Contrasting velocity-porosity relationships in differing tectonic regimes, Nankai Trough subduction zone, Japan; implications for pore pressure and effective stress estimation

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Author(s): Tudge, J.; Webb, S. I.; Tobin, H. J.
Author Affiliation(s): Primary:
University of Wisconsin-Madison, Madison, WI, United States
Volume Title: AGU 2012 fall meeting
Source: American Geophysical Union Fall Meeting, Vol.2012; American Geophysical Union 2012 fall meeting, San Francisco, CA, Dec. 3-7, 2012. Publisher: American Geophysical Union, Washington, DC, United States
Note: In English
Summary: The identification of areas of anomalously high porosity in subduction zones can have implications for fluid pressure, flow paths and the calculation of vertical effective stress in and under accretionary wedges. The relationship between p-wave velocity (Vp) and porosity is particularly useful for the estimation of fluid and solid material budgets in the subduction process because Vp is detectable with seismic reflection and refraction imaging. Data from cores and borehole logging can be used to develop quantitative Vp to porosity transforms, which in turn permit estimation of porosity from seismic reflection and refraction interval velocity. The relationship between Vp and porosity in sediments, however, is intrinsically linked to their burial history and tectonic evolution. Focusing on data from recent IODP drilling for the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) offshore Japan, we investigate the relationship between Vp and porosity for the different tectonic regions of a subduction zone accretionary complex, since universal transforms are shown to fit the data poorly. We demonstrate that each of the tectonic domains, Kumano forearc basin, accretionary wedge, and incoming Shikoku Basin sediments, exhibit very different Vp-porosity relationships. In addition, we show for sediments of the incoming plate (Shikoku Basin) section that correction of the core porosity data for smectite content results in a substantially modified Vp-porosity relationship. We use these new tectonic domain-specific Vp-porosity relationships to calculate estimated porosity from p-wave velocity models derived from seismic reflection data and OBS studies. By applying the specific Vp-porosity relationship in each tectonic region, a better-constrained estimate of distribution of porosity within the subduction zone accretionary prism complex, particularly across the main fault zones and décollement can be made. For example, when this approach is applied to the velocity reversal across the splay fault a smaller porosity contrast is observed. Using a single Vp-porosity transform on a single extracted seismic velocity trace there is a porosity increase of 4 - 9%, in contrast to an increase of 2 - 7% when domain-specific estimates are used. From this preliminary porosity estimate, in-situ pore pressure can be determined if the porosity-effective stress relationship is also constrained. This important in trying to understand the fluid flow dynamics, and fault strength within the Nankai Trough subduction zone. Since excess pore pressure results in fluid migration, which affects the overall strength of the sediments and may suppress stick-slip behavior of faults by reducing the effective normal stress. By constraining the Vp-porosity relationship in the undisturbed Shikoku Basin sediments with we can potentially gain more insights into the controls on fluid migration in the Nankai Trough, and constrain further the up-dip limit of the seismogenic zone.
Year of Publication: 2012
Research Program: IODP Integrated Ocean Drilling Program
Key Words: 16 Structural Geology; Body waves; Elastic waves; Integrated Ocean Drilling Program; Marine sediments; NanTroSEIZE; Nankai Trough; North Pacific; Northwest Pacific; P-waves; Pacific Ocean; Sediments; Seismic waves; Subduction zones; Tectonics; Velocity; West Pacific
Record ID: 2014082024
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data supplied by, and/or abstract, Copyright, American Geophysical Union, Washington, DC, United States

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