THE HYDROLOGY OF THE SEISMIC CYCLE PART I: COSEISMIC DILATION AND ITS IMPLICATIONS FOR FAULT MECHANICS AND ORE-MINERALIZATION PROCESSES (Invited Presentation)

Coseismic fracture dilation within fault zones is commonly argued to result in rapid decreases in local pore-fluid pressures, with implications for both fault mechanics and ore-mineralization processes. The mechanisms and consequences of this process, however, have only been examined in concept. This talk will provide a more quantitative hydrologic consideration of the fundamental physics and spatial and temporal scales of relevance associated with coseismic dilation. Using a series of theoretical analyses derived from first principles and basic hydrology, I will show that coseismic fracture dilation and associated decreases in pore-fluid pressure must necessarily result in a highly localized “singularity” in the hydraulic gradient surrounding any newly dilated fractures. The nature of this singularity implies that the maximum amount of fluid movement that can occur in response is limited to only that required to fill the fracture volume and equilibrate its pressure with the surrounding rock mass. If the magnitude of pressure decrease inferred by some previous work is correct, then it is likely that the magnitude of the hydraulic gradients near these underpressured fractures could exceed 100-102 MPa / m. This value is several orders of magnitude beyond that typical of a lithostatic pore-fluid pressure gradient in the crust, and as such, it is unlikely that underpressured fractures formed through coseismic dilation are operative within larger fault- or crustal-scale fluid flow pathways following earthquakes (i.e. fault-valve behavior). Indeed, the nature and magnitude of the hydraulic gradient singularity around rapidly dilated fractures indicates that the length scales of fluid transport into these structures is no more than a factor of 5 larger than the length scale of the fracture in any direction. These observations have two important implications for fault mechanics and ore-mineralization processes: 1) underpressured conditions produced through coseismic fracture dilation are volumetrically minor, highly localized, and unlikely to persist beyond a few hours following rupture; and 2) this process is unlikely to act as a major factor driving the formation of ore deposits, even incrementally, as the limited mass balance would require unrealistically large concentrations of dissolved minerals prior to failure.