UC Santa Cruz

Slow slip events are well documented in global subduction zones at depths of 30-50 km. Tectonic (non-volcanic) tremor is considered to be the seismic manifestation of slow slip and is spatiotemporally correlated with slip events in most regions. Along the northern Hikurangi Margin, New Zealand, where a seamount studded igneous plateau subducts beneath the North Island, slow slip occurs shallowly at depths <15 km. Here, slow slip is associated with increases in microseismicity levels and has previously been weakly linked to tectonic tremor. Over a six-year period, the spatiotemporal progression of slow slip events along the northern Hikurangi Margin with respect to tremor, earthquake occurrence, and Coulomb failure stress changes imparted on the megathrust is analyzed. In this study, the first comprehensive tremor catalog is presented for 2010-2015. The catalog demonstrates that tremor is temporally associated with shallow slow slip events and deep tremor episodes may indicate the occurrence of previously undetected long duration slip events. A slow slip event in 2014 was recorded by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) experiment, resulting in a detailed spatiotemporal analysis of various interplate slip processes with respect to a local subducted seamount. Coulomb failure stress change analysis of this event suggests that seamount subduction plays a dominant role in the stress state of the shallow megathrust, and that the northern Hikurangi Margin is weakly coupled and largely releases strain through slow slip events. A detailed analysis of the Coulomb failure stress change imparted on the shallow megathrust by seven slow slip events along the northern Hikurangi Margin between 2010-2014 demonstrates that stress changes from these events influence the along strike migration of slow slip event sequences. Additionally, the stress changes dictate the spatial relationship between tectonic tremor and slow slip, with onshore tectonic tremor occurring almost wholly within regions of stress increase. Over multiple slow slip events, the shallow-most part of the plate interface experiences a net stress increase and may promote failure in future shallow earthquakes, such as tsunami earthquakes, which impact seismic hazards along the east coast of New Zealand.