Improving upper mantle viscosity estimates: constraints from seismic and magnetotelluric data, and impacts on asthenospheric flow
This thesis is submitted in partial fulfillment of the requirements for the degree of Philosophiae Doctor at the University of Oslo, Norway and Doctor of Philosophy in Earth and Environmental Sciences at Macquarie University, Australia. The research presented here is conducted under the supervision of Professor Dr. Clinton P. Conrad and Dr. Kate Selway.
Shifting my interests from Physics to Geophysics started when I had my immersion in 2015 in the Philippine Institute of Volcanology and Seismology (PHIVOLCS) for six months, particulary in Volcano Monitoring and Eruption Prediction Division (VMEPD). I had fieldworks and the chance to visit the two most known active volcanoes, Taal and Mayon volcanoes, which unfortunately erupted in the past few years (after 2015). The fieldworks conducted are for monitoring purposes, where we measured the electrical conductivities and the magnetic fields on the flank of the volcano to detect any anomalies that may be related to a rising magma, as precursors for an eruption. The interpretations that we derived from these physical quantities are also coupled with geochemistry (e.g., steam/gas activity and levels) and geodetic interpretations (e.g., ground inflation), which are done by different research groups. During those six months, I was self-fulfilled because I was doing something good for the society and doing active research (which I really like to do!). From then on, I was convinced what path I want to take to feed my curiosity and fulfill my purpose. But, it was not an easy path, especially for someone like me who was not born with a silver spoon. To cut the long story short (since I need to discuss what this thesis is all about), I ended up in the International Centre for Theoretical Physics to have a background in Geophysics, and then doing PhD in University of Oslo and Macquarie University about a topic that I really like.
For three years, I have been working on mantle viscosity (an important property of a mantle), which controls surface expressions like plate tectonics (more in Chapter 1). When I was in highschool, I imagined that plates are like sheets of ice floating on a water. So, I thought that the mantle is in liquid form! I had this stupid and naive idea for 12 years, until I had to read about the structure of the Earth when I was in PHIVOLCS. Being naive about Earth is I guess related to the fact that I do not see its entirety but just specks of it, and Earth is part of my living. The same holds true for most of the people living in a place like Philippines where volcanic eruption and earthquake are merely part of our lives; it is normal that our curiousity about these phenomena is not ignited because there is something more important - that is, survival. However, in order to survive and plan for mitigation, it is important to educate ourselves about these phenomena. However, knowledge on the occurence of these phenomena is not enough in my opinion. Instead, the root cause has to be understood first, which leads me to study the mantle (below the Earth’ crust; the yellow meat of a mango as an analogy), specifically its viscosity.
In any Physics book, viscosity is defined as a measure of fluid’s resistance to deformation at a given rate. As an example, water is less viscous than a honey because it can easily be deformed by stirring. However, viscosity is usually a physical property of a fluid (liquid and gas), not of a solid. Then, why viscosity is considered as one of the physical properties of a mantle? Do mantle rocks flow? Going back to the provided definition, other keywords that need to be considered in understanding such property are deformation and rate. In order to deform an object, a force is required. The longer the time that the force is applied on an object, the more deformed it would be. For instance, your favorite pillow can lose fluffiness over time and become flat permanently. Similarly, mantle rocks in Earth’s interior can be deformed over time (at least thousands of years), but with extremely great amount of force (in mega to giga newtons) because they are more compact and much harder than your pillow. These enormous quantities (> 1000 years that is beyond our lifetime and > 106 N that is more than 2000 times my gravitational force) imply that mantle rocks are extremely viscous (~ 1020 times more than that of honey). Since the mantle is deformable and flowing as a viscous material, it can affect both the Earth’s surface and internal structure. Thus, studying the processes observed on the surface of the Earth can provide insights on the structure beneath us. We are somehow limited to do this because we cannot dig into the mantle and measure its viscosity.
In this thesis, a simple method is introduced in estimating upper mantle viscosity using geophysical observations, particularly seismic and magnetotelluric (MT) observations, which reflect mantle conditions relatable to viscosity. With the aid of rock deformation laboratory experiments, the most direct method to study a deforming mantle rock, we are able to understand and know what controls viscosity and deformation. Combining such an empirical knowledge with the mantle parameters that can be inferred from geophysical observations provides us with a good estimate on mantle viscosity (details in Chapter 2). Constraining mantle viscosity helps in geodynamic studies that strongly depend on viscosity, such as modeling flow patterns in the upper mantle to understand the observed seismic structures and the low-viscosity asthenosphere beneath the moving plates.
