Role of Basalt Alteration on Slip Behavior at the Hikurangi Subduction Zone

Abstract:

The northern Hikurangi margin hosts shallow slow slip events (SSEs) that propagate to within a few km or closer to the seafloor. The frictional properties and in situ conditions necessary for this observed slip behaviour are, as yet, poorly understood. International Ocean Discovery Program (IODP) Expedition 375 cored sediment on the subducting plate along the Hikurangi margin at Sites U1520 and U1526, which represent protolith of materials along the subduction megathrust. One key objective of drilling was to constrain the frictional regime of the materials along faults that host slow slip. Regional seismic reflection profiles spanning Sites U1520 and U1526 indicate that volcaniclastic facies and altered basalts found at these sites are entrained along the plate interface and likely participate in SSE nucleation and propagation. Here, we report on the frictional properties of clay-rich (up to 80% by weight) volcaniclastic sediments and powdered altered basalts (<125 um) from Sites U1520 and U1526 at stresses up to 25 MPa and pore-pressures up to 5 MPa. We conducted velocity stepping experiments from 0.1 – 300 µm/s to constrain the rate-state frictional behaviour of these materials. We also report on slide-hold-slide (SHS) experiments with holds of up to 3000 s in duration to measure frictional healing. Preliminary results indicate that the altered basalts are generally velocity-neutral to slightly velocity-strengthening at slip rates of ≈ 0.1 µm/s, consistent with peak slip rates of the northern Hikurangi SSEs, and become increasingly velocity strengthening at higher sliding rates. SHS experiments suggest near-zero healing rates, which, together with short recurrence intervals and small total slip relative to dynamic earthquakes, are consistent with the small stress drops observed for the shallow SSEs at Hikurangi and globally. We suggest that the high clay content of the volcaniclastic facies may play a key role in controlling slip behaviour. Additionally, the results indicate that basalt alteration may be a viable mechanism for arresting dynamic rupture and favouring slow slip due to its velocity-strengthening behaviour at higher slip rates.