Institut de Physique du Globe, Strasbourg, France

The character of the hydrological changes that follow major earthquakes has been investigated and found to be critically dependent on the style of fault displacement. In areas where fracture-flow in the crystalline crust communicates uninterrupted with the surface the most significant response is found to accompany major normal fault earthquakes. Increases in spring and river discharges peak a few days after the earthquake and typically excess flow is sustained for a period of 4 12 months. Rainfall equivalent discharges, have been found to ceed 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 M 7 normal fault earthquakes in the Western U.S.A. was 0.3-0.5 km3. In contrast, hydroIogical changes accompanying reverse fault earthquakes are either undetected or else involve falls in well-levels and spring-flows. The magnitude and distribution of the water-discharge for these events is compared with deformation models calibrated from seismic and geodetic information, and found to correlate with the crustal volume strain down to a depth of at least 5 km. Such relatively rapid drainage is only possible if the fluid was formerly contained in high aspect ratio fissures interconnected throughout much of the seismogenic upper crust. The rise and decay times of the discharge are shown to be critically dependent on crack widths, for which the «characteristic» or dominant cracks cannot be wider than 0.03 mm. 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. Seismohydraulic fluid flows have major implications for our understanding of the mechanical and chemical behaviour of crustal rocks, of the tectonic controls of fluid flow associated with petroleum migration, hydrothermal mineralisation and a significant hazard for underground waste disposal.

A model of stress transfer implies that earthquakes in 1933 and 1952 increased the Conlomb stress at the site of the 1971 San Fernando earthquake. The 1971 earthquake in turn raised stress and produced aftershocks at the site of the 1987 Whittier Narrows and 1994 Northridge ruptures. The Northridge main shock raised stress in areas where its aftershocks and surface faulting occurred. Together, M ? 6 earthquakes near Los Angeles since 1933 have stressed parts of the Oak Ridge, Sierra Madre, Santa Monica Mountains, Elysian Park, and Newport-Inglewood faults by > 1 bar. While too small to cause earthquakes, these stress changes can trigger events if the crust is already near failure, or advance future earthquake occurrence if it is not.