Seismic anisotropy and microseismicity: from crustal formation to subduction termination
The plate tectonic cycle is fundamental to our dynamic Earth, encompassing the formation and evolution of new lithosphere at divergent, mid-ocean ridges, all the way to its eventual return to, and re-equilibration with, the mantle in subduction zones. I investigate the structure of the crust and upper mantle of the Earth in two regions that represent different endmembers in this cycle through the analysis of microseismicity and seismic anisotropy, seeking to learn more about the stresses in these environments and how they are manifest in the structure of the subsurface. In Iceland, new oceanic crust is accreted episodically within the volcanic rift zones that delineate the subaerial portion of the Mid-Atlantic Ridge, the divergent margin between the North American and the Eurasian plates. Northern Borneo, conversely, exhibits the tectonic signatures of not one but two terminated subduction zones, where oceanic lithosphere was once being actively recycled into the Earth’s mantle. As part of this work, I have helped to deploy and service two passive seismic experiments—the Cambridge Volcano Seismology network in Iceland and the northern Borneo Orogeny Seismic Survey (nBOSS) network in Sabah—from which I have derived my results.
Seismic anisotropy is manifest on a vast range of scales, from swathes of the crust and mantle, all the way down to the scale of single mineral crystals. It has the potential to inform on the dynamic state of the mantle, the structural fabric of fault zones, layering in sedimentary basins, and the distribution of partial melt in the subsurface, to name a few applications. Here, I seek new insights into the stress state and structure of nascent oceanic crust as it is accreted at a mid-ocean ridge, and the volcanic systems found therein, in Iceland. I also look to piece together the interplay between past tectonic events, subduction termination, and the present-day state of the mantle in northern Borneo.
In the first part of my dissertation, I apply shear-wave splitting analysis to a microseismic catalogue (spanning the period 2008–2018) in the Northern Volcanic Zone of Iceland in order to investigate the relationship between seismic anisotropy and the tectonic stresses arising from extension and plate spreading, microseismicity, and the presence of melt. I find the upper 3–4 km of the Icelandic crust to be seismically anisotropic. Modelling of the stresses in the upper crust arising due to plate spreading and active deformation around Askja volcano, I find this anisotropic layer can be explained through a mechanism of stress-aligned microcracks within the porous crust. I then use earthquakes that occur within the lower, ductile crust—associated with the movement of melt within the volcanic plumbing system—and effective elastic media modelling to explore the role of melt in the generation of seismic anisotropy below 10 km depth, finding a strong correlation between my observations and the presence of melt as inferred from seismic tomography. I synthesise these observations with recent works examining the seismic anisotropic structure of the Icelandic crust from ambient noise observations to construct a conceptual model for the mechanisms generating anisotropy throughout the Icelandic crust.
In the second part of my dissertation, I step back in terms of scale to investigate how tectonic processes related to subduction (and post-subduction) are manifest in the form of seismic anisotropy by applying shear-wave splitting analysis to teleseismic core-refracted seismic phases. My results, in conjunction with a recent tomographic model of the region, constrain the seismic anisotropy to the lithosphere and reflect recent (past ∼20 Myr) events in the tectonic history of northern Borneo. In north-west Sabah, my observations of seismic anisotropy are aligned with the orogenic belt running down the north-west coast (the Crocker Range), which formed during the termination of subduction of the proto-South China Sea. Conversely, in south-west Sabah my observations suggest extension has played an important role in the subsequent tectonic evolution of northern Borneo following subduction of the Celebes Sea and the opening of the Sulu Sea. I supplement this work with the first detailed study of seismic activity on a network of faults around Mount Kinabalu, a 4100 m tall mountain within the Crocker Range of northern Borneo. For this, I developed and used QuakeMigrate, a new Python package for the automatic detection and location of earthquakes using waveform migration and stacking. These new observations indicate ongoing extension in the region around Mount Kinabalu and allow, for the first time, for this network of faults to be mapped and explored in detail.