GEOPHYSICAL EVIDENCE FOR AQUEOUS FLUIDS WITHIN THE EARTH'S CRUST AND MANTLE

We review geophysical evidence for aqueous fluids in the crust and mantle. Geophysical observations such as magnetotelluric (MT) data, DC resistivity measurements, seismic tomography, and seismic wave attenuation allow us to construct 2D and 3D models of Earth’s structure at depth. The joint interpretation of geophysical data allows us to infer fluid involvement in many important Earth processes. For example, high conductivity, seismic attenuation and low-velocity zones indicate that water is present in the mantle due to on-going degassing of the planet and the subduction of hydrated lithosphere. The presence of water in the mantle reduces viscosity and thus facilitates large-scale convection. Fluids in the crust influence active processes in every tectonic setting, from rifts to orogens. Subduction zones are one of the best-studied tectonic settings where evidence for crustal fluids has been found. The existence of fluids within these margins is based on anomalies such as low-velocity zones, bright seismic reflectors and highly conductive zones within an otherwise resistive crystalline crust. In the subduction zone beneath Honshu, Japan, 3-D ray tracing clearly show regions of low-velocity above, and high-velocity with the subducting slab. Spatial and temporal variations in fluid concentrations and pressures can play an important role in all types of fault slip behavior. Studies of major fault zones, such as the San Andreas and the Kunlun fault on the Tibetan plateau have provided evidence for the role of fluids in faulting. However, the precise role that fluids play in determining fault slip is a topic of considerable debate. Groundbreaking seismic and GPS observations that have identified seismic tremor and episodic slip offer exciting advances in the quest to better understand the role fluids play in earthquakes and other crustal processes.