HYDROLOGICAL SIGNATURES OF EARTHQUAKE STRAIN

The character of the hydrological changes that follow major earthquakes has been investigated and found to be dependent on the style of faulting. 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 6-12 months. In contrast, hydrological changes accompanying pure reverse fault earthquakes are either undetected or indicate lowering of well levels and spring flows. Strike-slip and oblique-slip fault movements are associated with a mixture of responses but appear to release no more than 10% of the water volume of the same sized normal fault event. For two major normal fault earthquakes in the western United States (those of Hebgen Lake on August 17, 1959, and Borah Peak on October 28, 1983), there is sufficient river flow information to allow the magnitude and extent of the postseismic discharge to be quantified. The discharge has been converted to a rainfall equivalent, which is found to exceed 100 mm close to the fault and to remain above 10 mm at distances greater than 50 km. The total volume of water released in these earthquakes was around 0.3 km3 (Borah Peak) and 0.5 km3 (Hebgen Lake). Qualitative information on other major normal fault earthquakes, in both the western United States and Italy, indicates that the size, duration, and range of their hydrological signatures have been similar. The magnitude and distribution of the water discharge for these events are compared with deformation models calibrated using seismic and geodetic information. The quantity of water released over a time period of 6-12 months suggests that crustal volume strain to a depth of at least 5 km is involved. The rise and decay times of the discharge are shown to be critically dependent on crack widths, and it is concluded that the dominant cracks have a high aspect ratio and cannot be much wider than 0.03 mm. Using the estimated depth to which water is mobilized, the modeled crack size, and the measured volumes of water expelled, it is concluded that even at distances of 50 km from the earthquake epicenters, cracks must be separated by no more than 10 or 20 m. In regions of highest discharge nearer the earthquake epicenters, separations of 1 or 2 m are required. These results suggest that water-filled cracks are ubiquitous throughout the brittle continental crust and that these cracks open and close throughout the earthquake cycle. The existence of tectonically induced fluid flows on the scale that we demonstrate has major implications for our understanding of the mechanical and chemical behavior of crustal rocks.