Frictional and Lithological Controls on Shallow Slow Slip at the
Northern Hikurangi Margin
Abstract
Slow slip events (SSEs) have been identified at subduction zones
globally as an important link in the continuum between elastodynamic
ruptures and stable creep. The northern Hikurangi margin is home to
shallow SSEs which propagate to within 2 km of the seafloor and possibly
to the trench, providing insights into the physical conditions conducive
to SSE behavior. We report on a suite of friction experiments performed
on protolith material entering the SSE source region at the Hikurangi
margin, collected during the International Ocean Discovery Program
Expedition 375. We performed velocity stepping and slide-hold-slide
experiments over a range of fault slip rates, from plate rate (5 cm/yr)
to ~1 mm/s and quantified the frictional velocity
dependence and healing rates for a range of lithologies at different
stresses. The friction velocity dependence (a-b) and critical
slip distance Dc increase with fault slip rate in
our experiments. We observe a transition from velocity weakening to
strengthening at slip rates of ~0.3 µm/s. This velocity
dependence of Dc could be due to a combination of
dilatant strengthening and a widening of the active shear zone at higher
slip rates. We document low healing rates in the clay-rich
volcaniclastic conglomerates, which lie above the incoming plate
basement at least locally, and relatively higher healing rates in the
chalk lithology. Finally, our experimental constraints on healing rates
in different input lithologies extrapolated to timescales of 1-10 years
are consistent with the geodetically-inferred low stress drops and
healing rates characteristic of the Hikurangi SSEs.