Gravity Datasets Research Articles

Data–driven prospectivity modelling of sediment–hosted Zn–Pb mineral systems and their critical raw materials

Demand for critical raw materials is expected to accelerate over the next few decades due to continued population growth and the shifting consumption patterns of the global economy. Sedimentary basins are important sources for critical raw materials and new discoveries of sediment–hosted Mississippi Valley–type (MVT) and/or clastic–dominated (CD) Zn–Pb deposits are likely required to mitigate future supply chain disruptions for Zn, Pb, Ag, Cd, Ga, Ge, Sb, and In. Herein we integrate public geoscience datasets using a discrete global grid to system to model the mineral potential for MVT and CD deposits across Canada, the United States of America, and Australia. Statistical analysis of the model results demonstrates that surface–wave tomography and derivative products from satellite gravity datasets can be used to map the most favourable paleo–tectonic settings of MVT and CD deposits inboard of orogenic belts and at the rifted edges of cratonic lithosphere, respectively. Basin development at pre–existing crustal boundaries was likely important for maintaining the low geothermal–gradients that are favourable for metal transport and generating the crustal fluid pathways that were reactivated during ore–formation, as suggested by the statistical association of both sediment–hosted mineral deposit types with the edges of upward–continued gravity and long–wavelength magnetic anomalies. Multivariate statistical analysis demonstrates that the most prospective combination of these geophysical datasets varies for each geological region and deposit type. We further demonstrate that maximum and minimum geological ages, coupled with Phanerozoic paleogeographic reconstructions, represent mappable proxies for the availability of oxidized, brine–generating regions that are the most likely source of ore–forming fluids (e.g., low– to mid–latitude carbonate platforms and evaporites). Ore deposition was likely controlled by interaction between oxidized, low–temperature brines and sulfidic and/or carbonaceous rocks, which, in some cases, can be mapped at the exposed surface or identified using the available rock descriptions. Baseline weights–of–evidence models are based on regional geophysics and are the least impacted by missing surface information but yield relatively poor results, as demonstrated by the low area–under–the–curve (AUC) for the spatially independent test set on the success–rate plot (AUC = 0.787 for MVT and AUC = 0.870 for CD). Model performance can be improved by: (1) using advanced methods that were trained and validated during a series of semi–automated machine learning competitions; and/or (2) incorporating geological and geophysical datasets that are proxies for each component of the mineral system. The best–performing gradient boosting machine models yield higher AUC for the test set (AUC = 0.983 for MVT and AUC = 0.991 for CD) and reduce the search space by >94%. The model results highlight the potential benefits of mapping sediment–hosted mineral systems at continental scale to improve mineral exploration targeting for critical raw materials.

Short‐wavelength Bouguer gravity anomaly and active fault distribution in the northeastern Japan arc

AbstractIn the northeastern (NE) Japan arc, active faults have a characteristic effect upon the short‐wavelength Bouguer anomaly (SWBA). Here, we create a SWBA map based on the Fourier analysis of the latest gravity dataset and evaluate the relationship between the SWBA and active faults in the arc. Ultimately, we found that active faults are associated with negative regions or grooves in the SWBA and that the SWBA does not clearly depend on underlying geology in the arc. Accumulated volumetric strain caused by faulting in the crust over the past 3 Ma of geological time has generated these negative gravity anomalies, estimated at around ‒10 mGal at a maximum. Strain concentration zones can have negative zones of gravity anomaly affecting the patterns of the SWBA throughout in the arc, and the SWBA reflects the amount of crustal deformation with the curvature.

Assessing model uncertainty for the scaling function inversion of potential fields

We have used the very fast simulated annealing algorithm to invert the scaling function along selected ridges, lying in a vertical section formed by upward continuing gravity data to a set of altitudes. The scaling function is formed by the ratio of the field derivative by the field itself, and it is evaluated along the lines formed by the zeros of the horizontal field derivative at a set of altitudes. We also use the same algorithm to invert gravity anomalies only at the measurement altitude. Our goal is to analyze the different models obtained through the two different inversions and to evaluate the relative uncertainties. One main difference is that the scaling function inversion is independent of density and the unknowns are the geometric parameters of the source. The gravity data are instead inverted for the source geometry and the density simultaneously. A priori information used for both the inversions is that the source has a known depth to the top. We examine the results over the synthetic examples of a salt dome structure generated by Talwani’s approach and real gravity data sets over the Mors salt dome (Denmark) and the Decorah Basin (USA). For all of these cases, the scaling function inversion yields models with better sensitivity to specific features of the sources, such as the tilt of the body, and reduced uncertainty. Finally, we analyze the density, which is one of the unknowns for the gravity inversion, and it is estimated from the geometric model for the scaling function inversion. The histograms over the density estimated at many iterations indicate a very concentrated distribution for the scaling function, whereas the density contrast retrieved by the gravity inversion, according to the fundamental ambiguity density/volume, is widely dispersed, making it difficult to assess its best estimate.

Gradient-boosted equivalent sources

SUMMARY The equivalent source technique is a powerful and widely used method for processing gravity and magnetic data. Nevertheless, its major drawback is the large computational cost in terms of processing time and computer memory. We present two techniques for reducing the computational cost of equivalent source processing: block-averaging source locations and the gradient-boosted equivalent source algorithm. Through block-averaging, we reduce the number of source coefficients that must be estimated while retaining the minimum desired resolution in the final processed data. With the gradient-boosting method, we estimate the sources coefficients in small batches along overlapping windows, allowing us to reduce the computer memory requirements arbitrarily to conform to the constraints of the available hardware. We show that the combination of block-averaging and gradient-boosted equivalent sources is capable of producing accurate interpolations through tests against synthetic data. Moreover, we demonstrate the feasibility of our method by gridding a gravity data set covering Australia with over 1.7 million observations using a modest personal computer.

Do Regional Trade Agreements Enhance International Tourism Flows? Evidence from a Cross-Country Analysis

This study investigates the effect of regional trade agreements (RTAs)—including preferential and free trade agreements, customs unions, and common markets—on bilateral tourism flows. We explore these effects using a panel gravity data set of 163 destination countries, 171 source countries, and 13,589 country-pairs from 1995 to 2015. This is the first large cross-country study to undertake such an integrated analysis using the gravity framework. Results show that all types of RTAs have a positive and significant effect on bilateral tourism flows. The overall indicator of RTAs that captures the combined effect of all types of RTAs on bilateral tourism flows is also positive and significant, on average, as well as when different regions are separately evaluated. These findings underscore the importance of strong economic integration in fostering international tourism flows. Policies aimed at improving a country’s economic integration with other countries can help promote international tourism flows.

Determinants of Foreign Trade in Nepal

This study examines the determinants of foreign trade in Nepal. Exports and imports of Nepal are the dependent variables. The selected independent variables are GDP of Nepal, GDP of trading partners, real effective exchange rate, distance, regional economic integration, per capita GDP of Nepal, per capital GDP of trading partners, economic freedom of Nepal and economic freedom of trading partners. The study is based on secondary data of 21 trading partners of Nepal with 210 observations for the period of 2010 to 2019. The data are collected from the Direction of Trade Statistics (DOTS) dataset of International Monetary Fund (IMF), World Development Indicator database of World Bank, CEPII gravity data set and the Heritage Foundation. The regression models are estimated to test the impact of various variables on the exports and imports of Nepal. The study showed that GDP of Nepal has a positive impact on exports of Nepal. It indicates that increase in GDP of Nepal leads to increase in exports of Nepal. Similarly, GDP of trading partners has a positive impact on exports. It indicates that increase in GDP of trading partners leads to increase in exports of Nepal. Likewise, distance has a negative impact on exports. It indicates that greater the distance with the trading countries, lower would be the exports of Nepal to those trading partners. Moreover, real effective exchange rate has a positive impact on exports. It indicates that increase in real effective exchange rate leads to increase in exports. In addition, SAFTA membership has a positive impact on exports. It indicates that SAFTA membership leads to increase in exports of Nepal. Furthermore, the study revealed that economic freedom index of trading partners has a positive impact on exports. It indicatesthat increase in economic freedom in dex of trading partners leads to increase in exports of Nepal. In addition, the study shows that GDP of Nepal has a positive impact on imports of Nepal. It indicates that increase in GDP of Nepal leads to increase in imports of Nepal.Similarly, GDP of trading partners has a positive impact on imports. It indicates that increase in GDP of trading partners leads to increase in imports of Nepal. Likewise, distance has a negative impact on imports. It indicates that greater the distance with the trading countries, lower would be the imports of Nepal from those trading partners. Similarly, real effectiveexchange rate has a positive impact on imports. It indicates that increase in real effective exchange rate leads to increase in imports. The study also reveals that GDP per capita differential has a positive impact on imports. It indicates that higher the GDP per capita differential, higher would be the imports of Nepal.

Singling out modified gravity parameters and data sets reveals a dichotomy between Planck and lensing

ABSTRACT An important route to testing general relativity (GR) at cosmological scales is usually done by constraining modified gravity (MG) parameters added to the Einstein perturbed equations. Most studies have analysed so far constraints on pairs of MG parameters, but here, we explore constraints on one parameter at a time while fixing the other at its GR value. This allows us to analyse various models while benefiting from a stronger constraining power from the data. We also explore which specific data sets are in tension with GR. We find that models with (μ = 1, η) and (μ, η = 1) exhibit a 3.9σ and 3.8σ departure from GR when using Planck18 + Supernovae type Ia (SNe) + Baryon acoustic oscillation (BAO), while (μ, η) shows a tension of 3.4σ. We find no tension with GR for models with the MG parameter Σ fixed to its GR value. Using a Bayesian model selection analysis, we find that some one-parameter MG models are moderately favoured over ΛCDM when using all data set combinations except Planck cosmic microwave background lensing and dark energy survey data. Namely, Planck18 shows a moderate tension with GR that only increases when adding any combination of redshift space distortion, SNe, or BAO. However, adding lensing diminishes or removes these tensions, which can be attributed to the ability of lensing in constraining the MG parameter Σ. The two overall groups of data sets are found to have a dichotomy when performing consistency tests with GR, which may be due to systematic effects, lack of constraining power, or modelling. These findings warrant further investigation using more precise data from ongoing and future surveys.

Predictive geologic mapping from geophysical data using self-organizing maps: A case study from Baie Verte, Newfoundland, Canada

Self-organizing maps (SOMs) are a type of unsupervised artificial neural networks clustering tool. SOMs are used to cluster large multivariate data sets. They can identify patterns and trends in the geophysical maps of an area and generate proxy geology maps, known as remote predictive mapping. We have applied SOMs to magnetic, radiometric, and gravity data sets compiled from multiple modern and legacy data sources over the Baie Verte Peninsula, Newfoundland, Canada. The regional and local geologic maps available for this area and knowledge from numerous geologic studies has enabled the accuracy of SOM-based predictive mapping to be assessed. Proxy geology maps generated by primary clustering directly from the SOMs and secondary clustering using a k-means approach reproduced many geologic units identified by previous traditional geologic mapping. Of the combinations of data sets tested, the combination of magnetic data, primary radiometric data and their ratios, and Bouguer gravity data gave the best results. We found that using reduced-to-the-pole residual intensity or using the analytic signal as the magnetic data were equally useful. The SOM process was unaffected by gaps in the coverage of some of the data sets. The SOM results could be used as input into k-means clustering because this method requires no gaps in the data. The subsequent k-means clustering resulted in more meaningful proxy geology maps than were created by the SOM alone. In regions where the geology is poorly known, these proxy maps can be useful in targeting where traditional, on-the-ground geologic mapping would be most beneficial, which can be especially useful in parts of the world where access is difficult and expensive.

The Effect of Migration on International Tourism Flows: The Role of Linguistic Networks and Common Languages

We investigate the effect of migration rates on international tourism flows and whether linguistic networks and common languages affect the relationship between migration rates and tourist flows. We utilize the gravity data set consisting of 166 origin and 30 destination countries, over the 1995–2010 period for the empirical analysis. The results show that increased migration rates lead to higher international tourism flows in destination countries. This effect becomes even stronger when the destination country has a larger linguistic network. Migration matters less, however, when a country-pair shares closer linguistic ties. The findings of this study will help to inform public, and tourism industry policies aimed at boosting sustainable tourism flows, especially post COVID-19 pandemic. Targeted immigration policies, such as those that seek to attract foreign workers with specific skill sets, tend to enlarge linguistic networks and contribute to multilingualism, which in turn can boost international tourism flows.

Collocation and FFT-based geoid estimation within the Colorado 1 cm geoid experiment

In the frame of the International Association of Geodesy Joint Working Group 2.2.2 “The 1 cm geoid experiment”, terrestrial and airborne gravity datasets along with GPS/leveling data were made available for the comparison of different geoid modeling methods and techniques in the wider area of Colorado, USA. We discuss the methods and procedures we followed for computing gravimetric quasi-geoid and geoid models and geopotential values from the available datasets. The procedures followed were based on the remove-compute-restore approach using XGM2016 as a reference geopotential model. The higher frequencies of the gravity field were computed via the residual terrain correction, using (a) the CGIAR-CSI SRTM digital elevation model with the classical technique and (b) a spectral one. Least-Squares Collocation was used for the downward continuation of the airborne data and for gridding. Finally, the geoid models were obtained by applying Least-Squares Collocation and spherical FFT-based methods, while the influence of the orthometric height correction on geoid heights was taken into account by employing simple and complete Bouguer reductions. All results were evaluated with available GPS/leveling benchmarks. Moreover, potential values were determined in support of the International Height Reference System/Frame. From the results acquired, a final accuracy of 5–7 cm for the determined geoid models was achieved depending on the adopted method and data combination, without considering the accuracy of the GPS/leveling data used for their evaluation. The contribution of the airborne gravity data was deemed as limited in combination solutions although the airborne only solution provided equal level of accuracy to the terrestrial and the combined ones. Better consistency was obtained on the points of the GSVS17 line, when compared to the GPS/leveling data, where an accuracy of 2.4 cm and 2.8 cm was reached for the FFT and LSC based methods, respectively.

High-resolution geoid modeling using least squares modification of Stokes and Hotine formulas in Colorado

The Colorado geoid experiment was initiated and organized as a joint study by the Joint Working Group (JWG) 2.2.2 (1-cm geoid experiment) of the International Association of Geodesy (IAG) in 2017, and different institutions and research groups contributed to this study. The aim of this experiment was to clarify the repeatability of gravity potential values as International Height Reference System (IHRS) coordinates from different geoid determination approaches carried out with the same input dataset. The dataset included the terrestrial and airborne gravity observations, a digital terrain model, the XGM2016 global geopotential model and GPS/leveling data for model validations belonging to a mountainous area of approximately 550 km × 730 km in Colorado, US. The dataset was provided by National Geodetic Survey (NGS) department. In this frame, this article aims providing a discussion on Colorado geoid modeling through individual experimental results obtained by Istanbul Technical University-Gravity Research Group (ITU-GRG). This contribution mainly focused on modeling the Colorado geoid using the least squares modifications of Stokes and Hotine integral formulas with additive corrections. The computations using each formula were carried out using ITU-GRG software, including the solution variants based on terrestrial-only, airborne-only and combined gravity datasets. Then, the calculated experimental geoid models were validated using historical and recently measured profile-based GPS/leveling datasets, and they were also compared with the official solutions submitted by different institutions for the “1-cm geoid experiment” of IAG JWG 2.2.2. For all validation results, the Hotine and Stokes integral formulas yielded similar performances in terms of geoid accuracy; however, the models computed using the combined data had better accuracy than those using the terrestrial-only and airborne-only solutions. The geoid model solutions using the combined data had an accuracy of 2.69 cm for the Hotine method and 2.87 cm for the Stokes method in the test results using GPS/leveling data of the GSVS17 (Geoid Slope Validation Survey 2017) profile. Airborne data from the Gravity for the Redefinition of the American Vertical Datum (GRAV-D) project contributed significantly towards improving the geoid model, especially in the mountainous parts of the area.

A high resolution edge detector for interpreting potential field data: A case study from the Witwatersrand basin, South Africa

Extracting the edges of gravity and magnetic data has been commonly used as an important tool for determining the lineaments of buried structures. There are many edge detectors based on potential field derivatives for highlighting the source edges. The universal restrictions of these detectors are that they cannot produce balanced images, or the results obtained from use of these filters have low resolution. To solve these problems, I introduce a new filter that uses the Heaviside step function of the vertical derivative of the potential field data to detect the source edges. The novel filter is demonstrated on synthetic and real gravity datasets over the Witwatersrand basin (South Africa) and the estimated edges are compared with those determined from popular detectors. The findings show that the proposed method had better performance than other detectors in outlining the geologic features which were not seen in the observed fields.

Interpretation of gravity data to delineate the subsurface structures and reservoir geometry of the Aluto–Langano geothermal field, Ethiopia

The Aluto–Langano geothermal field is located within the central-southern part of the Main Ethiopian Rift, about 200 km southeast of Addis Ababa. This study analyzes gravity data to define the geometry of the geothermal reservoir and subsurface geological structures controlling the geothermal system. A Bouguer anomaly map of the area was generated using a Bouguer density of 2.49 g/cm3, which was obtained from the gravity dataset using different optimization methods. In order to highlight the structures of interest in the calderic system, a regional-residual separation was conducted. The map produced shows two dominant high Bouguer anomalies underlying Aluto caldera associated with the presence of basaltic rocks and the occurrence of fumarolic activity. Analysis of horizontal gradients of gravity data showed that the area is characterized by the presence of high gradient anomalies associated with geothermal manifestations. A 3-D gravity model for assessing the geothermal heat source architecture within the Aluto caldera area is also presented. The model was constrained using the estimated densities of rock cuttings obtained from drilled wells. The results from 3-D modeling imaged a high-density body at depth surrounded by a less-dense body (∼2.55 g/cm3), which is interpreted to be the geothermal reservoir. The model was then interpreted together with surface manifestations, lithology, and geological structures. The less-dense body found west of the Adonsha fault lineament could be the expression of the second reservoir, which is likely deeper than the productive reservoir found along the NNE–SSW trending structure (i.e., the Artu Jawe Fault Zone) in the central area.

Joint inversion of ground gravity data and satellite gravity gradients between Nepal and Bhutan: New insights on structural and seismic segmentation of the Himalayan arc

Along-strike variation in the geometry of lithospheric structures is a key control parameter for the occurrence and propagation of major interplate earthquakes in subduction and collision zones. The lateral segmentation of the Himalayan arc is now well-established from various observations, including topography, gravity anomalies, exhumation rates, and present-day seismic activity. Good knowledge of the main geometric features of these segments and their boundaries is thus the next step to improve seismic hazard assessment in this area. Following recent studies, we focus our approach on the transition zone between Nepal and Bhutan where both M > 8 earthquakes and changes in the geometry of the Indian plate have been documented. Ground gravity data sets are combined with satellite gravity gradients provided by the GOCE mission (Gravity Field and Steady-State Ocean Circulation Explorer) in a joint inversion to assess the location and the geometry of this transition. We obtain a ca. 10 km wide transition zone located at the western border of Bhutan that is aligned with the Madhupur fault in the foreland and coincides with the Dhubri–Chungthang fault zone and the Yadong-Gulu rift in Himalaya and southern Tibet, respectively. This sharp segment boundary at depth can act as a barrier to earthquake rupture propagation. It can possibly restrict the size of large earthquakes and thus reduce the occurrence probability of M > 9 earthquakes along the Main Himalayan Thrust.

Gravity observations on Santorini island (Greece): Historical and recent campaigns

Santorini is located in the central part of the Hellenic Volcanic Arc (South Aegean Sea) and is well known for the Late-Bronze-Age “Minoan” eruption that may have been responsible for the decline of the great Minoan civilization on the island of Crete. To use gravity to probe the internal structure of the volcano and to determine whether there are temporal variations in gravity due to near surface changes, we construct two gravity maps. Dionysos Satellite Observatory (DSO) of the National Technical University of Athens (NTUA) carried out terrestrial gravity measurements in December 2012 and in September 2014 at selected locations on Thera, Nea Kameni, Palea Kameni, Therasia, Aspronisi and Christiana islands. Absolute gravity values were calculated using raw gravity data at every station for all datasets. The results were compared with gravity measurements performed in July 1976 by DSO/NTUA and absolute gravity values derived from the Hellenic Military Geographical Service (HMGS) and other sources. Marine gravity data that were collected during the PROTEUS project in November and December 2015 fill between the land gravity datasets. An appropriate Digital Elevation Model (DEM) with topographic and bathymetric data was also produced. Finally, based on the two combined datasets (one for 2012–2014 and one for the 1970s), Free air and complete Bouguer gravity anomaly maps were produced following the appropriate data corrections and reductions. The pattern of complete Bouguer gravity anomaly maps was consistent with seismological results within the caldera. Finally from the comparison of the measurements made at the same place, we found that, within the caldera, the inner process of the volcano is ongoing both before, and after, the unrest period of 2011–2012.

Delineation of structure elements and the basement depth at the Jifara plain NW LIBYA using integration application of potential field dataset

In this study, the analysis of ground gravity data (from the Libyan Petroleum Institute) and aeromagnetic data (from the African Magnetic Mapping Project) are used to delineate the structure elements and basement depth within the Jifara plain, northwest Libya. A high-pass filter and a reduction-to-the-pole (RTP) transformation are applied to the gravity and aeromagnetic data respectively. The Total horizontal gradient and Centre for Exploration Targeting (CET) grid analysis are applied to gravity datasets. Werner deconvolution is applied to the RTP magnetic data. One 2-D forward modelling profile is applied to gravity and magnetic dataset. The results show gravity anomaly highs and magnetic anomaly with different structure trends. Werner deconvolution profile shows a high-density intrusion (about 1.8 km from the surface) within the sedimentary sequence. The intrusion may be the result of the Eocene volcanic activities near the Gharyan region. The 2-D model shows that the basement depth of Jifara plain ranges from 2.8 to 3.0 km. The integration results of the gravity and magnetic dataset reflect various periods of tectonic activities within Jifara plain and the adjacent area.

Exploring structured scripting cartographic technique of GMT for ocean seafloor modeling: A case of the East Indian Ocean

This paper examines spatial variations in the geomorphology of the Ninety East Ridge (NER), located in the Indian Ocean. The NER is an extraordinary long linear bathymetric feature with topography reflecting complex geophysical setting and geologic evolution. The research is based on a compilation of high-resolution bathymetric, geological, and gravity datasets clipped for the study area extent (65° - 107°E, 35°S - 21°N): General Bathymetric Chart of the Oceans (GEBCO), Earth Gravitational Model (EGM2008, EGM96). The submarine geomorphology of the NER was modeled by digitized cross-sectional profiles using Generic Mapping Tools (GMT). The availability of the method is explained by 1) the free datasets; 2) the open source GMT toolset; 3) the available tutorials of the GMT and the codes explained in this work. Three segments of the NER were selected, digitized, and modeled: 1) northern 89°E, 7°S to 90°E, 7°N; 2) central 88.4°E, 14.7°S to 88.8°E, 8.2°S; 3) southern 87.9°E, 17°S to 87.5°E, 27°S. Measured depths were visualized in graphs, compared, and statistically analyzed by the histograms. The northern segment has a steepness of 21.3° at the western slopes, and 14.5° at the eastern slope. The slopes on the eastern flank have dominant SE orientation. The central segment has a bell-shaped form, with the highest steepness comparing to the northern and southern segments. The eastern flank has a steepness of 49.5°. A local depression at a distance of 50 km off from the axis (90°E) continues parallel to the NER, with the shape of the narrow minor trench. The western slope has a steepness of 57.6°, decreasing to 15.6°. The southern segment has a dome-like shape form. Compared to the northern and central segments, it has a less pronounced ridge crest, with a steepness of 24.9° on the west. The eastern flank has a steepness of 36.8° until 70 km, gradually becoming steeper at 44.23°. A local minor trench structure can be seen on its eastern flank (100 km off the axis). This corresponds to the very narrow long topographic depressions stretching parallel to this segment of the NER at 90.5°E. The study contributes to regional geographic studies of Indian Ocean geomorphology and cartographic presentation of GMT functionality for marine research and oceanographic studies.

The improvements of the Costa Rican ground-based gravity dataset as a result of a comprehensive attribute and spatial assessment of the historical databases

Improving the accuracy of gravity models and their derived products such as Geoid models requires not only accurate airborne, shipborne, and satellite gravity measurements but also a good distribution of ground-based gravity measurements. Since terrestrial gravity surveys are limited by access and may entail a great cost, scientists rely on historical data generated by past surveys. Such datasets are heterogeneous and often lack meta-data. This work achieved a homogenized gravity dataset for Costa Rica and characterizes the spatial distribution of existing, good-quality data in order to consider the focus of terrestrial gravity surveys. First, the gravity value quality control removed duplicates and identified outliers through a process of leave one out cross-validation. Then, we compared terrestrial gravity disturbance values with data from Combined Global Geopotential Models such as EGM2008, GECO, EIGEN-6C4, and the SGG-UGM-1 for quality assessment and consistency of the data. Furthermore, we evaluated a 5′ grid for Costa Rica considering the best spatial resolution expected for the current Combined Global Geopotential Models. Taking into account the spatial distribution of the validated historical dataset, we surveyed new gravity points to significantly improve the coverage, reducing the 5′ × 5′ cells lacking data from 30% to 15%. This resulted in a complete dataset on a 5′ grid except for areas that are exceedingly difficult to access such as national parks, indigenous areas, and restricted private properties.

Surface slip distributions and geometric complexity of intraplate reverse-faulting earthquakes

Abstract Earthquake ground surface ruptures provide insights into faulting mechanics and inform seismic hazard analyses. We analyze surface ruptures for 11 historical (1968–2018) moment magnitude (Mw) 4.7–6.6 reverse earthquakes in Australia using statistical techniques and compare their characteristics with magnetic, gravity, and stress trajectory data sets. Of the total combined (summative) length of all surface ruptures (∼148 km), 133 km (90%) to 145 km (98%) align with the geophysical structure in the host basement rocks. Surface rupture length (SRL), maximum displacement (MD), and probability of surface rupture at a specified Mw are high compared with equivalent Mw earthquakes globally. This is attributed to (1) a steep cratonic crustal strength gradient at shallow depths, promoting shallow hypocenters (∼1–6 km) and limiting downdip rupture widths (∼1–8.5 km), and (2) favorably aligned crustal anisotropies (e.g., bedrock foliations, faults, fault intersections) that enhanced lateral rupture propagation and/or surface displacements. Combined (modeled and observed) MDs are in the middle third of the SRL with 68% probability and either the ≤33rd or ≥66th percentiles of SRL with 16% probability. MD occurs proximate to or directly within zones of enhanced fault geometric complexity (as evidenced from surface ruptures) in 8 of 11 earthquakes (73%). MD is approximated by 3.3 ± 1.6 (1σ) × AD (average displacement). S-transform analyses indicates that high-frequency slip maxima also coincide with fault geometric complexities, consistent with stress amplifications and enhanced slip variability due to geometric and kinematic interactions with neighboring faults. Rupture slip taper angles exhibit large variations (−90% to +380% with respect to the mean value) toward rupture termini and are steepest where ruptures terminate at obliquely oriented magnetic lineaments and/or lithology changes. Incremental slip approximates AD between the 10th and 90th percentiles of the SRL. The average static stress drop of the studied earthquakes is 4.8 ± 2.8 MPa. A surface rupture classification scheme for cratonic stable regions is presented to describe the prevailing characteristics of intraplate earthquakes across diverse crustal structural-geophysical settings. New scaling relationships and suggestions for logic tree weights are provided to enhance probabilistic fault displacement hazard analyses for bedrock-dominated intraplate continental regions.

Mapping mean lake surface from satellite altimetry and GPS kinematic surveys

Lake water height is a key variable in water cycle and climate change studies, which is achievable using satellite altimetry constellation. A method based on data processing of altimetry from several satellites has been developed to interpolate mean lake surface (MLS) over a set of 22 big lakes distributed on the Earth. It has been applied on nadir radar altimeters in Low Resolution Mode (LRM: Jason-3, Saral/AltiKa, CryoSat-2) in Synthetic Aperture Radar (SAR) mode (Sentinel-3A), and in SAR interferometric (SARin) mode (CryoSat-2), and on laser altimetry (ICESat). Validation of the method has been performed using a set of kinematic GPS height profiles from 18 field campaigns over the lake Issykkul, by comparison of altimetry’s height at crossover points for the other lakes and using the laser altimetry on ICESat-2 mission. The precision reached ranges from 3 to 7 cm RMS (Root Mean Square) depending on the lakes. Currently, lake water level inferred from satellite altimetry is provided with respect to an ellipsoid. Ellipsoidal heights are converted into orthométric heights using geoid models interpolated along the satellite tracks. These global geoid models were inferred from geodetic satellite missions coupled with absolute and regional anomaly gravity data sets spread over the Earth. However, the spatial resolution of the current geoid models does not allow capturing short wavelength undulations that may reach decimeters in mountaineering regions or for rift lakes (Baikal, Issykkul, Malawi, Tanganika). We interpolate in this work the geoid height anomalies with three recent geoid models, the EGM2008, XGM2016 and EIGEN-6C4d, and compare them with the Mean Surface of 22 lakes calculated using satellite altimetry. Assuming that MLS mimics the local undulations of the geoid, our study shows that over a large set of lakes (in East Africa, Andean mountain and Central Asia), short wavelength undulations of the geoid in poorly sampled areas can be derived using satellite altimetry. The models used in this study present very similar geographical patterns when compared to MLS. The precision of the models largely depends on the location of the lakes and is about 18 cm, in average over the Earth. MLS can serve as a validation dataset for any future geoid model. It will also be useful for validation of the future mission SWOT (Surface Water and Ocean Topography) which will measure and map water heights over the lakes with a high horizontal resolution of 250 by 250 m.