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Okubo, Kurama

Publications and source records attributed to Okubo, Kurama.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Signature of transition to supershear rupture speed in the coseismic off-fault damage zone

Most earthquake ruptures propagate at speeds below the shear wave velocity within the crust, but in some rare cases, ruptures reach supershear speeds. The physics underlying the transition of natural subshear earthquakes to supershear ones is currently not fully understood. Most observational studies of supershear earthquakes have focused on determining which fault segments sustain fully grown supershear ruptures. Experimentally cross-validated numerical models have identified some of the key ingredients required to trigger a transition to supershear speed. However, the conditions for such a transition in nature are still unclear, including the precise location of this transition. In this work, we provide theoretical and numerical insights to identify the precise location of such a transition in nature. We use fracture mechanics arguments with multiple numerical models to identify the signature of supershear transition in coseismic off-fault damage. Furthermore, we then cross-validate this signature with high-resolution observations of fault zone width and early aftershock distributions. We confirm that the location of the transition from subshear to supershear speed is characterized by a decrease in the width of the coseismic off-fault damage zone. We thus help refine the precise location of such a transition for natural supershear earthquakes.

42 ENGINEERING↗
DOE OSTI2020

HOSS: an implementation of the combined finite-discrete element method

Nearly thirty years since its inception, the combined finite-discrete element method (FDEM) has made remarkable strides in becoming a mainstream analysis tool within the field of Computational Mechanics. FDEM was developed to effectively “bridge the gap” between two disparate Computational Mechanics approaches known as the finite and discrete element meth-ods. At Los Alamos National Laboratory (LANL) researchers developed the Hybrid Optimization Software Suite (HOSS) as a hybrid multi-physics platform, based on FDEM, for the simulation of solid material behavior complemented with the latest technological enhancements for full fluid–solid interaction. Furthermore, in HOSS, several newly developed FDEM algorithms have been implemented that yield more accurate material deformation formulations, inter-particle interaction solvers, and fracture and fragmentation solutions. Additionally, an explicit computational fluid dynamics solver and a novel fluid–solid interaction algorithms have been fully integrated (as opposed to coupled) into the HOSS’ solid mechanical solver, allowing for the study of an even wider range of problems. Advancements such as this are leading HOSS to become a tool of choice for multi-physics problems. Finally, HOSS has been successfully applied by a myriad of researchers for analysis in rock mechanics, oil and gas industries, engineering application (structural, mechanical and biomedical engineering), mining, blast loading, high velocity impact, as well as seismic and acoustic analysis. This paper intends to summarize the latest development and application efforts for HOSS.

58 GEOSCIENCES↗
DOE OSTI2019

Can supershear transition be seen in damage and aftershock pattern? Part one: Theory [Slides]

What we’ve learned from fracture mechanics and numerical modeling - Stress intensity at the crack tip evolves with the rupture velocity • As the rupture velocity approaches the Rayleigh wave speed, before transitioning to supershear, the stress intensity reduces • As a result, the region affected by the stress intensity decreases in size as well, leading to a sudden shrinkage of the near-fault damage zone.

58 GEOSCIENCES↗
DOE OSTI2019

Dynamics, Radiation, and Overall Energy Budget of Earthquake Rupture With Coseismic Off-Fault Damage

Earthquake ruptures dynamically activate coseismic off–fault damage around fault cores. Systematic field observation efforts have shown the distribution of off–fault damage around main faults, while numerical modeling using elastic–plastic off–fault material models have demonstrated the evolution of coseismic off–fault damage during earthquake ruptures. Laboratory–scale microearthquake experiments have pointed out the enhanced high–frequency radiation due to the coseismic off–fault damage. However, the detailed off–fault fracturing mechanisms, subsequent radiation, and its contribution to the overall energy budget remain to be fully understood because of limitations of current observational techniques and model formulations. Here, we constructed a new physics–based dynamic earthquake rupture modeling framework, based on the combined finite–discrete element method, to investigate the fundamental mechanisms of coseismic off–fault damage, and its effect on the rupture dynamics, the radiation and the overall energy budget. We conducted a 2–D systematic case study with depth and showed the mechanisms of dynamic activation of the coseismic off–fault damage. We found the decrease in rupture velocity and the enhanced high–frequency radiation in near field due to the coseismic off–fault damage. We then evaluated the overall energy budget, which shows a significant contribution of the coseismic off–fault damage to the overall energy budget even at depth, where the damage zone width becomes narrower. Here, the present numerical framework for the dynamic earthquake rupture modeling thus provides new insights into earthquake rupture dynamics with the coseismic off–fault damage.

58 GEOSCIENCES↗
DOE OSTI2018

Earthquake Damage Patterns Resolve Complex Rupture Processes

Fracture damage patterns around faults induced by dynamic earthquake rupture are an invaluable record to clarify the rupture process on complex fault networks. The 2016 M w 7.8 Kaikōura earthquake in New Zealand has been reported as one of the most complex earthquakes ever documented that ruptured at least 15 crustal faults. High-resolution optical satellite image displacement maps provide distinctive profiles of displacement across the faults and help visualize the off-fault damage pattern. They are combined with field observation and coupled with a numerical tool that captures the dynamics of the rupture and simultaneous activation of off-fault damage to allow the determination of the most likely rupture scenario. Lastly, this study demonstrates that complex rupture processes can be explained in a rather simple way via a synergetic combination of state-of-the-art observation and first principle physics-based numerical modeling of off-fault damage.

58 GEOSCIENCES↗