Journal of Geophysical Research: Solid Earth最新文献

Basin aquifers are important groundwater sources in the Western United States that are increasingly stressed due to growing populations, increased resource use, and the impacts of climate change. These aquifers are mainly recharged through melting snowpack in the surrounding mountains that infiltrates to the water table and flows directly into the basin (Mountain Front Recharge), or through deeper groundwater pathways that flow from the mountains directly into the basin aquifer (Mountain Block Recharge). However, the dominant system of recharge remains uncharacterized in many mountain basin aquifers. To address this challenge, near-surface geophysical methods are being implemented to efficiently measure properties that govern groundwater storage and movement. This study infers groundwater storage and recharge to the Casper Aquifer around Laramie, WY, building off past studies that relied solely on sparse monitoring well data and observation of rainfall events. In this study, we use a clustering analysis on airborne electromagnetic data to define hydrogeophysical layers within the Casper Aquifer. These layers, which represent significant changes in bulk subsurface electrical resistivity, are integrated with existing hydrologic, lithologic, and smaller scale geophysical datasets to build a more representative hydrogeophysical model. Through this analysis, we define two sub-aquifers within the larger Casper Aquifer system that are connected through structurally induced fractures and faults. This research highlights the importance of integrating geophysical data at multiple scales for defining hydrogeophysical layers that provide both a more complete understanding of basin aquifer recharge dynamics and constrain more detailed hydrologic models.

Shear stress levels on reverse faults are anticipated to be several times higher than on normal faults with the same pore pressure ratio. In addition, ruptures on normal faults release gravitational potential energy, whereas earthquakes on reverse faults expend work in uplifting rocks. In this study, I investigate the significance of these differences for earthquake cycles and I question whether the source of energy driving earthquakes is the same on reverse and normal faults. Based on the assumption that normal and reverse faults have the same background frictional properties and pore pressure states, I use numerical simulations with a two-dimensional dynamic elastic-plastic model to show that due to stress differences, earthquakes on reverse faults tend to occur less frequently, produce more coseismic slip and stress drop and involve higher slip rates than ruptures on normal faults with equivalent dimensions. The analysis also shows differences in the energy changes associated with earthquake cycles on reverse and normal faults. However, the earthquakes on both fault types result from abrupt release of elastic strain energy, which proceed and essentially drive variations in gravitational potential energy. Thus, ruptures on both normal and reverse faults are consistent with elastic rebound theory.

The intricate and changing stress conditions within complex fault networks pose challenges in understanding earthquake recurrence and seismic hazards. The Kongur Shan Extensional System (KSES) in the northeastern Pamir, characterized by complex fault geometries and potentially variable surface loads in its surroundings, offers an ideal research area. Here we investigate three paleoseismic sites in the northern KSES, including lacustrine seismites in the Muji Basin, secondary fault scarps of the northern Kongur Shan Fault (KSF), and an exposure of the eastern Muji Fault. The seismites, as a regional record, indicate two M_w 6–7.3 paleoearthquakes occurring 720–680 and 1030–940 cal yr BP, with an interval of 230–330 years. The other two on-fault sites reveal paleoearthquakes with compatible magnitudes but much longer average time intervals (ATI). Secondary fault scarps of the northern KSF indicate four M_w 6.9–7.2 paleoearthquakes since 4.8 ± 0.8 ka, with an ATI of 1,000–1,870 years. The fault exposure of the eastern Muji Fault indicates two M_w 6.8–7.3 paleoearthquakes since 2.7 ± 0.3 ka, with an ATI of 1,200–3,000 years. Using its previously constrained long-term slip rate of ∼6.3 mm/yr and a simple Monte Carlo simulation of earthquake cycles, the observed ATI of the eastern Muji Fault suggests a “clustered” earthquake recurrence behavior. Such potential irregular earthquake recurrence is likely due to the low maturity and fault interaction in the northern KSES, and climate-related changes in surface loads of paleolakes and glaciers.

The third-order elastic (TOE) model has been used to describe the widely observed nonlinear mechanical behaviors of earth materials. In addition to linear elastic constants (λ, μ), three nonlinear elastic moduli (A, B, C) are required for isotropic rocks. Contrary to previous research on dynamic TOE moduli, this study followed the protocol to measure strain and stress under uniaxial and hydrostatic compressive tests statically, which were later used to invert for the full set of TOE moduli for four standard rock types with differing pore structures of a porous oolitic limestone, quartz-rich sandstones, and a dense crystalline basalt. The applicability of the TOE model to characterize nonlinearity depends on the fulfillment of path-independence and small-strain assumptions. Using the measured static TOE moduli, the finite element model demonstrates that the stress in the vicinity of the wellbore is more amplified than the stress in the linear elastic case, which leads to a wider zone of rock failure around the wellbore. Due to the long-standing discrepancy between static and dynamic moduli, the rarely reported full set of static TOE moduli is necessary and will benefit future research in understanding the effect of rock nonlinearity on geophysical and geomechanical applications, such as long-term safe storage of CO₂ and generating process of geohazards.

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 toward interpreting geological history. We present a 3D crustal model of density and susceptibility distribution in the Wilkes Subglacial Basin (WSB) 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 inferred susceptibility and density, relating these quantities to the same gravity and magnetic sources to give an enhanced inversion result. Our model reveals a large body located in the interior of the WSB 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 bodies. The petrophysical relationship indicates that the postulated batholitic intrusion is granitic, but independent from the Granite Harbor Igneous Complex described previously in the TAM area. Emplacement of a large volume of intrusive granites can potentially elevate local geothermal heat flow significantly. Finally, we present a new conceptual tectonic model 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 intrusive events associated with the protracted Ross Orogen.

The mechanics of methane hydrate-bearing sediments (MHBS) have been broadly investigated over recent years in the context of methane-gas production or climate-change effects. Their mechanical investigation has mainly been carried out using models constructed from experimental data obtained for laboratory-formed MHBS. Along with the dominant effects of hydrate saturation and morphology within the host soil pores, this study recognizes the effective pressure at which the hydrate is formed as a key factor in the MHBS mechanics. A state-of-the-art experimental study has been conducted in order to isolate the effect of the hydrate formation pressure, for use as a model parameter. Two generalized mechanical prediction models that incorporate the effect of the hydrate formation pressure are developed in this work: (a) an analytical shear strength prediction, and (b) an empiric graphical model for predicting volumetric changes along a given stress path. The models are related to a novel data representation which enables the analysis of a few individual test outcomes as a whole, through a volume-change mapping that describes the complex influence of the volumetric effect of hydrate in MHBS, under combined hydrostatic and deviatoric loading scenarios. In this study, we delve into a specific configuration of hydrate morphology, hydrate saturation, and host soil type, enabling a distinctive fundamental geotechnical investigation and the development of a conceptual modeling approach. The paper describes the approaches by which the MHBS properties can be extracted for other MHBS samples (than those examined in this work) having different host soils and hydrate pore-space morphologies.

We analyzed refraction and reflection seismic data covering the Central Lau Spreading Center (CLSC) and the Tonga volcanic arc at 18°S in the Pacific Ocean to investigate tectonic inactivity in actively extending back-arc systems. Our P-wave travel time tomography shows a characteristic 6–8 km thick back-arc crust beneath and around the CLSC and a ∼13 km thick arc-crust at the eastern end of our profile, which corresponds to the Tonga volcanic arc. Lower crustal velocities increase to 7.5 km/s toward the volcanic arc, marking the transition from back-arc to arc crust. These high P-wave velocities can be explained by a high pyroxene content in the lower crust originating from depleted melts. Our seismic reflection data show steep normal faults below the CLSC and volcanic structures closer to the volcanic arc, without a tectonically inactive zone (also knows as the “diffuse plate boundary”) in between. Based on our results, we suggest that the Niuafo'ou and Tonga microplates should be treated as one tectonic plate with local zones of intra-plate deformation that are separated by zones that are tectonically quiet.

The Tonga-Kermadec subduction zone exhibits the fastest observed trench retreat and convergence near its northern end. However, a paradox exists: despite the rapid trench retreat, the Tonga slab maintains a relatively steep dip angle above 400 km depth. The slab turns flat around 400 km, then steepening again until encountering a stagnant segment near 670 km. Despite its significance for understanding slab dynamics, no existing numerical model has successfully demonstrated how such a distinct slab morphology can be generated under the fast convergence. Here we run subduction models that successfully reproduce the slab geometries while incorporating the observed subduction rate. We use a hybrid velocity boundary condition, imposing velocities on the arc and subducting plate while allowing the overriding plate to respond freely. This approach is crucial for achieving a good match between the modeled and observed Tonga slab. The results explain how the detailed slab structure is highly sensitive to physical parameters including the seafloor age and the mantle viscosity. Notably, a nonlinear rheology, where dislocation creep reduces upper mantle viscosity under strong mantle flow, is essential. The weakened upper mantle allows for a faster slab sinking rate, which explains the large dip angle. Our findings highlight the utilizing rheological parameters that lead to extreme viscosity variations within numerical models to achieve an accurate representation of complex subduction systems like the Tonga-Kermadec zone. Our study opens new avenues for further study of ocean-ocean subduction systems, advancing our understanding of their role in shaping regional and global tectonics.

Constraining stresses in the Earth's crust in volcanic regions is critical for understanding many mechanical processes related to eruptive activity. Dike pathways, in particular, are shaped by the orientation of principal stress axes. Therefore, accurate models of dike trajectories and future vent locations rely on accurate estimates of stresses in the subsurface. This work presents a framework for probabilistic constraint of the stress state of calderas by combining three-dimensional physics-based dike pathway models with observed past vent locations using a Monte Carlo approach. The retrieved stress state is then used to produce probability maps of future vent opening across a caldera. We test our stress inversion and vent forecast approach on synthetic scenarios, and find it successful depending on the distribution of the available vents and the complexity of the volcano's structural history. We explore the potential and limitations of the approach, show how its performance is sensitive to the assumptions in the models and available prior information, and discuss how it may be applied to real calderas.