The link between upper plate stress and subduction dynamics: Insights from the Hikurangi Subduction Margin

Characterisation of the contemporary in-situ stress state, orientation and magnitude, of the Earth’s crust is crucial to improve our understanding of active crustal deformation, geodynamic processes, and seismicity in tectonically active regions such as subduction dynamics. Earthquake occurrence and a variety of seismic slip behaviours are controlled by the interaction between in-situ principal stresses, mechanical and geometrical properties of crustal faults and fault rocks, and pore pressure within and around faults. Therefore, acquiring quantitative knowledge of the in-situ stress is an essential step to assess seismic and tsunamigenic hazards at subduction zones. This thesis seeks to explore the link between stress, structure, and subduction dynamics at the Hikurangi Subduction Margin (HSM). HSM, off the east coast of New Zealand's North Island, known to experiences strong, along-strike variations in megathrust slip behaviour, ranging from episodic slow slip events (SSEs) and creep at the northern and central HSM to deep interseismic locking (and stress accumulation) beneath the southern HSM. I explore spatial and temporal variations in the stress state within the overriding plate of HSM in order to (1) better understand their relationship to the far-field stress state and long-term tectonic deformations, and its potential link to observed along-strike variations in subduction slip behaviour and (2) determine critically stressed faults that are optimally oriented for slip and dilation in the HSM, investigate their impact on the permeability of the overriding plate’s faults, and investigate the role they play in driving SSEs. I analyse borehole image logs and oriented four-arm calliper logs acquired from thirteen boreholes, and integrate wireline data, leak-off test data, formation Integrity test data, and pore pressure data (mud weights and formation tests) from 44 wells. Then I present principal stress orientations and magnitudes, the first quantitative map of the faulting style (absolute and relative principal stress magnitudes), and its uncertainty along the HSM. Our results reveal a combination of contractional, transpressional strike-slip and/or normal faulting with SHmax orientations of 065°/245° (ENE-WSW) in the central and northern HSM. The southern HSM stress field becomes more contractional, showing a transpressional strike-slip and/or reverse faulting with a dominant 112°/292° (E-W) SHmax orientation. The variation of stress state along the HSM strike correlates spatially with observed along-strike variations in subduction interface slip behaviour. The variation of stress state along the HSM strike may result from along-strike variation in deformation style imposed by clockwise rotation of forearc. In the southern HSM, borehole-derived stress state in the overriding plate is inconsistent with stress state derived from focal mechanism solutions within the subducting plate, implying some degree of mechanical decoupling between the shallow hanging wall and subducting slab. This decoupling may reflect low shear stress on the subduction interface. This study found that faults in the central HSM in a strike-slip and compressional stress state have higher dilation tendencies compared to the faults in the southern HSM. Along-strike decrease in the vertical structural permeability of overriding plate may influence the degree of Pp on the subduction interface, which may influence duration of SSEs. Rate at which the transient Pp drops on the subduction interface is partly controlled by the time required for the permeability of high-dilating faults to increase in response to small stresses induced by SSEs. This transient increase in permeability allows the overpressure drop on the subduction interface, thus terminating slip in SSE episodes. This model may account for both short-term SSEs lasting 2-3 weeks in the northern and central HSM and long-term SSEs lasting over a year in the southern HSM.