Drilling at the northern Hikurangi subduction margin, New Zealand: The key to unlock the secrets of slow slip events
Abstract
Over the last decade, the surprising discovery of episodic slow slip events (SSEs) at subduction margins around the globe has led to an explosion of new theories about fault mechanics and subduction interface deformation mechanisms and rheology. The northern Hikurangi margin is the only well-documented place on Earth where recurrent SSEs take place on a subduction interface that is within range of existing drilling capabilities. We suggest that drilling, sampling, and monitoring of the northern Hikurangi margin SSE source area may be the best way to definitively test ideas regarding SSE occurrence, and unlock the secrets of slow slip. Northern Hikurangi SSEs recur every two years, and thus provide an excellent setting to monitor changes in deformation rate and physical properties within and surrounding the SSE source area throughout a slow slip cycle. In collaboration with a broad group of proponents, we have recently submitted a pre-proposal to IODP to target the Hikurangi SSE source area. We propose a staged drilling plan with 2 phases: (1) Three to four shallow (~200-1000 m depth) non-riser holes to collect samples, make measurements, and install observatory equipment to monitor changes in strain, tilt, seismicity and other physical properties throughout an SSE cycle and characterize the target SSE source area with very high fidelity; (2) A deep hole (~6 km below the sea floor) that penetrates the subduction interface and directly samples material from the SSE source region, and measures physical properties in the SSE region and throughout the upper plate. Drilling and instrumenting these sites can resolve competing hypotheses regarding the generation of slow slip and the mechanics of subduction interface thrusts. Do SSEs occur under near-lithostatic fluid pressures, or are weak and shallow interseismic coupling and shallow SSEs caused by low fluid pressures and high structural permeability within the upper plate? Do short-term hydrological property variations facilitate SSEs or is there no relationship? What control does temperature have on the down-dip limit of interseismic coupling and the depth to slow slip events? Is the character of the subduction interface where weak, shallow coupling occurs fundamentally different from that of deeply, strongly coupled subduction margins? The establishment of observatory facilities and the monitoring of changes in hydrological properties, strain rate and seismicity within and near the SSE source area throughout a two-year SSE cycle will elucidate the role that short-term variations of physical conditions play in the occurrence of aseismic vs. seismic slip. Comparison of properties of the interface between the northern Hikurangi margin (weak shallow interseismic coupling, characterized by moderate, shallow subduction thrust events) and the Nankai margin (strongly coupled, produces Great megathrust earthquakes) has the potential to solve the long-standing mystery of why some subduction margins rupture in Great megathrust earthquakes while others do not.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.U43A0019B
- Keywords:
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- 3036 MARINE GEOLOGY AND GEOPHYSICS / Ocean drilling;
- 3060 MARINE GEOLOGY AND GEOPHYSICS / Subduction zone processes;
- 8170 TECTONOPHYSICS / Subduction zone processes