Dynamic Fragmentation and Earthquake Energy Partitioning

University Of Maine, Orono ME

Investigators

Abstract

Strike-slip faults like the San Andreas Fault in California represent major threats to life and property owing to the repeated generation of large earthquakes. Less than 20% of the energy released during an earthquake radiates away from the source as elastic waves that cause ground shaking. A large portion of the energy is dissipated around the fault as frictional heat and through a variety of processes. These processes control how the earthquake rupture propagates. During each earthquake, the rocks surrounding the active fault are fragmented by cracking of individual mineral grains. They form the so-called damage zone. Over many earthquake cycles, damage increases in this zone causing changes in seismic-wave direction and speed. Understanding these processes is critical to evaluate the energy budget through the entire seismogenic zone. It is also critical to assess seismic hazards. Here, the researchers combine field-based results and modeling to improve the understanding of seismically active faults. They quantify processes causing fragmentation in the deeper reaches of the seismogenic zone. They develop wave speed models that can be used to better predict ground shaking directions and intensities. The interdisciplinary project supports a postdoctoral associate, as well as the training of undergraduate and graduate students at University of Maine. The developed codes and analytical protocols are made openly available through public portals. These outcomes can be applied beyond Seismology in Material Sciences and Engineering, notably to investigate brittle fragmentation in ceramics and advanced composite materials. The objectives of this project are: (1) to characterize the microfracture density and fragment size distributions for fragmented minerals in two deeply exhumed seismogenic faults/shear zones; (2) to use the fragment size distributions to estimate the energy that was consumed by fragmentation; (3) to apply physics-based fragmentation modeling to estimate strain rates associated with the fragmentation; (4) to better constrain the energy partitioning in the earthquake source volume; (5) to develop and disseminate protocols for using electron backscatter diffraction techniques to analyze mineral and rock fragmentation. Addressing the above objectives provides an opportunity to test hypotheses related to the source energy budget, styles of rupture propagation, and the relations between loading conditions and resulting rock microstructures in the deeper seismogenic zone. The study also allows evaluating whether intense fragmentation of brittle minerals can be used as a seismogenic signature at depth, as it is at the surface. The chosen approach applies models and concepts that originate largely in the physics and engineering communities, and mostly constitutes novel research in the geosciences. Recognition of the importance of microfracturing for macroscopic behavior and the ability to treat it quantitatively is growing in both Earth Sciences and Materials Engineering. Outcomes of this project will provide a framework for future efforts in both fields, and for collaborations between them. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.