On the implementation of faults in finite-element glacial isostatic adjustment models

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Palaeoseismological analyses of northern and central Germany

Northern Germany is an intraplate region and has been regarded as a low seismicity area for a long time. However, historic sources show the occurrence of several significant natural earthquakes in northern and central Germany since the 10th century. In recent years natural earthquakes as well as earthquakes in the vicinity of active gas fields, likely to have been associated with the recovery of hydrocarbons, have been repeatedly instrumentally recorded in northern Germany. In central Germany, which is exposed to a higher earthquake hazard than northern Germany, historically and instrumentally recorded earthquakes accumulate in a N-S trending zone. However, the seismic record of Germany is limited and solely goes back to the year 800 CE. Long periods of seismic quiescence alternating with fault activity for a short geological period of time can falsify the seismic hazard of an intraplate region. Seismic hazard can be underestimated because of seismic quiescence or overestimated because of the detection of periodical clustering, migrating and infrequent seismicity. Therefore, palaeoseismology is the missing link for an accurate assessment of the seismic hazard estimation of a continental low strain area like Germany. Northern and central Germany were repeatedly affected by glaciations and periglacial processes during the Pleistocene. The main difficulty is to distinguish the vast glaciotectonic deformation structures that are present in northern Germany from neotectonic deformation structures. Processes like cryoturbation, depositional loading in water saturated sediments and rapid rates of deposition can generate soft-sediment deformation structures that may also be mistaken for earthquake-induced structures. The analysis of neotectonic activity in northern and central Germany is challenging because recently observed vertical crustal movements along NWSE- striking faults do not commonly correspond to visible morphological features and fault scarps are rapidly destroyed by climatic conditions. Seven WNW-ESE trending major basement faults with a high potential for reactivation due to glacial isostatic adjustment were analysed with regard to neotectonic fault activity. In addition, in central Germany the controversially discussed seismically active part of the Regensburg-Leipzig-Rostock fault system between Leipzig and Cheb and surroundings was analysed with regard to pre-historic activity. Deformation bands and seismites in Palaeogene and Pleistocene deposits exposed in sand and gravel pits are indicators for neotectonic activity. Luminescence dating, shear-wave reflection seismics, electrical resistivity tomography and lineament analysis were applied to support neotectonic activity in the study area. Evidence for neotectonic movements, indicated by the occurrence of deformation bands in Middle to Late Pleistocene sediments, was identified along five of the seven major basement faults that were analysed in northern Germany. Evidence was found at the Aller Fault, the Halle Fault, the Harz Boundary Fault, the Steinhuder Meer Fault and the Osning Thrust. In the area around the Regensburg-Leipzig-Rostock fault system neotectonic movements are indicated by deformation bands in Palaeocene and Middle Pleistocene sediments at fault intersections of mainly NW-SE oriented faults like the Lusatian Thrust and the Finne-Gera-Jáchymov fault system and fault intersections of minor faults in the vicinity to the cities Leipzig and Dresden. It was possible to estimate the timing of neotectonic activity of faulted Pleistocene sediments by means of luminescence dating at two basement faults (Harz Boundary Fault, Steinhuder Meer Fault). The estimated ages of faulted debris-flow deposits at the Harz Boundary Fault (15.2 ± 0.8 and 14.2 ± 0.8 ka ka) point to fault movements after ~15 ka corresponding with the reactivation of the Osning Thrust. The estimated age of growth strata at the Steinhuder Meer Fault (189 ± 5 ka and 158 ± 4 ka) indicates fault movements in Middle Pleistocene Saalian times. At the Harz Boundary Fault shear-wave reflection seismic surveys and electrical resistivity tomography profiles support the neotectonic activity in the Lateglacial. The timing of fault movements implies that the seismicity in northern and parts of central Germany is likely induced by varying lithospheric stress conditions related to glacial isostatic adjustment. For the Harz Boundary Fault and the Osning Thrust this is supported by numerical simulations of Coulomb failure stress variations. Thus, the faults can be classified as glacially-induced faults. Along the Regensburg-Leipzig-Rostock fault system, focal mechanisms of deep-seated earthquakes partly show NW-SE trending nodal planes. The focal mechanisms indicate activity along NW-SE oriented faults that intersect the N-S striking Regensburg-Leipzig-Rostock fault system. This supports the seismotectonic importance of NWSE oriented faults and intersecting faults in the study area of northern and central Germany

Deformation of salt structures by ice-sheet loading: insights into the controlling parameters from numerical modelling

Subsurface salt flow is driven by differential loading, which is typically caused by tectonics or sedimentation. During glaciations, the weight of an ice sheet represents another source of differential loading. In salt-bearing basins affected by Pleistocene glaciations, such as the Central European Basin System, ice loading has been postulated as a trigger of young deformation at salt structures. Here, we present finite-element simulations (ABAQUS) with models based on a simplified 50-km long and 10-km-deep two-dimensional geological cross-section of a salt diapir subject to the load of a 300-m-thick ice sheet. The focus of our study is to evaluate the sensitivity of the model to material parameters, including linear and non-linear viscosity of the salt rocks and different elasticities. A spatially and temporarily variable pressure was applied to simulate ice loading. An ice advance towards the diapir causes lateral salt flow into the diapir and diapiric rise. Complete ice coverage leads to downward displacement of the diapir. After unloading, displacements are largely restored. The modelled displacements do not exceed few metres and are always larger in models with linear viscosity than in those with non-linear viscosity. Considering the low stresses caused by ice-sheet loading and the long time-scale, the application of linear viscosity seems appropriate. The elastic parameters also have a strong impact, with lower Young's moduli leading to larger deformation. The impact of both the viscosity and the elasticity highlights the importance of a careful parameter choice in numerical modelling, especially when aiming to replicate any real-world observations

A Sea Level Equation for seismic perturbations

Large earthquakes are a potentially important source of relative sea level variations, since they can drive global deformation and simultaneously perturb the gravity field of the Earth. For the first time, we formalize a gravitationally self-consistent, integral sea level equation suitable for earthquakes, in which we account both for direct effects by the seismic dislocation and for the feedback from water loading associated with sea level changes. Our approach builds upon the well-established theory first proposed in the realm of glacio-isostatic adjustment modelling. The seismic sea level equation is numerically implemented to model sea level signals following the 2004 Sumatra–Andaman earthquake, showing that surface loading from ocean water redistribution (so far ignored in post-seismic deformation modelling) may account for a significant fraction of the total computed post-seismic sea level variatio

Recent intraplate earthquakes in Northwest Germany : glacial isostatic adjustment and/or a consequence of hydrocarbon production?

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Active Tectonics In Southern Alaska And The Role Of The Yakutat Block Constrained By Gps Measurements

Thesis (Ph.D.) University of Alaska Fairbanks, 2011GPS data from southern Alaska and the northern Canadian Cordillera have helped redefine the region's tectonic landscape. Instead of a comparatively simple interaction between the Pacific and North American plates, with relative motion accommodated on a single boundary fault, the margin is made up of a number of small blocks and deformation zones with relative motion distributed across a variety of structures. Much of this complexity can be attributed to the Yakutat block, an allochthonous terrane that has been colliding with southern Alaska since the Miocene. This thesis presents GPS data from across the region and uses it to constrain a tectonic model for the Yakutat block collision and its effects on southern Alaska and eastern Canada. The Yakutat block itself moves NNW at a rate of 50 mm/yr. Along its eastern edge, the Yakutat block is fragmenting into small crustal slivers. Part of the strain from the collision is transferred east of the Fairweather -- Queen Charlotte fault system, causing the region inboard of the Fairweather fault to undergo a distinct clockwise rotation into the northern Canadian Cordillera. About 5% of the relative motion is transferred even further east, causing small northeasterly motions well into the northern Cordillera. Further north, the GPS data and model results indicate that the current deformation front between the Yakutat block and southern Alaska runs along the western side of the Malaspina Glacier. The majority of the ~37 mm/yr of relative convergence is accommodated along a narrow band of thrust faults concentrated in the southeastern part of the St. Elias orogen. Near the Bering Glacier, the tectonic regime abruptly changes as crustal thrust faults give way to subduction of the Yakutat block beneath the western St. Elias orogen and Prince William Sound. This change aligns with the Gulf of Alaska shear zone, implying that the Pacific plate is fragmenting in response to the Yakutat collision. The Bering Glacier region is undergoing internal deformation and may represent the final stage of accretion of the Yakutat block sedimentary layers. Further west, modeled block motions suggest the crust is laterally escaping along the Aleutian forearc

Ice melting and earthquake suppression in Greenland

It has been suggested that the Greenland ice sheet is the cause of earthquake suppression in the region. With few exceptions, the observed seismicity extends only along the continental margins of Greenland, which almost coincide with the ice sheet margin. This pattern has been put forward as further validation of the earthquake suppression hypothesis. In this review, new evidence in terms of ice melting, post-glacial rebound and earthquake occurrence is gathered and discussed to re-evaluate the connection between ice mass unloading and earthquake suppression. In Greenland, the spatiotemporal distribution of earthquakes indicates that seismicity is mainly con- fined to regions where the thick layer of ice is absent and where significant ice melting is presently occurring. A clear correlation between seismic activity and ice melting in Greenland is not found. However, earthquake locations and corresponding depth distributions suggest two distinct governing mechanisms: post-glacial rebound promotes moderate-size crustal earthquakes at Greenland’s regional scale, while current ice melting promotes shallow low magnitude seismicity locall

Investigating Strike-Slip Faulting Parallel to the Icelandic Plate Boundary Using Boundary Element Models

Most faults in Iceland strike roughly parallel to the divergent plate boundary, a part of the North American-Eurasian plate boundary, which would be expected to lead to primarily normal faulting. However, several studies have observed a significant component of rift-parallel strike-slip faulting in Iceland. To investigate these fault kinematics, we use the boundary element method to model fault slip and crustal stress patterns of the Icelandic tectonic system, including a spherical hotspot and uniaxial stress that represents rifting. On a network of faults, we estimate the slip required to relieve traction imposed by hotspot inflation and remote rifting stress and compare the model results with observed slip kinematics, crustal seismicity, and geodetic data. We note a good fit between model-predicted and observed deformation metrics, with both indicating significant components of normal and strike-slip faulting and consistency between recent data and longer-term records of geologic fault slip. Possible stress permutations between steeply plunging σ1 and σ2 axes are common in our models, suggesting that localized stress perturbations may impact strike-slip faulting. Some increases in model complexity, including older hotspot configurations and allowing fault opening to simulate dike intrusion, show improvement to model fit in select regions. This work provides new insight into the physical mechanisms driving faulting styles within Iceland away from the current active plate boundary, implying that a significant portion of observed strike-slip faulting is likely caused by the combined effects of tectonic rifting, hotspot impacts, and mechanical interactions across the fault network

Adjoint models of mantle convection with seismic, plate motion, and stratigraphic constraints: North America since the Late Cretaceous

We apply adjoint models of mantle convection to North America since the Late Cretaceous. The present-day mantle structure is constrained by seismic tomography and the time-dependent evolution by plate motions and stratigraphic data (paleoshorelines, borehole tectonic subsidence, and sediment isopachs). We infer values of average upper and lower mantle viscosities, provide a synthesis of North American vertical motions (relative sea level) from the Late Cretaceous to the present, and reconstruct the geometry of the Farallon slab back to the Late Cretaceous. In order to fit Late Cretaceous marine inundation and borehole subsidence, the adjoint model requires a viscosity ratio across 660 km discontinuity of 15:1 (reference viscosity of 10^(21) Pa s), which is consistent with values previously inferred by postglacial rebound studies. The dynamic topography associated with subduction of the Farallon slab is localized in western North America over Late Cretaceous, representing the primary factor controlling the widespread flooding. The east coast of the United States is not stable; rather, it has been experiencing continuous dynamic subsidence over the Cenozoic, coincident with an overall eustatic fall, explaining a discrepancy between sea level derived from the New Jersey coastal plain and global curves. The east coast subsidence further constrains the mantle viscosity structure and requires an uppermost mantle viscosity of 10^(20) Pa s. Imposed constraints require that the Farallon slab was flat lying during Late Cretaceous, with an extensive zone of shallow dipping Farallon subduction extending beyond the flat-lying slab farther east and north by up to 1000 km than previously suggested