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Electrical resistivity of sediments was analyzed using samples recovered during International Ocean Discovery Program (IODP) Expedition 362, during which the input materials of the north Sumatran subduction zone were drilled to investigate the material properties linked to shallow seismogenic slip. Electrical resistivity is a valuable indicator for sediment consolidation, pore/grain structures, and distribution of fluid, which can affect the mechanical properties of the forearc wedge. Sediments were recovered from the seafloor to 1415.35 meters below seafloor (mbsf ) at Site U1480 and from 1149.7 to 1500 mbsf at Site U1481. They consist of thick sequences of the Bengal-Nicobar Fan (Lithologic Units I–II) underlain by a thin pelagic/igneous sequence (Units III–V). In this study, electrical resistivity was measured on 35 sediment samples from Site U1480 with an Agilent 4294A component analyzer using the bridge method with a two-terminal circuit. Measured resistivity values range from 0.20 to 7.45 Ωm and generally increase with depth. Sample measurements are consistent with the downhole resistivity logs acquired during Expedition 362. Formation factor was calculated from sediment and seawater resistivities, and Archie’s coefficients (cementation [m] and tortuosity [b]) were examined from the relationship between formation factor and porosity. When plotting the sample resistivity in this study together with resistivity logs and shipboard porosity from Sites U1480 and U1481, a contrast in Archie’s coefficients are inferred between the Bengal-Nicobar Fan and pelagic sediments, where the former (m = 3.4–3.8) is characterized by higher m values compared to the latter (m = 2.2). These coefficients show differences in consolidation trend in the input sediments, providing improved equations to estimate porosity from resistivity logs. Introduction The input materials of the North Sumatran subduction zone were drilled during International Ocean Discovery Program (IODP) Expedition 362, which was designed to investigate the material properties and causes for the shallow seismogenic slip and forearc prism structures responsible for the Mw 9.2 earthquake and tsunami that occurred in 2004 (McNeill et al., 2017b) (Figure F1A). Sediment consolidation is one of the important factors that can affect the mechanical properties of the forearc wedge, and it is a focus of this study. Electrical resistivity is a valuable indicator for sediment consolidation, pore/grain structures, and distribution of fluid (e.g., Kozlov et al., 2012; Cai et al., 2017). The consolidation trend of sediments can be illustrated from the correlation between resistivity and porosity, known as Archie’s law (Archie, 1942): F = Reff/Rf = bφ−m, (1) where F = formation factor, Reff = resistivity (Ωm) of fluid-saturated sediment, Rf = resistivity (Ωm) of fluid in the sediment, φ = porosity (ratio) of sediment, m = cementation coefficient (Archie’s coefficient; empirical parameter), and b = tortuosity coefficient (Archie’s coefficient; empirical parameter). Generally, m ranges from 1.0 to 3.0 for natural rocks (e.g., Cai et al., 2017), representing the slope of the porosity-resistivity curve on a logarithmic scale, where larger m values (larger slope) represent M. Hamahashi Data report: electrical resistivity of sediments from Site U1480 IODP Proceedings 2 Volume 362 higher resistivity for a given porosity, implying higher consolidation. Previous studies have documented the relationships between m, porosity, and grain geometry (e.g., Mendelson and Cohen, 1982; Daigle and Reece, 2015). For example, Mendelson and Cohen (1982) showed in their numerical model that m increases with increases in the grain aspect ratio, which could be caused by the presence of high–aspect ratio clay mineral grains. It is noted, however, that Archie’s law (Equation E1) is only strictly valid in the limit of zero surface conductivity on the grains (e.g., Revil et al., 1998), and the prefactor b should not take any value except b = 1 because any other value would imply that the resistivity of the porous medium does not approach the water resistivity as porosity reaches 100% (e.g., Glover, 2009). Indeed, b values that are not equal to 1 are typically the result of not considering surface conductivity or the presence of a percolation threshold (Winsauer et al., 1952; Sen et al., 1981; Ghanbarian et al., 2014). Nonetheless, Archie’s law is widely applied in various geophysical observations and is useful and practical for comparing resistivity and porosity, particularly from downhole logs. One of the important applications of Archie’s law is to estimate in situ porosity from resistivity logs. This paper reports on the data and analysis of electrical resistivity measured on discrete sediment samples from Site U1480 in comparison with downhole resistivity logs and shipboard porosity data from Sites U1480 and U1481 acquired during Expedition 362. To investigate the consolidation trend of the input sediments, the relationship between formation factor, porosity, and Archie’s coefficients were examined. Study sites Coring at Site U1480 (3°2.04ʹN, 91°36.35ʹE; 4148 m water depth; ~250 km southwest of the subduction zone) reached 1431.6 meters below seafloor (mbsf), penetrating ~17 m into igneous basement (Figure F1A, F1B) (McNeill et al., 2017c). The full sedimentary succession was recovered from the seafloor to 1415.35 mbsf and consists of predominantly siliciclastic sediments deposited from various sediment gravity flows (Lithologic Units I–II), interpreted to be the Nicobar Fan, underlain by mixed tuffaceous and pelagic sediment (Unit III) and thin intervals of intercalated pelagic and igneous material overlying oceanic crust (Units IV and V) (McNeill et al., 2017c) (Figure F1C). The composition of the Nicobar Fan is similar to the Bengal Fan and is sourced mainly from the Himalayan-derived Ganges-Brahmaputra river system and the Indo-Burman range/West Burma (McNeill et al., 2017a). The sediment section encompasses the Late Cretaceous to recent, marked by an increase in deposition rate of the Nicobar Fan since the late Miocene (McNeill et al., 2017c; McNeill et al., 2017a; Backman et al., 2019). At Site U1481 (2°45.29ʹN, 91°45.58ʹE; 4178 m water depth; 35 km southeast of Site U1480), cores were recovered from 1149 to Figure F1. A. Regional map of study area modified from McNeill et al. (2017a) showing Sunda subduction zone and surrounding eastern Himalayan provinces, Bengal-Nicobar submarine fan system, rupture area of 2004 Mw 9.2 earthquake (black outline), and location of Sites U1480 and U1481 (red dots). BB = Bengal Basin, SP = Shillong Plateau, SB = Surma Basin, IBR = Indo-Burman range, R = river. Blue lines = major river systems, yellow line = location of seismic profiles in B. Relative plate velocities are from Shearer and Bürgmann (2010). Inset = summarized lithostratigraphy, Site U1480. B. Seismic Profiles BGR06-101 and BGR06102 with location of Site U1480 (see location in A) (McNeill et al., 2017b; data from McNeill et al., 2016). Blue line = unconformable boundary between trench wedge and underlying Nicobar Fan sediments, green line = transition from reflective to less reflective stratigraphy, dashed red line = high-amplitude reflector having negative polarity toward subduction zone, overlying oceanic basement. CDP = common depth point, TWT = two-way traveltime. C. Lithology, stratigraphic ages, and sample P-wave velocity and porosity data obtained during Expedition 362 (McNeill et al., 2017b), Site U1480. Seismic panel is based on timedepth tie at the seafloor and at 1431 mbsf (McNeill et al., 2017b).

Data report: electrical resistivity of sediments from Site U1480, IODP Expedition 362, Sumatra subduction zone