= Solution
\b[Probably yes in connected fractures or transiently opened <pore space>, especially where a <transform fault> admits circulating <seawater>; a uniform water-filled porous layer several kilometres thick is not guaranteed.] <Porosity> and <permeability of a porous medium> generally decrease as cracks close or seal, while ongoing <faulting> can reopen pathways. Fluid can occur at high <pressure> and <temperature>, including a supercritical state; it need not resemble cold surface <water>.
Several complementary tests address presence and origin:
* \b[Geological:] recover fresh cores or dredged <fault rocks>. Hydrous alteration assemblages, veins, <serpentinite> replacing <peridotite>, and altered <gabbro> record <water-rock interaction>. Sealed <fluid inclusions> can preserve former fluids. These establish fluid access in the past; they do not by themselves demonstrate currently connected free <pore fluid>.
* \b[Geochemical:] analyse sampled fluids and <fluid inclusions> for salinity and chlorine, oxygen and hydrogen <isotope ratios>, and strontium or boron <isotope ratios> in alteration products. Compare with <seawater>, host <rocks>, and possible magmatic or metamorphic sources. A seawater-like input modified systematically by <water-rock interaction> supports deep circulation from the <ocean>. Exchange, <isotope fractionation>, mixing and phase separation mean that altered fluids need not retain the exact surface composition.
* \b[Geophysical:] combine <seismic tomography>, <Compressional-to-shear wave speed ratio> and <seismic attenuation> with <magnetotelluric sounding> or controlled-source electrical measurements. A connected saline fluid network can reduce <electrical resistivity> and change elastic <wave speeds>. Sample <hydrothermal vents>, measure <heat flux> and, if drilling is possible, make direct <pore pressure> and permeability measurements.
\b[No one anomaly uniquely identifies water.] <Seismic velocity> can also reflect cracks, composition and <temperature>; <electrical conductivity> can reflect <melt> or conductive <minerals>. <Serpentinization> demonstrates incorporation of <water> into <minerals>, which must be distinguished from fluid currently occupying <pore space>. At an oceanic <transform fault>, an external <seawater> supply is the natural first hypothesis, but <isotope ratios> and chemical budgets can test contributions from deep fluids rather than presuming the source. A research example of coupled deformation and hydrothermal alteration is <Mass transfer and fluid flow during detachment faulting and development of an oceanic core complex, Atlantis Massif by Chiara Boschi and others>; a detachment system illustrates the methods without proving identical flow at every <transform fault>.
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