Subducting topography generates upper plate porosity that promotes slow slip at the Hikurangi Margin, New Zealand

Abstract:

How subducted topography hinders or promotes megathrust slip remains a widely debated question in subduction dynamics. Proposed models illustrate the potential for subducting topography to create complex fracture networks in the upper plate that may promote creep and thus prevent large earthquakes, yet testing such a hypothesis remains elusive. The Hikurangi Margin, a subduction zone located east of New Zealand’s North Island, provides a natural setting to study the relationship between rough subduction and earthquake slip behavior as numerous seamounts and knolls are evident on the incoming Pacific Plate. In December 2018 - January 2019, we collected seafloor magnetotelluric and controlled-source electromagnetic (EM) data at 3 trench-crossing profiles along the Hikurangi Margin to constrain the electrical resistivity of the incoming plate and forearc. At the northern section of the margin, a 90 km profile extends from a knoll on the incoming plate to a subducted knoll-like feature beneath the forearc. The collocation of our profile with previously collected seismic reflection data, IODP sites, absolute pressure gauges, and ocean bottom seismometers has allowed for joint interpretation of these datasets. Because seawater is orders of magnitude more conductive than crystalline and lithified rock, EM data are particularly sensitive to regions of high porosity, such as faults and fractures in lithosphere. Regularized joint inversions of the data show that the incoming plate knoll is characterized by a resistive cap that traps a conductive matrix of porous material over a resistive, intrusive core. We posit that the resistive cap allows underlying fluids to subduct with the topography while the resistive core maintains the structural competence necessary to fracture the upper plate. We also show that two prominent conductive anomalies are above the subducting knoll in the upper plate. The coincidence of the more conductive anomaly with the location of repeating earthquakes and microseismicity from a slow slip event during September to October 2014 provides unequivocal evidence that directly links the effects of upper plate damage from subducting topography to the creation of a highly porous zone in the forearc that promotes localized seismicity and SSEs.