Project Number1925965
Collaborative Research: The Mechanics of Intermediate Depth Earthquakes: a Multiscale Investigation Combining Seismological Analyses, Laboratory Experiments, and Numerical Modeling
Zhigang Peng (Principal Investigator)
Host InstitutionGeorgia Tech Research Corporation
Year Published2019
End Date2022-11-30
Funding OrganizationUS-NSF
SubtypeStandard Grant
Project Funding529866(USD)
Abstract in EnglishIt has been nearly a century since deep earthquakes, below about 50 kilometers, were definitively detected. Due to the difficulty observing such events, however, the mechanisms that control deep earthquakes are still poorly understood compared to shallower events. The pressures at depths greater than roughly 50 kilometers prohibit the frictional sliding and weakening mechanism that is essential to shallow events. This project focuses on the study of intermediate-depth earthquakes, those between approximately 50 and 300 km depth. The collaborative research combines small-scale scale laboratory experiments conducted at high pressure and temperature, seismological analysis of intermediate-depth earthquakes in well instrumented areas, and physics-based computer simulations to bridge the scales between the laboratory experiments and field observations. The hypothesis, generated from preliminary laboratory data, is that the minerals typical in subducting plates that produce deep earthquakes undergo one or more transformations that lead to mechanical instability as pressure and temperature increase. Minerals can densify when changing phase, reducing pressure locally and allowing for fracture propagation. Other reactions may create weak surfaces between or within grains. Under the right conditions, these instabilities can slip, generating heat and causing a runaway reaction leading to an earthquake. Laboratory data can reproduce these reactions at the small scale. Computer models will be created using the laboratory data to reproduce these experiments and determine parameters for the models. The results will then be systematically scaled up to simulate intermediate depth earthquakes in actual subduction zones. These large-scale simulations will be compared to the characteristics of observed earthquakes, with improved sensitivity to detect micro-earthquakes based on novel template matching and machine learning techniques. The combined research will help to demonstrate, for the first time, whether the same processes observed in small-scale laboratory specimens can account for large intermediate-depth earthquakes in subduction zones. Intermediate-depth earthquakes, while less common than shallow earthquakes, do result in casualties and significant damage. Understanding the mechanisms that cause such events will help to better characterize the potential hazard and risks to seismically active areas. The interdisciplinary experimental, numerical, and seismological work has the potential to transform our understanding of deep seismic events and deep Earth interior that are difficult to observe directly. The project will support postdoctoral researchers, graduate students, and undergraduate students further their education.

It has been nearly a century since deep earthquakes, defined as below about 50 km depths, were first unequivocally discovered. The pressures at these depths preclude the frictional sliding that dominates shallow earthquakes and mechanisms of deep earthquakes remain poorly understood. Many challenges surround the study of deep earthquakes, including the inability to physically examine the fault structure and directly observe earthquake slip in the deep Earth interior. Here, this project investigate the mechanisms behind intermediate-depth earthquakes, defined as those between about 50 and 300 km depths, by integrating three key approaches: (1) detailed seismological investigation over a few well-studied tectonic settings, (2) controlled laboratory experiments on candidate mineral/rock groups with potential mechanical instabilities triggered by high-pressure, high-temperature reactions with acoustic emission monitoring and quantitative waveform analyses, and (3) micromechanics-based mathematical and physical models with a multiple scaling scheme to cover rupture processes from mm to km scales.

Emerging new seismological tools such as template matching and machine learning allow detection of microevents in subduction zones with unprecedented spatial and amplitude resolution. The more than 10-fold increase in event detection provides much more illumination of fault areas than previously available. With such advances, this study will focus on the subduction zones in Central and Northern Japan, to examine event distribution, frequency magnitude statistics, aftershock productivities, source properties, fault orientation and stress drops. Experimentally, a number of major constituents of subducting slabs such as partially serpentinized olivine, eclogitization of lawsonite blueschist facies rocks, and even harzburgite, are now known to produce mechanical instability. Several physical mechanisms have been proposed for intermediate-depth earthquakes based on these observations. Development of experimental devices have increased sample linear dimensions by a factor of about 10. New developments in broadband acoustic emission technology have permitted quantitative analyses of acoustic emission events ("labquakes") using state-of-the-art seismological tools. Therefore, earthquakes and labquakes can be treated in a unified fashion in seismological analyses, allowing direct and better comparison with observations at very different scales. Thermo-poro-mechanical models will account for phase transformations, and formation of nano-shear or reaction bands as observed in the experiments. Simulations will be conducted in three stages: (1) Simulate the small-scale experiments. Detailed scans of experiments will allow us to mimic the perturbation in material that will initiate the transformation. The experiments at this stage will be used to validate and improve the model, as well as fit model parameters. (2) Mathematically upscale the model, homogenizing the small-scale behavior, so that the model can be used to simulate earthquakes in plates. (3) Simulate the Japan subduction zone. The models will then be compared with seismological observations to valid the results.

The study will complete one of the last pieces in the puzzle of intermediate-depth earthquakes: verifying whether observed phenomena in small-scale experiments can quantitatively be the principal mechanism for regular intermediate-depth earthquakes observed at large depths. The combined seismological, experimental, and multiscale numerical work has the potential to truly transform the approach to the study of intermediate-depth earthquakes. The research will also lead to the development of new numerical methods and their application to geophysical processes.

The subject of intermediate-depth earthquakes bears enormous societal impact with great scientific significance to the Earth and planetary science community. Large intermediate-depth earthquakes are capable of producing significant damage and casualties. Hence, an improved understanding of their mechanisms helps mitigate seismic hazard from these events. The mechanics of solids under high pressures is also of interest to physicists and material scientists. The interdisciplinary nature of the work has diverse applications throughout science and engineering.

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.
Document Type项目
Recommended Citation
GB/T 7714
Zhigang Peng .Collaborative Research: The Mechanics of Intermediate Depth Earthquakes: a Multiscale Investigation Combining Seismological Analyses, Laboratory Experiments, and Numerical Modeling.2019.
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