The habitat of slow slip phenomena in the Hikurangi subduction zone
Final Report Abstract
In the last decade our knowledge of fault slip behavior has tremendously increased – showing that the plate boundary thrust in subduction zones not only host destructive “fast” earthquakes but also “slow” earthquakes that can last for months. The hydrogeological and diagenetic factors controlling the fault slip mode is still subject to debate. In 2018 International Ocean Drilling Program (IODP) Expedition 375 sampled the Hikurangi margin to investigate the conditions of the plate interface in the northern Hikurangi subduction zone. Using samples from the expedition the funded research investigated the linkage between water-rock interaction and fluid flow processes that influence the hydro-mechanical state of the subduction system. The conducted research encompassed boron isotope analysis of fluids sampled at Expedition 375 reference Site U1520 outboard the subduction system. The B and B isotopic composition shows a large spread in measured B and 11B values, which stems mainly from the combination of organic matter diagenesis, volcanic ash/silicate mineral diagenesis and potential lateral fluid flow. The U1520 results are compared to previously reported B and 11B composition of spring fluids sampled along the subaerially exposed forearc of the Hikurangi subduction zone. Assessment of the two data sets discards the common explanation of claywater interaction at temperatures <120°C for the observed differences. Instead, the B isotopic data supports a fluid origin in part from the subducting oceanic crust. Based on these findings the processes are discussed that drive the fault-valve behavior / slip mode of the plate boundary interface. The consolidation state of reference Site U1520 was also investigated using laboratory oedometer tests in which intact specimens cut from whole-round samples were re-loaded. Specimen recovered from the upper 500 mbsf show an underconsolidated state due to excess pore pressure caused by rapid sediment accumulation through mas wasting processes. In contrast, specimen below 550 mbsf are overconsolidated, which is mainly the result of calcite and zeolite cements in sediments. The implication of sediment lithification on frictional behavior was investigated by consolidating mixtures of halite and shale powders with halite-saturated brine, which were then desiccate. The desiccation allowed precipitation of halite as cement, creating synthetic rocks. Analysis of the individual rate-dependent friction parameters showed that the occurrence of velocity weakening is due to relatively low values of a for lithified samples. Larger velocity weakening is associated with cohesion of >~1 MPa, and porosity reduction of >~50 vol%. Microstructural images reveal that the shear surfaces for powders tend to exhibit small cracks not seen on the lithified sample shear surfaces. Our results suggest that lithification via cementation and porosity loss can facilitate slip instability, supporting the hypothesis that lithification of subductiong sediments is required for seismogenic slip.
Publications
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(2019). Mixed deformation styles observed on a shallow subduction thrust, Hikurangi margin, New Zealand. Geology, 47(9), 872-876
Fagereng, Å., Savage, H. M., Morgan, J. K., Wang, M., Meneghini, F., Barnes, P. M., Bell., R., KItajima, H., Saffer, D.M., Wallace, L.M., Petronotis, K., LeVay, L. & IODP Expedition 372/375 Scientists
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(2020). Slow slip source characterized by lithological and geometric heterogeneity. Science advances, 6(13), eaay3314
Barnes, P.M., Wallace, L.M., Saffer, D.M., Bell, R.E., Underwood, M.B., Fagereng, A., Meneghini, F., Savage, H.M., Rabinowitz, H.S., Morgan, J.K., Kitajima, H., Kutterolf, S., Hashimoto, Y., Engelmann de Oliveira, C.H., Noda, A., Crundwell, M.P., Shepherd, C.L., Woodhouse, A.D., Harris, R.N., Wang, M., Henrys, S., Barker, D.H.N., Petronotis, K.E., Bourlange, S.M., Clennell, M.B., Cook, A.E., Dugan, B.E., Elger, J., Fulton, P.M., Gamboa, D., Greve, A., Han, S., Hüpers, A., Ikari, M.J., Ito, Y., Kim, G.Y., Koge, H., Lee, H., Li, X., Luo, M., Malie, P.R., Moore, G.F., Mountjoy, J.J., McNamara, D.D., Paganoni, M., Screaton, E.J., Shankar, U., Shreedharan, S., Solomon, E.A., Wang, X., Wu, H.Y., Pecher, I.A., LeVay, L.J., and the IODP Expedition 372 Scientists
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(2021). Velocity-weakening friction induced by laboratorycontrolled lithification. Earth and Planetary Science Letters, 554, 116682
Ikari, M. J. and Hüpers, A.
