ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉The Wasatch fault zone defines the eastern boundary of the actively extending Basin and Range Province (Utah, western United States) and poses a significant seismic hazard to the metropolitan areas along the Wasatch Range. A wealth of paleoseismological data documents ∼24 surface-rupturing M〈sub〉w〈/sub〉 ≥ 7 earthquakes along the Wasatch fault during the past 6400 yr. Here, we simulated the Holocene earthquake sequence on the Wasatch, Oquirrh–Great Salt Lake, and West Valley faults using three-dimensional finite-element forward modeling with the goal to calculate coseismic and postseismic Coulomb stress changes and to evaluate the slip and magnitude of hypothetical present-day and future earthquakes. Our results show that a good fit between modeled and observed paleoevents and time-integrated slip rates can be achieved within the uncertainties of the paleoseismological record and model parameters like the fault geometry. The Coulomb stress change analysis for selected paleoearthquakes showed that maximum positive stress changes are induced on faults located along strike of the source fault, while faults parallel to the source fault are generally located in stress shadow zones. Postseismic viscoelastic relaxation considerably modifies the coseismic stress changes; the resulting transient stress changes are recognizable for more than 100 yr after an earthquake. The modeled present-day state of Coulomb stress changes shows that the Brigham City, Salt Lake City, and Provo segments of the Wasatch fault are prone to failure in a M〈sub〉w〈/sub〉 ≥ 6.8 earthquake. Our study shows that simulation of an entire earthquake sequence based on a paleoseismological record is feasible and facilitates identification of possible gaps and inconsistencies in the paleoseismological record. Therefore, forward modeling of earthquake sequences may ultimately contribute to improved seismic hazard estimates.〈/span〉

Description: Glacial-isotactic adjustment (GIA) is one of the key processes considering relative sea-level (RSL) and paleo-topography during the last glacial cycle. Especially in former ice-covered regions the subsidence of the solid Earth due to ice loads can reach more than 500 m and contributes to the stability of ice-sheets (e.g. position of grounding line and ice-sheet elevation), whereas at the coasts of the world oceans the deformation is governed by global RSL fall of more than 100 m. Because the viscoelastic response of the solid Earth is governed by its viscosity structure, the effect of lateral viscosity variations on deformations due to GIA has to be estimated. The importance was already shown for the differences in earth structure below the glacial ice sheets of Fennoscandia and Laurentide, as well as for a number of peripheral and far-field regions. One open question arises: Can the 3D earth properly be parameterized by locally optimized 1D earth structures? In this study, we apply a 3D Earth structure which we derived from seismic tomography and further geodynamic constraints as an a priori estimation of the Earth viscosity distribution. Applying a standard glaciation history, we compare the response characteristics of 1D and 3D earth parameterizations and discuss the limits of optimized 1D earth parametrizations. We will focus on reconstructions of RSL during the last deglaciation in view of sea level index points which are generally used for validating the GIA process.

Description: We present a new set of global and local sea‐level projections at example tide gauge locations under the RCP2.6, RCP4.5 and RCP8.5 emissions scenarios. Compared to the CMIP5‐based sea‐level projections presented in IPCC AR5, we introduce a number of methodological innovations, including: (i) more comprehensive treatment of uncertainties; (ii) direct traceability between global and local projections; (iii) exploratory extended projections to 2300 based on emulation of individual CMIP5 models. Combining the projections with observed tide gauge records, we explore the contribution to total variance that arises from sea‐level variability, different emissions scenarios and model uncertainty. For the period out to 2300 we further breakdown the model uncertainty by sea‐level component and consider the dependence on geographic location, time horizon and emissions scenario. Our analysis highlights the importance of variability for sea‐level change in the coming decades and the potential value of annual‐to‐decadal predictions of local sea‐level change. Projections to 2300 show a substantial degree of committed sea‐level rise under all emissions scenarios considered and highlights the reduced future risk associated with RCP2.6 and RCP4.5 compared to RCP8.5. Tide gauge locations can show large (〉 50%) departures from the global average, in some cases even reversing the sign of the change. While uncertainty in projections of the future Antarctic ice dynamic response tends to dominate post‐2100, we see a substantial differences in the breakdown of model variance as a function of location, timescale and emissions scenario.

Description: Based on the latest GFZ release 06 of monthly gravity fields from GRACE satellite mission, area-averaged barystatic sea-level is found to rise by 2.02 mm/a during the period April 2002 until August 2016 in the open ocean with a 1000 km coastal buffer zone when low degree coefficients are properly augmented with information from satellite laser ranging. Alternative spherical harmonics solutions from CSR, JPL and TU Graz reveal rates between 1.94 and 2.08 mm/a, thereby demonstrating that systematic differences among the centers are much reduced in the latest release. The results from the direct integration in the open ocean can be aligned to associated solutions of the sea-level equation when fractional leakage derived from two differently filtered global gravity fields is explicitly considered, leading to a global mean sea-level rise of 1.72 mm/a. This result implies that estimates obtained from a 1000 km coastal buffer zone are biased 0.3 mm/a high due the systematic omission of regions with below-average barystatic sea-level rise in regions close to substantial coastal mass losses induced by the reduced gravitational attraction of the remaining continental ice and water masses.

Description: Earthquakes on intra-continental faults pose substantial seismic hazard to populated areas. The interaction of faults is an important mechanism of earthquake triggering and can be investigated by the calculation of Coulomb stress changes. Using three-dimensional finite-element models, co- and postseismic stress changes and the effect of viscoelastic relaxation on dip-slip faults are investigated. The models include elastic and viscoelastic layers, gravity, ongoing regional deformation as well as source and receiver fault zones. A parameter study with a systematic fault geometry, which is independent of a specific earthquake, shows that high coseismic stress increase occurs in along-strike prolongation of the source fault and in small areas parallel to the source fault. The coseismic slip and coefficient of friction influence the magnitude of stress changes, while the fault dip also influences the distribution. The stress changes can be explained by the spatial distribution of the coseismic strain. Differences in normal and thrust fault models are mainly caused by the different fault dips. The postseismic stress changes – caused by viscoelastic relaxation and interseismic stress increase – modify the coseismic stress changes that stress-triggering zones can change to stress-shadow zones and vice versa. Stress changes induced by viscoelastic relaxation can outweigh the interseismic stress increase so that negative stress changes can persist for decades. The lower the viscosity of the lower crust or lithospheric mantle, the more pronounced is the effect of viscoelastic relaxation in the first years. Layers with low viscosity define the area of highest postseismic velocities and hence determine relaxation and stress changes. The application of the model to the active Wasatch fault system in the eastern Basin and Range Province (Utah) is the first study in which an entire series of earthquakes on a natural fault system is simulated in a finite-element model using realistic fault geometries and palaeo-seismological data to investigate the co- and postseismic Coulomb stress changes for palaeo-earthquakes and the future evolution. The coseismic stress changes extend over all modelled fault segments. The postseismic stress changes and velocities show that the postseismic relaxation dominates the first years after the earthquake, while in the hundredth year the stress increase by the regional stress field dominates. The analysis of the stress changes since the last event per fault segment shows that the Brigham City segment (~780 bar) and Salt Lake City segment (~510 bar) have accumulated the most stress since the last earthquake. Modelled hypothetical present-day earthquakes suggest that present-day ruptures on the Brigham City segment or Salt Lake City segment could experience M ~7.1 or M ~7.0 earthquakes, respectively, which pose high seismic hazard for the metropolitan areas.

Description: Earthquakes in the brittle upper crust induce viscoelastic flow in the lower crust and lithospheric mantle,which can persist for decades and lead to significant Coulomb stress changes on receiver faults located in the surrounding of the source fault. As most previous studies calculated the Coulomb stress changes for a specific earthquake in nature, a general investigation of postseismic Coulomb stress changes independent of local geological conditions is still lacking for intra-continental dip-slip faults. Here we use finite-element models with normal and thrust fault arrays, respectively, to show that postseismic viscoelastic flow considerably modifies the original coseismic Coulomb stress patterns through space and time. Depending on the position of the receiver fault relative to the source fault, areas with negative coseismic stress changes may exhibit positive postseismic stress changes and vice versa. The lower the viscosity of the lower crust or lithospheric mantle, the more pronounced are the transient stress changes in the 1st years, with the lowest viscosity having the largest effect on the stress changes. The evolution of postseismic Coulomb stress changes is further controlled by the superposition of transient stress changes caused by viscoelastic relaxation (leading to stress increase or decrease) and the interseismic strain accumulation