The southeastern margin of the Tibetan Plateau, situated at the tectonic intersection of the oblique collision between the Indian and Eurasian plates, exhibits prominent features of crustal shortening, lateral block extrusion, and partial melting associated with Cenozoic intracontinental deformation. It constitutes a key area for studying the formation and evolution of the Tethys tectonic belt, providing crucial constraints on the uplift and growth of the plateau. Although there are many seismic velocity models with variable resolutions for the region, the effective integration between seismic velocity models and geological interpretations remains limited, hindering a more complete understanding of lithospheric structure and tectonic evolution. To address this issue, we developed a new body-wave travel time tomography method constrained by the variation of information, which enhances the correlation among different velocity properties. By applying this method to the southeastern margin of the Tibetan Plateau, we obtained intrinsically consistent Vp, Vs, and Vp/Vs models with a lateral resolution of 0.2 degrees . Compared with previous studies, our results exhibit significantly improved correlation between Vp and Vp/Vs models. To further interpret the velocity models, we applied the feature-weighted clustering algorithm to identify physically coherent domains. Based on the clustering results, we can divide the lithosphere of the region into distinct geological domains, which is helpful for constructing the tectonic framework of the southeastern Tibetan Plateau. Overall, this new tomography method, combined with the clustering analysis, establishes a geological modeling framework that can bridge the gap between seismic velocity models and geological models.

Fluids in the crust may build pressure through a variety of mechanisms. This buildup can activate nearby faults, which then serve as conduits for fluid flow and as valves for pressure release. The rapid pressure drop promotes mineral precipitation and fault sealing, allowing the cycle to restart. While evidence of this cyclical interplay between faults and fluids is abundant in exhumed faults, whether these processes can be identified in the seismological record remains unknown. Here, we have detected a complete sequence of fluid pressure build-up, fluid migration through local faults, and pressure release by means of a joint magnetotelluric and seismicity survey in the Southern Andes. We identified intense seismicity along a previously unmapped WNW-striking fault, with seismic swarm activity concentrated at the edge of a low-resistivity crustal zone, interpreted as a fluid reservoir. Hypocenters delineate two fault planes from the reservoir toward higher-resistivity crust. We linked distinct seismicity patterns to each sequence stage: fluid pressure buildup is marked by relatively high b-values (1.2-1.4) and low magnitudes (M-L < 1.5), while fluid migration and pressure release are characterized by a sudden drop in b-values (<1). Our findings capture the seismic signature of a fluid migration pulse along faults, linking active fault-fluid interactions with long-term geological evidence of fluid compartmentalization and cyclical along-fault fluid flow.

The surface geology of Greenland is only known along the ice-free coast. The remaining 80% of Earth's largest island are covered by ice that masks the surface geology and makes direct observation nearly impossible. Interpolation of the known coastal geology over the inland ice, combined with expert knowledge, can provide a first, but not well-constrained picture. In contrast, the surface geology in Scandinavia is well-studied. The formerly adjacent northeastern part of Greenland belongs to the same Caledonian orogeny and is expected to be somewhat similar to Scandinavia. Therefore, we use Scandinavia as a case study to set up a workflow of joint inversion of potential field data to find physical relations for the known geological structure and apply this workflow to NE Greenland.Results from individual inversion of potential field data are non-unique and have limited depth resolution. Combining gravity and magnetic data in a joint inversion can minimise the non-uniqueness and improve the depth resolution. The coupling furthermore creates comparable anomaly patterns for both inverted parameters. As coupling method, a variation of information (VI) constraint is used in the inversion. The VI creates representative parameter relationships where different branches reflect the numerous combinations of density and susceptibility for various rock types. Thus, the inverted parameter relationship can be used to map the surface geology.Crucial parts in the workflow setup are how deeper sources are handled for the gravity data and at which resolution and height the magnetic data are required. The simultaneous analysis of the well-studied surface geology in Scandinavia helps to verify the analysis, providing higher confidence in the resulting sub-ice geology for NE Greenland.

Seismic tomography is a useful tool to obtain the velocity structure of the Earth's interior. Compared with the Vp and Vs models, the Vp/Vs model is of great significance for studying the properties of subsurface structure, such as fluid saturation and porosity. However, due to different data quality and quantity for P- and S-wave, Vp and Vs models generally have different resolutions and uncertainties, leading to some artifacts in the Vp/Vs model. Tryggvason and Linde (2007) proposed to use the structural similarity in Vp and Vs models to better constrain the Vp/Vs model. However, the clustering relationship between Vp and Vp/Vs models is not optimized, which limits the further geological interpretations based on velocity models. In this study, we aim at developing a new seismic tomography method based on the variation of information for Vp and Vp/Vs models. This method follows joint inversion of magnetotelluric and gravity data based on the variation of information (Moorkamp, 2022), which can improve the clustering relationship between electrical resistivity and density. The 2021 Ms6.4 Yangbi earthquake is located at the intersection of the Red river fault and the nearly north-south trending Lijiang-Dali fault system on the southwestern boundary of the Sichuan-Yunnan block. This earthquake has the classic characteristic of a "foreshock-mainshock-aftershock" sequence. In this study, we have developed a new seismic tomography method based on the variation of information to couple the Vp/Vs model with the Vp model to obtain more reliable Vp, Vs, and Vp/Vs models in the source region of the Yangbi earthquake. Our results show that the foreshocks occur in structures with low Vp, high Vs and low Vp/Vs, while the main shock occurs in the area with high Vp, high Vs and low Vp/Vs. Based on the cross-plot analysis and petrophysical experimental data, we suggest that long-term stress accumulation causes shearing in areas with high quartz content at a depth of 10km.

The solid earth structure beneath Greenland, meaning the rocky part of Earth from the ice-bed interface to depth, has gained increased interest in recent years as it provides a critical boundary condition for the dynamic evolution of the Greenland ice sheet (GrIS), one of the largest sources of sea-level rise contributions since the early 2000s. However, no consensus has been reached regarding the key internal or surface earth properties influencing this boundary condition and thus GrIS behaviour. One important surface property is the subglacial heat flow, which affects sliding conditions of the ice sheet including the onset of major ice streams and is related to subglacial geology. Lithospheric architecture and mantle viscosity structure are internal properties that influence ice sheet evolution through changes in the height and slope of the ice-bed interface caused by glacial isostatic adjustment. Because there is no general agreement regarding crustal and lithospheric structures, some glaciological studies use an ensemble of solid earth models to incorporate uncertainties into their GrIS predictions, but it is unclear how these variations ultimately affect estimates of future sea-level rise. Here we describe the main solid earth properties that are important for GrIS evolution (heat flow, temperature, viscosity), from the base of the ice sheet to the upper mantle, and we provide some perspectives on how future collaborative efforts and integrated studies could lead to better agreement regarding these key characteristics.

The main goal of volcano monitoring is to understand processes occurring within the magmatic system from there geophysical response signatures that informs on the eruptive behavior of the system. Traditional volcano monitoring approaches have relied primarily on changes in seismicity, and geodetic response which depend on active changes such as magma migration. The magnetotelluric method, informs the physical property electrical conductivity, which within a magmatic context is sensitive to the presence of fluids, temperature, and melt fraction. High quality magnetotelluric measurements allow identification of conductivity variations related to the physical changes in magmatic systems, making it possible to determine temporal changes in magma content and temperature. Thus, magnetotellurics may be a valuable additional monitoring tool able to detect static phase changes of the magmatic fluids within the system, contributing to our understanding of the dynamics occurring within magmatic systems absent of and/or preceding magma movement within the system. Mount St Helens as perhaps the most studied volcanic system including a dense 3D MT data set collected during the 2004-2008 dome building eruption provides the opportunity to investigate the difference within the magmatic system in different eruptive states (i.e. eruptive 2004-2008 and current quiescence). This is been achieved by a complete repeat of the initial survey (67 sounding locations) collected in 2005-06 and establishment of 4 ‘continuous’ telemetered systems. Given the current non-eruptive state of Mount St Helens a second system has been selected, Stromboli, currently in an eruptive cycle for installation of 4 continuous telemetered MT systems. Stromboli volcano, maintains a persistent eruptive activity, with near daily minor explosions typically punctuated by 1-2 larger events a year. Initial evaluation of the streamed measurements has been completed using Phase Tensor parameters, which are immune to galvanic distortions, which can occur from seasonal near-surface conductivity changes (e.g. variations in soil moisture content). The Phase Tensor represents conductivity gradients in the subsurface, and Phase Tensor differences between two measurement times hence reflects the changes in conductivity structure in time. The temporal application of magnetotellurics provides a mechanism to determine short-term and long-term conductivity changes within the magmatic system representative of physical changes within the system, correlatable to active eruptive behavior, properties of the system during periods of quiescence and eruptive phases.

The shared tectonic history of southwestern Australia and East Antarctica facilitates the exchange of geological insights between the regions. In this study, we present coupled susceptibility and density models obtained through the joint inversion of magnetic and gravity data. By assuming a common geological source for both signals, our coupling method minimizes misfits and variation in information, thereby enhancing a correlation between susceptibility and density. The resulting anomalies demonstrate structural continuity between the continents, aligning closely with major shear zones and seismic reflectors. Combining these results with machine learning, geochemical, and petrophysical databases, we predict a high‐resolution (10 km) heat production map for East Antarctica. Utilizing a Markov Chain Monte Carlo (MCMC) algorithm, we further develop a geothermal heat flow map with greater spatial variability than previous studies, yielding an average of mW/ in East Antarctica and mW/ in southwestern Australia. Our results provide a crucial high‐resolution boundary condition for ice sheet simulations, enabling more realistic estimates of basal meltwater production and ice temperatures.

In easternmost Papua New Guinea, some of the most metal-enriched continental crust has formed, hosting some of the world’s largest copper and gold deposits that formed over the last 2-3 million years. The area is marked by a dynamic and complex geologic history at a convergent margin with arc-continent collision, subduction reversals producing a complex microplate mosaic, and continuous metasomatism of the mantle wedge. In 2023, the SO299 DYNAMET expedition set sail to target the New Ireland Basin with special focus on the South Lihir Volcanic Field and investigate the source of the youngest volcanic activity in this area. To detect potential ascending melts and aqueous fluids focusing in the suspected trans-lithospheric faults, our team deployed 16 ocean-bottom magnetotelluric stations (OBMT) with a bottom-time of up to 3 weeks in the New Ireland Basin south and west of Lihir Island. Given the assumed geological complexity, the stations were positioned in a 3D array with approximately 15 km spacing, including a dense cluster around the volcanic seamounts south of Lihir. After the deployment we found 12 stations recorded valid data which were segmented, filtered and robustly processed to electromagnetic impedances for periods between 60 and 20,000 s. Preliminary results based on analyzing the 1D Berdichevsky average response function, the 2D response functions and 1D inversions show anomalies for several stations close to Lihir island on an NW-SE pointing profile in approximately 20-30 kilometers depth. Dimensionality analysis points towards an at least 2D structured anomaly with a possible northern strike direction. However, the area's complex geometry and proximity to nearby islands introduce significant uncertainties and favors 3D inversion, which is currently work in progress.

Heterogeneities in subglacial geology and crustal properties can play a major role in determining the boundary conditions at the crucial interface between the solid earth and the cryosphere in Antarctica. Geothermal heat flow, a parameter closely tied to regional geology, can particularly influence the behaviour of the overriding icesheet. However, direct geological samples which could inform understanding of heat flow and other geological parameters are limited to ice free regions along the coast, high mountain ranges or isolated nunataks, while the origin of geological material transported by glaciers themselves is often ambiguous. Geophysical joint inversion of gravity and magnetic data can therefore play a key role in constraining the geological and crustal properties of the rocks hidden beneath the ice.We present a 3D crustal model of Wilkes Subglacial Basin and Transantarctic Mountains based on joint inversion of airborne gravity and magnetic data using the “Variation of Information” inversion algorithm incorporated in the software JIF3D. The applied “Variation of Information” technique enforces a coupling in the objective function between inverted susceptibility and density distribution during the inversion. The objective function is minimized iteratively until a reasonable mismatch between observed and inverted data is reached. The coupling ensures that identical geometries in the inverted density and susceptibility distribution are found relating to shared gravity and magnetic sources. This technique provides an enhanced inversion result for interpreting subglacial geology since inverted geometries relate to both petrophysical quantities compared to separately inverting for density and susceptibility distributions.Our model reveals a large body located in the interior of Wilkes Subglacial Basin interpreted as a batholithic intrusive structure, as well as a linear dense body at the margin of the Terre Adélie Craton. Density and susceptibility relationships of the anomalous bodies, together with their shapes in 3-dimensions are used to inform the composition and the origin of these crustal bodies. Comparing the density and susceptibility values recovered by our inversion with measurements on Antarctic rock samples indicates that the postulated batholitic intrusion is granitic in composition, but distinct from the Granite Harbour Igneous Complex described previously in the Transantarctic Mountains area. Emplacement of such a large volume of intrusive granite can potentially elevate local geothermal heat flow significantly, due to relative concentration of radiogenic heat producing elements such as U, K and Th. Finally, we present a new conceptual tectonic model for the region based on the inversion results, which includes passive margin development ~670 Ma, with the emplacement of the batholith intrusion before the Granite Harbour Suite.

Subglacial geology remains largely unknowns in Antarctica. Direct geological samples are limited to ice free regions along the coast, high mountain ranges or isolated nunataks, while the origin of geological material transported by glaciers themselves is often ambiguous. 3D singular and joint inversions of airborne gravity and magnetic data recovers subsurface density and susceptibility distribution. The relationship between both inverted petrophysical quantities provide crucial insight for subglacial geology and rock provinces interpretations. Validation of indirect derived subglacial geology models are critical but very challenging in Antarctica due to the sparsity of rock samples. We present 324 new density and susceptibility measurements on rock samples from Northern Victory Land, East Antarctica. 251 samples have been measured at the National Polar Sample Archive (NAPA) from the Federal Institute for Geosciences and Natural Resources (BGR) in Berlin-Spandau, Germany and additional 73 samples were measured at the BGR in Hannover, Germany. We use the petrophysical measurements to validate our recent regional scale 3D joint inversion model of the Wilkes Subglacial Basin and the Transantarctic Mountains. Furthermore, we validate inversion results on a local scale of singular magnetic inversion based on high resolution airborne magnetic data with a flight line spacing of 500m in the Mesa Range. We demonstrate that we can provide reliable discrimination between Ferrar Dolerites, Kirkpatrick Basalt and Granite Harbour intrusion rocks based on our local inversion model and that the recovered susceptibilities agree with those measured at rock samples from the study area. Furthermore, we show that regional scale inversion model of gravity and susceptibility distribution agrees for samples of the dominant crustal rock types. However, densities of small-scale dense intrusion bodies like Ferrar Dolerites are underestimated by the regional scale inversion, while the susceptibility range is correctly recovered. Constraining subglacial geology with joint inversion of airborne potential field data is heavily depended on the resolution of the airborne survey, flight line coverage, the inversion scale, and the scale of the target feature. Regional scale inversion is adequate for large scale geological heterogeneities, which underestimate petrophysical quantities for small scale structures, while local scale inversions are able to resolve such structures but are more computational demanding and in the case of Antarctica lack ultra-high resolution airborne gravity data with a line spacing below 1000 – 500m.

Direct geological information in Antarctica is limited to ice free regions along the coast, high mountain ranges or isolated nunataks. Therefore, indirect methods are required to reveal subglacial geology and heterogeneities in crustal properties, which are critical steps towards interpreting geological history. We present a 3D crustal model of density and susceptibility distribution in the Wilkes Subglacial Basin and the Transantarctic Mountains (TAM) based on joint inversion of airborne gravity and magnetic data. The applied “variation of information” technique enforces a coupling between gravity and magnetic sources to give an enhanced inversion result. Our model reveals a large-scale body located in the interior of the Wilkes Subglacial Basin interpreted as a batholithic intrusive structure, as well as a linear dense body at the margin of the Terre Adélie Craton. Density and susceptibility relationships are used to inform the interpretation of petrophysical properties and the reconstruction of the origin of those crustal blocks. The petrophysical relationship indicates that the postulated batholitic intrusion is granitic, but independent from the Granite Harbour Igneous Complex previous described in the TAM area. Emplacement of a large volume of intrusive granites can potentially elevate local geothermal heat flow significantly. Finally, we present a tectonic evolution sketch based on the inversion results, which includes development of a passive continental margin with seaward dipping basalt horizons and magmatic underplating followed by two distinct intrusion events in the Wilkes Subglacial Basin with Pan-African ages (700 - 551 Ma) and Ross ages (550 - 450 Ma).

SUMMARY Greenland’s tectonic history is complex, and the resulting lithospheric structure is, although extensively researched, not well constrained. In this study, we model the lithospheric structure of Greenland in a consistent, integrated framework with three steps. First, we build a lithospheric background model by forward modelling, adjusted to gravity gradient data and shear wave velocities from a regional tomography model. Subsequently, we jointly invert for the upper crustal density and susceptibility structure by minimizing the gravity residuals and magnetic total field anomaly misfit. The last modelling step searches for upper crustal thermal parameters to fit our model to the most recent geothermal heat flow predictions for Greenland. Finally, we present 3-D models of the density, temperature and velocity structure for the lithosphere as well as thermal parameters and susceptibilities for the upper crust. Our model also includes the depth of the Moho and LAB in Greenland. A comparison between inverted crustal parameters and surface geology shows a clear correlation. The novelty of our model is that all these results are consistent with each other and simultaneously explain a wide range of observed data.

Summary Bedrock geology from Antarctica remains largely unknown since it is hidden beneath thick ice sheets. Geophysical methods such as gravity and magnetic inverse modelling provide a framework to infer crustal rock properties indirectly in Antarctica. However, due to limited availability of rock samples, validation against direct geological information is challenging. We present a new rock property catalogue containing density and susceptibility measurements on 320 rock samples from northern Victoria Land. This catalogue is used to assess the reliability of local and regional scale inverse results, including a new local high resolution magnetic inversion in the Mesa Range region and a previously published regional scale joint inversion of gravity and magnetic data in northern Victoria Land and the Wilkes Subglacial Basin. We compare our density and susceptibility measurements to global and local measurements from the literature to access the correlation to rock types and geological units. Furthermore, the measured values are compared against inverted values. The close correspondence between inverted and measured rock properties allows us to predict locations of rock types where currently such information is missing. The utility of measured susceptibility and density relationships for interpreting inversion output provides a strong incentive to incorporate local rock samples into geophysical studies of subglacial geology across Antarctica.

: In this chapter, I discuss an alternative perspective on interpreting the results of joint and constrained inversions of geophysical data. Typically such inversions are performed based on inductive reasoning (i.e. we fit a limited set of observations and conclude that the resulting model is representative of the Earth). While this has seen many successes, it is less useful when, for example, the specified relationship between different physical parameters is violated in parts of the inversion domain. I argue that in these cases a hypothesis testing perspective can help to learn more about the properties of the Earth. I present joint and constrained inversion examples that show how we can use violations of the assumptions specified in the inversion to study the subsurface. In particular I focus on the combination of gravity and magnetic data with seismic constraints in the western United States. There I see that high velocity structures in the crust are associated with relatively low density anomalies, a possible indication of the presence of melt in a strong rock matrix. The concepts, however, can be applied to other types of data and other regions and offer an extra dimension of analysis to interpret the results of geophysical inversion algorithms.

The tectonic history of southern Africa includes Archean craton formation, multiple episodes of subduction and rifting and some of the world's most significant magmatic events. Lithospheric models based on seismological and magnetotelluric data show highly heterogeneous crustal structure, significant anomalies in the lithospheric mantle and strong variations of the depth to the lithosphere-asthenosphere boundary. While some of the spatial patterns agree between different geophysical methods, there are also significant differences in the geometry and location of many structures. I perform a joint inversion of magnetotelluric and satellite gravity data to reconcile these apparent discrepancies. Resistivity and density are coupled through a newly developed Variation of Information constraint which strives to establish a one-to-one relationship between the two quantities. This allows the resistivity-density relationship to evolve data driven during the inversion. The final combined resistivity-density model shows detailed lithospheric and sub-lithospheric structure below the Kaapvaal Craton and adjacent mobile belts. In addition, the retrieved parameter relationship exhibits several branches indicating strong variations in composition. Compared to results from a similar inversion in the western United States, the inversion indicates significantly less fluid related low resistivity anomalies and a dominance of high-density low-resistivity structures. This corroborates earlier ideas that fluids are difficult to contain within in the Earth over long time scales. I will discuss the implications for the tectonic evolution of the region and for the interpretation of low resistivity anomalies world-wide.

&lt;p&gt;Geophysical methods such as seismology, magnetotellurics and gravity are key to reconstructing the structure of the upper mantle and inferring its composition. However, the relationship between composition and geophysical parameters, e.g. seismic velocity, resistivity or density, is complex and depends on other factors such as temperature, for example. This makes it difficult to untangle the various effects from inversions based on single parameters. Joint inversion establishes quantitative relationships between different geophysical parameters and thus provides additional information that can be interpreted in terms of composition and temperature. I use 3D joint inversion of surface wave, gravity and magnetotelluric data to construct integrated models in a data driven way. The relationship between the different quantities is recovered as part of the inversion through a Variation of Information based constraint. This constraint aims at establishing a one-to-one relationship between each parameter pair. Results from the western United States, Germany, Southern Africa and Australia show that this approach can retrieve highly detailed and strongly coupled results that can be interpreted, for example, in terms of hydration of the lithosphere. Comparison of results from different geologic domains indicates significantly different relationships depending on formation age. I will discuss how we can use data driven parameter relationships to infer the state of the lithosphere. In addition, I will outline the road towards robust quantitative inference based on these relationships.&lt;/p&gt;

The Wilkes Subglacial Basin hosts potentially the largest unstable sector of the East Antarctica Ice Sheet due to the depth of the ice bed below sea level. Ice covering such basins poses a potentially high, but poorly constrained risk for future sea-level rise, as it is more vulnerable to melting by warming of the surrounding ocean. Such melting could potentially trigger mechanisms of unstable retreat. The neighbouring Transantarctic Mountains are the largest non-contractional mountain range on Earth. Traditionally, the Transantarctic Mountains are viewed as dividing the ancient East Antarctic craton from the younger West Antarctic Rift system. However, petrological samples and previous geophysical mapping suggest that the craton boundary is further west, following the western edge of the Wilkes Subglacial Basin. Subglacial geology influences geothermal heat flow and bed roughness, and therefore to better understand the past, present and possible future behaviour of the East Antarctic Ice Sheet improved understanding of the subglacial geology on which it flows, especially in the Wilkes Subglacial Basin and Transantarctic Mountains region, is important.We present a new 3D crustal model of the Wilkes Subglacial Basin and the Transantarctic Mountains based on joint inversion of airborne gravity and magnetic data using the mutual information inversion algorithm incorporated in the software JIF3D. Our model shows a large intrusive body located in the interior of the Wilkes Subglacial Basin and suggests a tectonically complex area west of the Basin, which could potentially indicate the transition zone at the margin of the Terre Adélie Craton. Geological units are inferred by clustering of inverted susceptibility and density distribution and are validated against sparse petrological samples from the Transantarctic Mountains region and along the George V Land and Terre Adélie coasts. Our inferred crustal properties model can provide crucial insight into the heterogeneity of subglacial geology in terms of thermal conductivity and crustal heat production, which could influence the geothermal heat flow in this area and therefore make the overlying ice sheet more vulnerable than commonly thought.

On 3 August 2014, a destructive earthquake with a magnitude of 6.5 occurred in the Ludian region in Yunan, China, causing heavy casualties. However, the seismogenic structure of the Ludian earthquake is still unclear. To address this issue, we have developed a three‐dimensional joint inversion algorithm for magnetotelluric data and seismic body wave arrival times based on the cross gradient structural similarity constraint, which utilizes the complementary advantages of the two different datasets. We use arrival times of P ‐ and S ‐waves from 6,369 local earthquakes recorded by 30 seismic stations and magnetotelluric impedances from 127 MT stations to perform the joint inversion to construct a combined 3‐D P ‐wave velocity ( Vp ), S ‐wave velocity ( Vs ), and resistivity models and the locations of relocated aftershocks in the source region of the Ludian earthquake. Our models show significant low Vp / Vs ratios and low resistivity in the upper crust of the main shock zone, and the relocated aftershocks form an L‐shaped zone. Based on petrophysical studies and analysis of the cross‐plots of different physical properties, we infer the existence of cracks/fractures and fluids as well as high quartz contents in the source region. These fluids may play a key role in promoting the Ludian earthquake by reducing the effective normal stress of the fault zone, and the existence of high quartz contents further makes the crustal rocks in the source region more brittle. Our study provides a new evidence how the properties of the crust in the focal area have some control on the genesis of moderate to large earthquakes.