PandSwave velocity measurements of water-rich sediments from the Nankai Trough, Japan

Abstract

Acoustic velocities were measured during triaxial deformation tests of silty clay and clayey silt core samples from the Nankai subduction zone (Integrated Ocean Drilling Program Expeditions 315, 316, and 333). We provide a new data set, continuously measured during pressure increase and subsequent axial deformation. A new data processing method was developed using seismic time series analysis. Compressional wave velocities (V p ) range between about 1450 and 2200 m/s, and shear wave velocities (V s ) range between about 150 and 800 m/s. V p slightly increases with rising effective confining pressure and effective axial stress. Samples from the accretionary prism toe show the highest Vp, while fore-arc slope sediments show lower Vp. Samples from the incoming plate, slightly richer in clay minerals, have the lowest values for V p . V s increases with higher effective confining pressures and effective axial stress, irrespective of composition and tectonic setting. Shear and bulk moduli are between 0.2 and 1.3 GPa, and 3.85 and 8.41 GPa, respectively. Elastic moduli of samples from the accretionary prism toe and the footwall of the megasplay fault (1.50 and 3.98 GPa) are higher than those from the hanging wall and incoming plate (0.59 and 0.88 GPa). This allows differentiation between normal and overconsolidated sediments. The data show that in a tectonosedimentary environment of only subtle compositional differences, acoustic properties can be used to differentiate between stronger (accretionary prism toe) and weaker (fore-arc slope, incoming plate) sediments. Especially V p /V s ratios may be instrumental in detecting zones of low effective stress and thus high pore fluid pressure.

Laboratory measurements quantifying elastic properties of accretionary wedge sediments: Implications for slip to the trench during the 2011 Mw 9.0 Tohoku-Oki earthquake

“…The laboratory velocities are consistent with seismicreflection models that indicate the velocity should range between 1.5-3.0 km/s in the frontal prism sediments in the Japan Trench (Murauchi and Ludwig, 1980;Miura et al, 2001Miura et al, , 2005Nakamura et al, 2014). They also agree with ultrasonic wave-speed measurements of prism sediments from other subduction zones (Carson and Bruns, 1980;Tobin et al, 1994Tobin et al, , 1995Tobin and Moore, 1997;Gettemy and Tobin, 2003;Hashimoto et al, 2010;Raimbourg et al, 2011;Schumann et al, 2014). For the purposes of this study, we are mainly interested in the average elastic properties of the prism.…”

“…At other subduction zones, it has been suggested that the occurrence of shallow coseismic slip is due to the presence of unusually competent and strong sediments which allow the accumulation and release of elastic energy (Gulick et al, 2011). To test this hypothesis, we examined ultrasonic velocity measurements that have been collected on shallow accretionary prism sediments retrieved from other subduction zone margins by IODP, DSDP, and Ocean Drilling Program (ODP) projects (Tobin et al, 1994(Tobin et al, , 1995Tobin and Moore, 1997;Gettemy and Tobin, 2003;Hashimoto et al, 2010;Raimbourg et al, 2011;Schumann et al, 2014). We compared our computed elastic properties to published data from other accretionary prisms to address whether a difference in the elastic properties could account for the occurrence of coseismic slip to the trench during the Tohoku earthquake.…”

“…While it is difficult to identify distinct rigidity-depth trends in the data, there are some differences among the prisms. The majority of data presented in Figure 7 This study Nankai Trough Schumann et al (2014) Sonic log (Webb, 2017) Costa Rica Margin Gettemy and Tobin (2003) come from the Nankai Trough, where multiple boreholes have been drilled at different locations in the accretionary prism as part of the NanTroSEIZE project. These data show variations in dynamic elastic moduli related to the setting from which each sample was taken within the accretionary prism volume.…”

“…This study Barbados Ridge Prism Moore (1997) Oregon Prism Tobin et al (1994;1995) Nankai Trough Raimbourg et al (2011) Hashimoto et al (2010) Schumann et al (2014 Webb (2017) Costa Rica Margin Gettemy and Tobin (2003) Laboratory shear modulus at 70 MPa fraction study (Miura et al, 2001(Miura et al, , 2005 to create a cross section of shear modulus variation within the Japan Trench. The refraction data were acquired on the MY 102 seismic line, located north of Site C0019 (Fig.…”

Texture Development of Clay‐Rich Sediments Across the Costa Rica Subduction Zone

“…During compaction, platy grains typically rotate from a less ordered orientation with predominantly edge‐to‐edge and edge‐to‐face contacts towards an alignment parallel to the seafloor with predominantly face‐to‐face contacts resulting in crystallographic preferred orientations (textures; Bennett et al, ; Bennett & Hulbert, ; Milliken & Reed, ). As for platy phyllosilicates crystallographic and shape preferred orientations are closely linked, textures can control the physical properties and deformation behavior of such sediments (e.g., Carson et al, ; Hashimoto et al, ; Kock & Huhn, ; Mondol et al, ; Oertel, ; Schumann, Stipp, Behrmann, et al, ), hence the strain distribution in active continental margins and also the frictional behavior in the forearc wedge as well as in the subduction channel along the plate boundary interface. Sediment strength, friction, and related seismogenic behavior are governed by, among other parameters, the mineralogical composition and fabric and their relevant physical properties (e.g., Kopf, ; Stipp et al, ; Schumann, Stipp, Leiss, et al, ).…”