crustal strain and seismic moment accumulation, is crucial for understanding the growth and distribution of seismic hazards along major fault systems. Here, we develop a methodology to integrate 4.5 years (2015–2019.5) of Sentinel‐1 Interferometric Synthetic Aperture Radar (InSAR) and continuous Global Navigation Satellite System (GNSS) time series to achieve 6 to 12‐day sampling of surface displacements at ∼500 m spatial resolution over the entire San Andreas fault system. Numerous interesting deformation signals are identified with this product (video link: https://www.youtube.com/watch?v=SxNLQKmHWpY). We decompose the line‐of‐sight InSAR displacements into three dimensions by combining the deformation azimuth from a GNSS‐derived interseismic fault model. We then construct strain rate maps using a smoothing interpolator with constraints from elasticity. The resulting deformation field reveals a wide array of crustal deformation processes including, on‐ and off‐fault secular and transient tectonic deformation, creep rates on all the major faults, and vertical signals associated with hydrological processes. The strain rate maps show significant off‐fault components that were not captured by GNSS‐only models. These results are important in assessing the seismic hazard in the region. Seismic hazard models rely on accurate measurements of small motion over large areas on the Earth's crust. Traditional geodetic models based on Global Navigation Satellite System (GNSS) data cannot resolve small scale deformation patterns, mainly due to expensive and limited station deployment. Interferometric Synthetic Aperture Radar (InSAR) has become the emerging tool for mapping surface deformation, with its advantages of low‐cost and full‐coverage. Yet InSAR measurements, compared to GNSS, come with larger biases from the atmospheric noise, especially over length scales greater than 80 km. Here, we combined the two methods to resolve fine spatial scales and achieve high accuracy. Our results are presented as deformation time‐series over the entire San Andreas fault system (video link: https://www.youtube.com/watch?v=SxNLQKmHWpY). From these deformation time series, we have estimated fault creep rates and strain accumulation. One important finding is that there is significant off‐fault strain, though we suspect this is mainly due to hydrological processes. These results will advance our knowledge of the earthquake cycle, strain/moment accumulation, and the associated seismic hazards. A practical approach is developed to integrate Sentinel‐1 Interferometric Synthetic Aperture Radar and Global Navigation Satellite System time‐series over the entire San Andreas fault systemThe product is used to estimate fault creep and three components of horizontal crustal strain which shows notable off‐fault portionChallenges remain in separating tectonic and hydrologic sources and whether hydrologic strain will increase seismic hazards A practical approach is developed to integrate Sentinel‐1 Interferometric Synthetic Aperture Radar and Global Navigation Satellite System time‐series over the entire San Andreas fault system The product is used to estimate fault creep and three components of horizontal crustal strain which shows notable off‐fault portion Challenges remain in separating tectonic and hydrologic sources and whether hydrologic strain will increase seismic hazards

Rheologic variations in the Earth's crust (like elastic plate thickness [EPT] or crustal rigidity) modulate the rate at which seismic moment accumulates for potentially hazardous faults of the San Andreas Fault System (SAFS). To quantify rates of seismic moment accumulation, Global Navigation Satellite Systems, and Interferometric Synthetic Aperture Radar data were used to constrain surface deformation rates of a four‐dimensional viscoelastic deformation model that incorporates rheological variations spanning a 900 km section of the SAFS. Lateral variations in EPT, estimated from surface heat flow and seismic depth to the lithosphere‐asthenosphere boundary, were converted to lateral variations in rigidity and then used to solve for seismic moment accumulation rates on 32 fault segments. We find a cluster of elevated seismic moment rates (11–20 × 1015Nm year−1km−1) along the main SAFS trace spanning the historical Mw7.9 1857 Fort Tejon earthquake rupture length; present‐day seismic moment magnitude on these segments ranges from Mw7.2–7.6. We also find that the average plate thickness in the Salton Trough is reduced to only 60% of the regional average, which results in a ∼60% decrease in moment accumulation rate along the Imperial fault. Likewise, a 30% increase of average plate thickness results in at least a ∼30% increase in moment rate and even larger increases are identified in regions of complex plate heterogeneity. These results emphasize the importance of considering rheological variations when estimating seismic hazard, suggesting that meaningful changes in seismic moment accumulation are revealed when considering spatial variations in crustal rheology. The earthquake potential of faults (i.e., moment magnitude) is determined by a fault's deep slip rate, the area of the fault, the time since the last major rupture, and the average crustal rigidity surrounding the fault. Here, we explore how earthquake potential is affected by variations in crustal rheology along the San Andreas Fault System (SAFS). We use measurements of surface heat flow and seismic velocity to estimate lateral variations in crustal rigidity and tectonic plate thickness. We then estimate seismic potential rate for 32 segments of the SAFS using surface deformation measurements from Global Navigation Satellite Systems and radar interferometry as constraints. We find high seismic potential rates along several segments of the San Andreas fault that also participated in the Mw7.9 1857 Fort Tejon earthquake and have not ruptured in over 160 years. We also find that the change in earthquake potential scales almost linearly with plate thickness variations surrounding the fault. For example, the Salton Trough region has a plate thickness that is only 60% of the regional average, which lowers the earthquake potential along the Imperial fault by roughly 60%. These findings suggest that variations in crustal rheology have an important impact on earthquake magnitude forecasts. Variable rheology was implemented in a four‐dimensional viscoelastic model to explore earthquake cycle deformation of the San Andreas Fault SystemElastic plate thickness variations are estimated from heat flow and depth to the lithosphere‐asthenosphere boundaryVariations in earthquake magnitude forecasts should be carefully considered when implementing variable crustal parameters Variable rheology was implemented in a four‐dimensional viscoelastic model to explore earthquake cycle deformation of the San Andreas Fault System Elastic plate thickness variations are estimated from heat flow and depth to the lithosphere‐asthenosphere boundary Variations in earthquake magnitude forecasts should be carefully considered when implementing variable crustal parameters

The segmentation of mid‐ocean ridges is a defining characteristic of seafloor spreading, yet some tectonic processes operating at segment boundaries remain poorly understood. Here, we analyze new satellite‐derived vertical gravity gradient (VGG) data, which reveal an abundance of off‐axis seafloor features that are oblique to ridges and transform faults and thus reflect the occurrence of ridge propagation at some segment boundaries. However, unlike many propagating ridges, these features commonly reverse direction leaving W‐shaped signatures in the seafloor which we refer to as “seesaw propagators” (SSPs). Using the VGG, we have documented these globally and find that: (1) SSPs are ubiquitous on seafloor that formed at half spreading rates between 10 and 40 mm year−1and their total length is about 1/3 that of fracture zones. (2) The lithospheric age offset across SSPs (0–2.5 Ma) is less than transform faults (2–10 Ma), which likely reflects a rheological threshold, whereby only young and weak lithosphere allows for “non‐rigid” SSP behavior. (3) Isostatic modeling of well‐surveyed SSPs confirms that they formed on young and thin (3 km) lithosphere. (4) The directional changes of SSPs in both time and space appear largely uncorrelated and cannot be explained by previous regional‐scale models invoked to explain unidirectional ridge propagation and thus require a different driving force. The process of “seesaw” ridge propagation is ubiquitous on seafloor that forms at half spreading rates between 10 and 40 mm yr−1Globally, the total length of seesaw propagators is approximately one‐third that of fracture zonesDriving mechanisms previously proposed to explain unidirectional ridge propagation fail account for the behavior of seesaw propagators The process of “seesaw” ridge propagation is ubiquitous on seafloor that forms at half spreading rates between 10 and 40 mm yr−1 Globally, the total length of seesaw propagators is approximately one‐third that of fracture zones Driving mechanisms previously proposed to explain unidirectional ridge propagation fail account for the behavior of seesaw propagators

Geoid data from Geosat and subsatellite basement depth profiles of the Kane Fracture Zone in the central North Atlantic were used to examine the correlation between the short-wavelength geoid (?=25–100 km) and the uncompensated basement topography. The processing technique we apply allows the stacking of geoid profiles, although each repeat cycle has an unknown long-wavelength bias. We first formed the derivative of individual profiles, stacked up to 22 repeat cycles, and then integrated the average-slope profile to reconstruct the geoid height. The stacked, filtered geoid profiles have a noise level of about 7 mm in geoid height. The subsatellite basement topography was obtained from a recent compilation of structure contours on basement along the entire length of the Kane Fracture Zone. The ratio of geoid height to topography over the Kane Fracture Zone valley decreases from about 20–25 cm km-1 over young ocean crust to 5–0 cm km-1 over ocean crust older than 140 Ma. Both geoid and basement depth of profiles were projected perpendicular to the Kane Fracture Zone, resampled at equal intervals and then cross correlated. The cross correlation shows that the short-wavelength geoid height is well correlated with the basement topography. For 33 of the 37 examined pro-files, the horizontal mismatches are 10 km or less with an average mismatch of about 5 km. This correlation is quite good considering that the average width of the Kane Fracture Zone valley at median depth is 10–15 km. The remaining four profiles either cross the transverse ridge just east of the active Kane transform zone or overlie old crust of the M-anomaly sequence. The mismatch over the transverse ridge probably is related to a crustal density anomaly. The relatively poor correlation of geoid and basement depth in profiles of ocean crust older than 130–140 Ma reflects poor basement-depth control along subsatellite tracks.

A region west of the southern East Pacific Rise (SEPR), between the Marquesas and Austral Fracture Zones has previously been found to exhibit anomalous depth-age behavior, based on gridded bathymetry and single-beam soundings. Since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism, the magnitude and regional extent of depth anomalies over the young eastern flank of the so called ‘South Pacific Superswell’ are re-examined using a mode-seeking estimation procedure on data obtained from several recent multibeam surveys. The modal technique estimates a representative seafloor depth, based on the assumption that bathymetry from non-edifice and edifice-populated seafloor has a low and a high standard deviation, respectively. Flat seafloor depth values are concentrated in a few bins which correspond to the mode. This method estimates a representative seafloor value even on seafloor for which more than 90% of coverage is dominated by ridge and seamount clusters, where the mean and median estimates may be shallow by hundreds of meters. Where volcanism-related bias is moderate, the mode, mean and median estimates are close.

The very high correlation of geoid height and topography on Venus, along with the high geoid topography ratio, can be interpreted as local isostatic compensation and/or dynamic compensation of topography at depths ranging from 50 to 350 km. For local compensation within the lithosphere, the swell-push force is proportional to the first moment of the anomalous density. Since the long-wavelength isostatic geoid height is also proportional to the first moment of the anomalous density, the swell push force is equal to the geoid height scaled by −g2/2πG. Because of this direct relationship, the style (i.e., thermal, Airy, or Pratt compensation) and depth of compensation do not need to be specified and can in fact vary over the surface. Phillips (1990) showed that this simple relationship between swell-push force and geoid also holds for dynamic uplift by shear traction on the base of the lithosphere caused by thermal convection of an isoviscous, infinite half-space mantle. Thus for all reasonable isostatic models and particular classes of dynamic models, the geoid height uniquely determines the magnitude of the swell-push body force that is applied to the venusian lithosphere.

A region west of the southern East Pacific Rise (SEPR), between the Marquesas and Austral Fracture Zones has previously been found to exhibit anomalous depth-age behavior, based on gridded bathymetry and single-beam soundings. Since gridded bathymetry has been shown to be unsuitable for some geophysical analysis and since the area is characterized by unusually robust volcanism, the magnitude and regional extent of depth anomalies over the young eastern flank of the so called ‘South Pacific Superswell’ are re-examined using a mode-seeking estimation procedure on data obtained from several recent multibeam surveys. The modal technique estimates a representative seafloor depth, based on the assumption that bathymetry from non-edifice and edifice-populated seafloor has a low and a high standard deviation, respectively. Flat seafloor depth values are concentrated in a few bins which correspond to the mode. This method estimates a representative seafloor value even on seafloor for which more than 90% of coverage is dominated by ridge and seamount clusters, where the mean and median estimates may be shallow by hundreds of meters. Where volcanism-related bias is moderate, the mode, mean and median estimates are close.Depth-age results indicate that there is only a small anomaly (< 200 m) over 15–35 Ma Pacific Plate seafloor with little age-dependent shallowing, suggesting that the lithosphere east of the main hot-spot locations on the ‘superswell’ is normal. An important implication is that, in sparsely surveyed areas, depths from ETOPO-5 are significantly different from true depths even at large scales (∼ 1000 km) and thus are unsuitable for investigations of anomalies associated with depth-age regressions. We find that seafloor slopes on conjugate profiles of the Pacific and Nazca Plates from 15 to 35 Ma are both slightly lower than normal, but are within the global range. Proximate to the SEPR, seafloor slopes are very low (218 m Myr−12) on the Pacific Plate (0–22 Ma) and slightly high (∼ 410 m Myr−12) on the Nazca Plate (0–8 Ma); slopes for older Pacific seafloor (22–37 Ma) are near normal (399 m Myr−12). Seafloor slopes are even lower north of the Marquesas Fracture Zone but are highly influenced by the Marquesas Swell. We find that the low subsidence rate on young Pacific seafloor cannot be explained by a local hot-spot or a small-scale convective model exclusively and a stretching/thickening model requires implausible crustal thickness variation (∼ 30%).

We present a preliminary tectonic chart of the Indian Ocean based on a joint compilation of bathymetric data, magnetic anomaly data and Geosat altimetry data. Satellite altimeters such as Geosat map the topography of the equipotential sea surface or marine geoid. Our interpretation of the GEOSAT data is based on an analysis of the first derivative of the geoid profiles (i.e. deflection of the vertical profiles). Because of the high correlation between the vertical deflection (at wavelength <200 km) and the seafloor topography, the Geosat profiles can be used to delineate accurately numerous tectonic features of the ocean floor such as fracture zones, seamounts and spreading ridges. The lineations in the Geosat data are compared with bathymetric data and combined with magnetic anomaly identifications to produce a tectonic fabric chart of the Indian Ocean floor.

To provide easy access to the large number of Seastat and Geosat altimeter observations collected over the last decade, we have plotted these satellite altimeter profiles as overlays to the General Bathymetric Chart of the Oceans (GEBCO). Each of the 32 overlays displays along-track gravity anomalies for either ascending (southeast to northwest) or descending (northeast to southwest) altimeter passes. Where Seasat and Geosat profiles coincide, only the more accurate Geosat profiles were plotted. In poorly charted southern ocean areas, satellite altimeter profiles reveal many previously undetected features of the seafloor.

The global spherical harmonic model of Venus' gravity field MGNP60FSAAP, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct positive gravity anomalies. No corona has been found to have a negative gravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regio with the structures themselves is remarkable and argues for a near-surface or lithospheric origin of the gravity signals over such relatively small features. Apparent depths of compensation (ADCs) of the prominent gravity anomalies at Artemis, Latona, and Heng-o Coronae are about 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Latona, and Heng-o Coronae lie in the range 32 to 35 m km^-1. The large ADCs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shallowly compensated topography and deep positive mass anomalies due to subduction or underthrusting at these coronae. At arcuate segments of Hecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fatua Corona, four coronae in Eastern Eistla Regio, and an arcuate segment of Western Parga Chasma are about 75 km. The GTRs at Fatua Corona, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m km^-1. The ADCs and GTRs of these coronae and arcuate chasmata segments are generally too large to reflect compensation by crustal thickness variations. Instead, they suggest compensation by thermally induced thickness variations in a moderately thick (≈100 km) lithosphere. Alternatively, the gravity signals at these sites could originate from deep positive mass anomalies due to subduction or underthrusting. Weighted linear least squares fits to GTR vs h (long-wavelength topography) data from Heng-o and Fatua Coronae, the four coronae in eastern Eistla Regio, and the arcuate segments of Hecate, Parga, and western Parga Chasmata are consistent with compensation by thermally induced thickness variations of a dense lithosphere above a less dense mantle; the fits imply an average lithosphere thickness of about 180 km and an excess lithospheric density of about 0.5 to 0.7%. Gravity anomalies at the arcuate segments of Dali and Diana Chasmata that form Latona Corona, at Artemis Chasma, and other arcuate segments of Parga and Hecate Chasmata occur on the concave sides of the arcs. By analogy with gravity anomalies of similar horizontal scale (600 km-several thousand kilometers) on the concave sides of terrestrial subduction zone arcs, which are due in large part to subducted lithosphere, it is inferred that the gravity anomalies on Venus are consistent with retrograde subduction at Artemis Chasma, along the northern and southern margins of Latona Corona, and elsewhere along Parga and Hecate Chasmata. Copyright 1994, 1999 Academic Press

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. 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. We provide detailed new observations that constrain variations in average creep rates following regional earthquakesA new model is proposed for understanding dynamic/static stress changes and short‐ and long‐term creep rate variations on the shallow SSAFOur observations suggest that static stress change has a long‐lasting effect on fault creep, even in the presence of transient processes

Three approaches are used to reduce the error in the satellite‐derived marine gravity anomalies. First, we have retracked the raw waveforms from the ERS‐1 and Geosat/GM missions resulting in improvements in range precision of 40% and 27%, respectively. Second, we have used the recently published EGM2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean. Third, we have used a biharmonic spline interpolation method to construct residual vertical deflection grids. Comparisons between shipboard gravity and the global gravity grid show errors ranging from 2.0 mGal in the Gulf of Mexico to 4.0 mGal in areas with rugged seafloor topography. The largest errors of up to 20 mGal occur on the crests of narrow large seamounts. The global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate. For rates less than about 60 mm/a the typical ridge segment is 50–80 km long while it increases dramatically at higher rates (100–1000 km). This transition spreading rate of 60 mm/a also marks the transition from axial valley to axial high. We speculate that a single mechanism controls both transitions; candidates include both lithospheric and asthenospheric processes.

Topographic flexural signatures on Venus are generally associated with the outer edges of coronae, with some chasmata and with rift zones. Using Magellan altimetry profiles and grids of venusian topography, we identified 17 potential flexure sites. Both 2-D cartesian and 2-D axisymmetric, thin-elastic plate models were used to establish the flexural parameter and applied load/bending moment. These parameters can be used to infer the thickness, strength and possibly the dynamics of the venusian lithosphere. Numerical simulations show that the 2-D model provides an accurate representation of the flexural parameter as long as the radius of the feature is several times the flexural parameter. However, an axisymmetric model must be used to obtain a reliable estimate of load/bending moment. 12 of the 17 areas were modelled with a 2-D thin elastic plate model, yielding best-fit effective elastic thicknesses in the range 12 to 34 km. We find no convincing evidence for flexure around smaller coronae, though five possible candidates have been identified. These five features show circumferential topographic signatures which, if interpreted as flexure, yield mean elastic thicknesses ranging from 6 to 22 km. We adopt a yield strength envelope for the venusian lithosphere based on a dry olivine rheology and on the additional assumption that strain rates on Venus are similar to, or lower than. strain rates on Earth. Many of the flexural signatures correspond to relatively high plate-bending curvatures so the upper and lower parts of the lithosphere should theoretically exhibit brittle fracture and flow, respectively. For areas where the curvatures are not too extreme, the estimated elastic thickness is used to estimate the larger mechanical thickness of the lithosphere. The large amplitude flexures in Aphrodite Terra predict complete failure of the plate, rendering mechanical thickness estimates from these features unreliable. One smaller corona also yielded an unreliable mechanical thickness estimate based on the marginal quality of the profile data. Reliable mechanical thicknesses found by forward modelling in this study are 21 km-37 km, significantly greater than the 13 km-20 km predictions based on heat-flow scaling arguments and chondritic thermal models. If the modelled topography is the result of lithospheric flexure, then our results for mechanical thickness, combined with the lack of evidence for flexure around smaller features. are consistent with a venusian lithosphere somewhat thicker than predicted. Dynamical models for bending of a viscous lithosphere at low strain rates predict a thick lithosphere, also consistent with low temperature gradients. Recent laboratory measurements indicate that dry crustal materials are much stronger than previously believed. Corresponding time-scales for gravitational relaxation are 108-109 yr. making gravitational relaxation an unlikely mechanism for the generation of the few inferred flexural features. If dry olivine is also found to be stronger than previously believed, the mechanical thickness estimates for Venus will be reduced, and will be more consistent with the predictions of global heat scaling models.

Closely spaced satellite altimeter profiles (>5 km) collected during the Geosat Geodetic Mission (Geosat/GM), and those planned for the extended ERS-1 mission, are easily converted to grids of vertical gravity gradient and gravity anomaly. As profile spacing decreases, it becomes increasingly difficult to perform a crossover adjustment on the original geoid height profiles without introducing large cross-track gradients. If one is only interested in the horizontal and vertical derivatives of the gravitational potential, however, adjustment of the profile is unnecessary. The long-wavelength radial orbit error is suppressed well below the noise level of the altimeter by simply taking the along-track derivative of each profile. Ascending and descending slope profiles are then interpolated onto separate uniform grids. These two grids are summed and differenced to form comparable grids of east and north vertical deflection. Using Laplace's equation, the vertical gravity gradient is calculated directly from the vertical deflection grids. Fourier analysis is required to construct gravity anomalies from the two vertical deflection grids. These techniques are applied to high-density (∼2 km profile spacing) Geosat/GM profiles in Antarctic waters (60°S to 72°S). Gridding and interpolation are performed using the method of projection onto convex sets where the smoothness criteria corresponds to upward continuation through 4 km of ocean. The resultant gravity grids have resolution and accuracy comparable to shipboard gravity profiles. After adjustment of a DC shift in the shipboard gravity profiles (∼5 mGal) the rms difference between the ship and satellite gravity is 5.5 mGal. Many interesting and previously uncharted features are apparent in these new gravity maps including a propagating rift wake and a large ‘leaky transform’ along the Pacific-Antarctic Rise.