Onboard measurements of the seismic velocity and electrical resistivity of dredged samples from the Southern Mariana region collected during cruise KH-23-9

Abstract

We measured the seismic velocity and electrical resistivity of 57 dredged rock samples collected in the Southern Mariana region during cruise KH-23-9. Measurements were carried out on board under both dry and wet conditions. The mass and solid volume were also measured in the laboratory to calculate the grain density and porosity. Seismic velocity varied with lithology, with peridotite yielding the lowest velocity, suggesting significant serpentinization. The Vp/Vs ratio was ∼1.8–2.0 for all lithologies with no significant variation. Electrical resistivity was related primarily to porosity, and the low-porosity samples yielded nearly constant resistivity, suggesting reduced connected porosity. To evaluate the effects of porosity on seismic velocity and electrical resistivity, we applied effective medium theory and a modified Archie's law to the geophysical data. This analysis indicated the presence of fractures and faults in both the forearc and the Pacific Plate, suggesting extensive fluid infiltration and hydration have occurred in these regions.