Seismic precursors linked to highly compressible fluids at oceanic transform faults

• Published in: Nature geoscience Vol. 7; no. 10; pp. 757 - 761
• Main Authors: Géli, Louis, Piau, Jean-Michel, Dziak, Robert, Maury, Vincent, Fitzenz, Delphine, Coutellier, Quentin, Henry, Pierre, Broseta, Daniel, Steele-MacInnis, Matthew, Driesner, Thomas
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 2014; Nature Publishing Group
• Subjects: 704/2151/241; 704/2151/508; 704/2151/562; Earth Sciences; Earth System Sciences; Engineering Sciences; Fluids mechanics; Geochemistry; Geology; Geophysics; Geophysics/Geodesy; letter; Mechanical engineering; Mechanics; Mechanics of materials; Physics; Sciences of the Universe
• ISSN: 1752-0894; 1752-0908
• DOI: 10.1038/ngeo2244

Earthquakes on oceanic transform faults are often preceded by foreshock swarms. A theoretical model suggests that circulating hydrothermal fluids, which compress as the fault rocks expand and deform, cause this precursor seismic activity. Large earthquakes on mid-ocean ridge transform faults are commonly preceded by foreshocks 1 , 2 , 3 and changes in the seismic properties of the fault zone 3 . These seismic precursors could be linked to fluid-related processes 2 , 3 . Hydrothermal fluids within young, hot crust near the intersection of oceanic transform faults become significantly more compressible with decreasing pressure, with potential impacts on fault behaviour. Here we use a theoretical model to show that oceanic transform faults can switch from dilatant and progressive deformation to rupture in response to fluid-related processes. We assume that the fault core material behaves according to a dilatant and strain-softening or contractant and strain-hardening constitutive law (Cam-clay-type 4 ), depending on the effective stress state. According to our model, after the initial purely rigid-elastic phase, dilatancy is found to occur within the fault core, causing pore pressure to decrease, hence fluid compressibility to increase. The plastic regime starts with a stable phase, with effective stresses gradually increasing over a timescale of years in response to tectonic loading. The fault then evolves into a metastable phase, lasting a few days, as the pore pressure decreases, inducing large variations or even a discontinuous jump in fluid compressibility, depending on fluid properties. This in turn triggers fault-slip instability that creates foreshock swarms. In the final phase, the fault fails in the mainshock rupture. Our results imply that seismic precursors are caused by the decrease in fluid pressure which results in an increase in fluid compressibility, in response to fault core dilatancy just before rupture.