arXiv subjects

Thomas Ulrich

Publications and source records attributed to Thomas Ulrich.

7 recordsLinked to original sources

arXiv2025

Servo navigation and phase equalization enhanced by run-time stabilization (PEERS) for 3D EPI time series

Purpose: To enhance time-resolved segmented imaging by synergy of run-time stabilization and retrospective, data-driven phase correction. Methods: A segmented 3D EPI sequence for fMRI time series is equipped with servo navigation based on short orbital navigators and a linear perturbation model, enabling run-time correction for rigid-body motion as well as bulk phase and frequency fluctuation. Complementary retrospective phase correction is based on the repetitive structure of the time series and serves to address residual phase and frequency offsets. The combined approach is termed phase equalization enhanced by run-time stabilization (PEERS). Results: The proposed strategy is evaluated in a phantom and in-vivo. Servo navigation is found to diminish motion confound in raw data and maintain k-space consistency over time series. In turn, retrospective phase equalization is found to eliminate shot-wise phase and frequency offsets relative to the navigator, which are attributed to eddy-currents and vibrations from phase encoding. Retrospective phase equalization reduces the precision requirements for run-time frequency control, supporting the use of short navigators. Relative to conventional volume realignment, PEERS achieved tSNR improvements up to $30\%$ for small motion and in the order of $10\%$ when volunteers tried to hold still. Retrospective phase equalization is found to clearly outperform phase correction based solely on navigator-based frequency estimates. Conclusion: Servo navigation achieves high-precision run-time motion correction for 3D EPI fMRI. Coarse frequency tracking based on short navigators is supplemented by precise retrospective frequency and phase correction. Fully automatic and self-calibrated, PEERS offers effective plug-and-play motion and phase correction for 3D fMRI.

eess.IV↗

arXiv2025

A Discontinuous Galerkin Method for Simulating 3D Seismic Wave Propagation in Nonlinear Rock Models: Verification and Application to the 2015 Mw 7.8 Gorkha Earthquake

The nonlinear mechanical responses of rocks and soils to seismic waves play an important role in earthquake physics, influencing ground motion from source to site. Continuous geophysical monitoring, such as ambient noise interferometry, has revealed co-seismic wave speed reductions extending tens of kilometers from earthquake sources. However, the mechanisms governing these changes remain challenging to model, especially at regional scales. Using a nonlinear damage model constrained by laboratory experiments, we develop and apply an open-source 3D discontinuous Galerkin method to simulate regional co-seismic wave speed changes during the 2015 Mw7.8 Gorkha earthquake. We find pronounced spatial variations of co-seismic wave speed reduction, ranging from <0.01% to >50%, particularly close to the source and within the Kathmandu Basin. The most significant reduction occurs within the sedimentary basin and varies with basin depths, while wave speed reductions correlate with the fault slip distribution near the source. By comparing ground motions from simulations with elastic, viscoelastic, elastoplastic, and nonlinear damage rheologies, we demonstrate that the nonlinear damage model effectively captures low-frequency ground motion amplification due to strain-dependent wave speed reductions in soft sediments. We verify the accuracy of our approach through comparisons with analytical solutions and assess its scalability on high-performance computing systems. The model shows near-linear strong and weak scaling up to 2048 nodes, enabling efficient large-scale simulations. Our findings provide a physics-based framework to quantify nonlinear earthquake effects and emphasize the importance of damage-induced wave speed variations for seismic hazard assessment and ground motion predictions.

physics.geo-ph↗

arXiv2024

Ground Motion Characteristics of Cascading Earthquakes in a Multiscale Fracture Network

Fault zones exhibit geometrical complexity and are often surrounded by multiscale fracture networks within their damage zones, influencing rupture dynamics and near-field ground motions. We investigate the ground-motion characteristics of cascading ruptures across damage zone fracture networks of moderate-sized earthquakes using high-resolution 3D dynamic rupture simulations. Our models feature a listric fault surrounded by over 800 fractures, emulating a major fault and its associated damage zone. We analyze three cases: a cascading rupture propagating within the fracture network, a non-cascading main-fault rupture with off-fault fracture slip, and a main-fault rupture without a fracture network. Cascading ruptures within the fracture network produce distinct ground-motion signatures with high-frequency content, arising from simultaneous slip of multiple fractures and parts of the main fault, resembling source coda-wave-like signatures. This case shows elevated near-field characteristic frequency (fc) and stress drop, approximately an order of magnitude higher than the estimation directly on the fault of the dynamic rupture simulation. The inferred fc of the modeled vertical components reflects the complexity of the radiation pattern and rupture directivity of cascading earthquakes. We show that this is consistent with observations of strong azimuthal dependence of corner frequency in the 2009-2016 Central Apennines, Italy, earthquake sequence. Simulated ground motions from cascading ruptures also show pronounced azimuthal variations in peak ground acceleration (PGA), peak ground velocity, and pseudo-spectral acceleration, with average PGA nearly double that of the non-cascading cases. Such outcomes emphasize the critical role of fault-zone complexity in affecting rupture dynamics and seismic radiation and have important implications for physics-based seismic hazard assessment.

physics.geo-ph↗

arXiv2023

Rapid 3D dynamic rupture modeling of the February 6, 2023, Kahramanmara\c{s}, Turkey, $M_W$7.8 and $M_W$7.7 earthquake doublet

The 2023 Turkey Earthquake sequence involved unexpected ruptures across numerous fault segments, challenging data interpretation efforts. We present rapid, 3D dynamic rupture simulations to illuminate the complexities of the $M_W$7.8 and $M_W$7.7 earthquake doublet. Constrained by observations available within days of the sequence, our models deliver timely, mechanically consistent explanations for the unforeseen rupture paths, diverse rupture speeds, multiple slip episodes, locally strong shaking, and fault system interactions. We reconcile regional seismo-tectonics, rupture dynamics, and ground motions of a fault system represented by ten curved dipping segments and a heterogeneous stress field. Our simulations link both events matching geodetic and seismic observations. The $M_W$7.8 earthquake features delayed backward branching from a steeply intersecting splay fault, not requiring supershear speeds. The asymmetrical dynamics of the distinct, bilateral $M_W$7.7 event is explained by heterogeneous fault strength, prestress orientation, fracture energy, and static stress changes from the previous event. Our models explain the northward deviation of its western rupture and the minimal slip observed on the S\"urg\"u fault. Rapidly developed 3D dynamic rupture scenarios can elucidate unexpected observations shortly after major earthquakes, providing timely insights for data-driven analysis and hazard assessment toward a comprehensive, physically consistent understanding of the mechanics of multi-fault systems.

physics.geo-ph↗

arXiv2023

Rupture Dynamics of Cascading Earthquakes in a Multiscale Fracture Network

Fault-damage zones comprise multiscale fracture networks that may slip dynamically and interact with the main fault during earthquake rupture. Using 3D dynamic rupture simulations and scale-dependent fracture energy, we examine dynamic interactions of more than 800 intersecting multiscale fractures surrounding a listric fault, emulating a major fault and its damage zone. We investigate ten distinct orientations of maximum horizontal stress, probing the conditions necessary for sustained slip within the fracture network or activating the main fault. Additionally, we assess the feasibility of nucleating dynamic rupture earthquake cascades from a distant fracture and investigate the sensitivity of fracture network cascading rupture to the effective normal stress level. We model either pure cascades or main fault rupture with limited off-fault slip. We find that cascading ruptures within the fracture network are dynamically feasible under certain conditions, including: (i) the state-evolutional distance scales with fracture and fault size, (ii) favorable relative pre-stress of fractures within the ambient stress field, and (iii) close proximity of fractures. We find that cascading rupture within the fracture network discourages rupture on the main fault. Our simulations suggest that favorable relative pre-stress fractures within a fault damage zone may lead to cascading earthquake rupture reaching off-fault moment magnitudes up to $Mw \approx 5.6$, shadowing the main fault slip. Our findings offer fundamental insights into physical processes governing cascading earthquake dynamic rupture within multiscale fracture networks. Our results have implications for the seismic hazard of naturally activated fracture or fault networks and earthquakes induced in geo-energy exploitation activities.

physics.geo-ph↗

arXiv2022

How does thermal pressurization of pore fluids affect 3D strike-slip earthquake dynamics and ground motions?

Frictional heat during earthquake rupture raises the pressure of fault zone fluids and affects the rupture process and its seismic radiation. Here, we investigate the role of two key parameters governing thermal-pressurization of pore fluids -- hydraulic diffusivity and shear-zone half-width -- on earthquake rupture dynamics, kinematic source properties and ground-motions. We conduct 3D strike-slip dynamic rupture simulations assuming a rate-and-state dependent friction law with strong velocity-weakening coupled to thermal-pressurization of pore fluids. Dynamic rupture evolution and ground-shaking are densely evaluated across the fault and Earth surface to analyze variations of rupture parameters (slip, peak slip-rate PSR, rupture speed Vr, rise time Tr), correlations among rupture parameters, and variability of peak ground velocity (PGV). Our simulations reveal how variations in thermal-pressurization affect source properties. We find that mean slip and Tr decrease with increasing hydraulic diffusivity, whereas mean Vr and PSR remain almost constant. Mean slip, PSR and Vr decrease with increasing shear-zone half-width, whereas mean Tr increases. Shear-zone half-width distinctly affects the correlation between rupture parameters, especially for parameter pairs slip-Vr, PSR-Vr and Vr-Tr. Hydraulic diffusivity has negligible effects on these correlations. Variations in shear-zone half-width primarily impact Vr, which then may affect other rupture parameters. We find negative correlation between slip and PSR, in contrast to simpler dynamic rupture models, whereas trends for other parameter pairs are in agreement. Mean PGVs decrease faster with increasing shear-zone half-width than with hydraulic diffusivity, whereas ground-motion variability is similarly affected by both parameters.

physics.geo-ph↗

arXiv2021

3D Acoustic-Elastic Coupling with Gravity: The Dynamics of the 2018 Palu, Sulawesi Earthquake and Tsunami

We present a highly scalable 3D fully-coupled Earth & ocean model of earthquake rupture and tsunami generation. We model seismic, acoustic and surface gravity wave propagation in elastic (Earth) and acoustic (ocean) materials sourced by physics-based non-linear earthquake dynamic rupture. Complicated geometries, including high-resolution bathymetry, coastlines and segmented earthquake faults are discretized by adaptive unstructured tetrahedral meshes. A Discontinuous Galerkin discretization with ADER local time-stepping (ADER-DG) yields petascale computational efficiency and high-order accuracy in time and space. We compare the 3D fully-coupled approach to a benchmark problem for 3D-2D linked models that use 2D shallow-water modeling. We present a large-scale fully-coupled model of the 2018 Sulawesi events that links the dynamics from supershear earthquake faulting to elastic and acoustic waves in Earth and ocean to tsunami gravity wave propagation in the narrow Palu Bay. And we demonstrate scalability and performance of the MPI+OpenMP parallelization on three petascale supercomputers.

physics.comp-ph↗