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Geological Society, London, Special Publications; 1994; v. 78; p. 85-98;
DOI: 10.1144/GSL.SP.1994.078.01.08
© 1994 Geological Society of London

Deformation and Fluid Flow

Earthquakes, strain-cycling and the mobilization of fluids

R. Muir Wood

EQE, Newbridge House, Clapton, Gloucestershire GL54 2LG, UK

Employing empirical observations of the hydrological changes that follow major earthquakes, it becomes possible to predict subsurface fluid flows that accompany tectonic activity. Coseismic hydrological changes are found to be critically dependent on the style of fault displacement. Normal faults involve post-seismic compressional elastic rebound and displace large volumes of fluid from the crust: rainfall equivalent discharges have been found to exceed 100 mm close to the fault and remain above 10 mm at distances greater than 50 km; the total volume of water released in two M7 normal fault earthquakes in western USA was 0.3–0.5 km3. In contrast, reverse fault earthquakes involve extensional elastic rebound that increases crustal porosity, drawing fluids into the crust. The magnitude and distribution fo the water-discharge for normal fault earthquakes has been compared with deformation models calibrated from seismic and geodetic information, and found to correlate with the crustal volume strain down to a depth of up to 5 km.

These results suggest that fluid-filled cracks are ubiquitous throughout the brittle continental crust, and that these cracks open and close through the earthquake cycle. By repeatedly drawing fluids into microcracks distributed throughout a very large volume of rock, strain-cycling can achieve very significant chemical fractionation. Different classes of tectonics impose particular configurations of fractures and according to the origin of fluid recharge, lead to a range of different types and styles of mineralization. Seismic strain-cycling can also produce primary and secondary oil migration within a single causative mechanism.





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