Journal Article

Spatial Heterogeneity of Pore Structure in the Crustal Section of the Samail Ophiolite: Implications for High VP/VS Anomalies in Subducting Oceanic Crust

Akamatsu, Y., Kuwatani, T., Katayama, I.

2024

Overview

Seismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies. Plain Language Summary Seismic studies along subduction zones have identified unusually high ratios of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust, which indicates the presence of water‐filled cracks and pores. The close link between pore water and local seismic activity highlights the importance of quantitatively interpreting these seismic anomalies in terms of pore characteristics. Previous interpretations have assumed that the microscopic pore structure is quite homogeneous, even at macroscopic scales as large as the seismic wavelength. However, our analysis of a bulk density profile of the crustal section of the Samail ophiolite, Oman, which is a fossilized oceanic plate preserved on land, indicates that the pore structure is more heterogeneous than previously assumed. This means that the fluid flow within the unit volume that represents the macroscopic physical properties of the rock cannot follow the wave‐induced loading during seismic wave propagations. This results in a relatively small impact of water on the seismic velocity, as inferred from theoretical models that predict the effective elastic properties of rock containing fluid‐filled cracks. Therefore, microscopic cracks may not have a large impact on the high VP/VS values of subducting oceanic crust, whereas large‐scale cracks may play a more significant role. Key Points The bulk density of the crustal section of the Samail ophiolite is more spatially heterogeneous than previously assumed The effect of fluid‐saturated microcracks on low‐frequency seismic velocities is modeled as an unrelaxed condition for this heterogeneity The high VP/VS anomaly in the subducting oceanic crust can be explained by both microcracks and large‐scale cracks

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Anomalies / Bulk density / crack / Cracks / Density profiles / Drilling / Elastic properties / Fluid flow / Fossils / Gamma rays / Heterogeneity / Laboratories / Oceanic crust / P-waves / Patchiness / Physical properties / Pore water / Pores / Radiation / Rock properties / Rocks / S waves / Seismic activity / Seismic propagation / Seismic studies / Seismic surveys / Seismic velocities / seismic velocity / Seismic wave propagation / Seismic waves / Seismology / Spatial heterogeneity / Subduction / Subduction (geology) / subduction zone / Subduction zones / upscaling / Velocity / Wave propagation / Wave velocity / Wavelength