The controls on whether a subduction zone plate interface will generate earthquakes or creep steadily is a major topic of discussion. The question is further complicated by the discovery, over the last two decades, of ‘slow earthquakes’ that accommodate slip at speeds intermediate between steady creep and earthquakes. The Hikurangi margin, along the east cost of North Island, New Zealand, shows a sharp along-strike transition from interseismic locking (and potential for large megathrust earthquakes) to fully creeping. The margin also hosts slow earthquakes at depths ranging from > 30 km in the locked region, to near the trench in the creeping segment. The Hikurangi margin therefore lends itself to detailed study of the full range of fault slip styles and the parameters that may control their occurrence.
Recent International Ocean Discovery Program (IODP) drilling in Expeditions 372 and 375 logged, sampled, and instrumented an across-strike transect of the northern, interseismically creeping, Hikurangi margin. Based on these data and the rich geophysical data sets also available for this margin, I will discuss two observations and their implications for how faults slip in subduction zones.
1) Time-dependent deformation along shallow faults in turbidites.
In the northern Hikurangi margin, recent sea floor geophysical observations highlight a range of slip styles, including near-trench slow slip, in an area of historical tsunami earthquakes. IODP drilling across the intraprism Pāpaku splay fault in this same area revealed a mixture of brittle and ductile deformation structures in a 60 m wide fault zone. These observations imply that conditions leading to formation of different structures occurred on the same fault. Although geodetic observations neither necessitate it nor rule it out, this geological evidence for multiple deformation modes on the Pāpaku fault suggests that it may host multiple slip styles in the same location. Time-variable behavior of shallow splays should therefore be considered in interpretation of slow earthquakes and in seismic and tsunami hazard models.
2) Input sediments, fluid budgets, and the seismic style of the megathrust.
IODP sampling showed that the northern Hikurangi input sequence involves turbidites, carbonaceous mud, and volcaniclastic sediments. Whereas the turbidite sequences are largely or completely accreted, the carbonaceous and volcaniclastic sediments likely host the plate interface at depths of more than about 10 km. In the input sequence, prior to subduction, the carbonaceous sediments host both faults and stylolites, indicating they can host mixed brittle and ductile deformation. The volcaniclastics, on the other hand, show highly variable clay-alteration, calcite and zeolite cementation, and porosity, reflecting heterogeneous strength and crystal-bound fluid content. This heterogeneity in the input sediments is likely to produce a variable rheology and fluid pressure distribution at potentially seismogenic depths. In comparison, the southern and uniformly locked part of the margin has relatively homogeneous inputs.