Collaborative Research: Exploring the Role of Rapid Cementation, Cohesion, and Pore Fluid Pressure Evolution in the Dynamics of Slow Slip Events

About This Grant

The Earth’s faults can rupture abruptly causing hazardous earthquakes, but some can also slip slowly, in events that can last hours to years. In the Cascadia Subduction Zone, slow slip events (SSEs) have durations of days to weeks and occur at depths between 30 and 45 km, which are deeper than typical earthquakes. These events produce a weak seismic signal known as tectonic tremor. Despite their status as one of the most significant discoveries in geophysics, the physical mechanism(s) responsible for SSEs remain enigmatic, and the effects of deep slip on seismic hazards are unclear. This project explores processes that affect the strength of the fault through time, one of the factors thought to control SSEs. Areas in the Earth where SSEs occur are typically hot and under high pressure. These conditions facilitate chemical reactions, suggesting that rapidly growing minerals could act like a quick-setting glue, binding faults together through cementation, and promoting rapid fault strengthening between SSEs. To explore this possibility, the team will conduct experiments where small rock and mineral samples are subjected to the temperature and pressure conditions of SSEs. The team will then measure the material properties, such as their strength and permeability to fluids, of the experimental products as they vary with experiment duration. Using the experimental results, the team will develop a mathematical description of rapid fault healing for incorporation into numerical models exploring the influence of cementation on SSEs. Results of these simulations will be compared to real-world observations to determine whether this process plays a fundamental role in the generation of SSEs. This project will catalyze interdisciplinary research in subduction zone geoscience through the training and mentorship of undergraduate and graduate students, plus postdoctoral researchers in the fields of seismology, rock mechanics, and geochemistry. The team leaders will enable interaction between the Cascadia Region Earthquake Science Center (CRESCENT) and Subduction Zones in 4 Dimensions (SZ4D) to facilitate coordination between these two efforts to achieve common goals relevant to geohazards. This proposal explores the role of cohesion, which is normal stress independent fault strength, via cementation and pore fluid pressure evolution in the dynamics of SSEs. Cementation is commonly observed in exhumed faults zones and is thought to play a key role in fault healing during the interseismic period. The high temperatures (~500°C) and pressures (1 GPa) present in SSE environs should favor relatively rapid cementation. There is also abundant evidence for fluids in the SSE source region that appear to play a crucial role in the generation of SSEs. The team proposes that pore-fluid pressure evolution and cementation can explain several enigmatic features of slow slip events, including radiative phenomena like tremor and low-frequency earthquakes, the tendency for the same section of the megathrust to re-rupture on short timescales during an SSE in so-called secondary slip fronts, the lack of sensitivity to tidally induced normal stresses, and the existence of fault strength in environments inferred to have nearly lithostatic pore fluid pressure. This work leverages interdisciplinary expertise from the fields of petrology, geochemistry, rock mechanics, observational seismology, fault mechanics, and numerical methods to explore the role of cementation and resulting cohesion in SSEs. This team will constrain the mechanisms of cementation, mineralogy and petrology of the cement, and the resulting time-dependent strengthening by performing a suite of piston-cylinder experiments at pressure, temperature, fluid compositions, and other conditions relevant for Cascadia SSEs. The project will determine the resulting cohesion and permeability using deformation experiments and contact area using microscopy. The results will provide quantitative constraints on time-dependent fault strengthening and permeability evolution. Constraints from these laboratory experiments will be used to develop a mathematical framework for cohesion and fluid flow. This framework will be implemented in numerical simulations to determine the impact of rapid cementation and cohesion on SSEs. The models will be validated with observables including propagation speeds, spatial scales, and time scales representative of SSEs and secondary fronts. This project will catalyze interdisciplinary research in subduction zone geoscience through the training and mentorship of undergraduate and graduate students, plus postdoctoral researchers in the fields of seismology, rock mechanics, and geochemistry. The team leaders will enable interaction between the Cascadia Region Earthquake Science Center (CRESCENT) and Subduction Zones in 4 Dimensions (SZ4D) to facilitate coordination between these two efforts to achieve common goals relevant to geohazards. This project is funded by the Frontier Research in Earth Science (FRES) program as well as Education and Human Resources (ERF) in support of Research Experiences for Undergraduates and Postdoctoral Scholars. 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.