Hydrological Signatures of Earthquake Strain
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B12, PAGES 22,035-22,068, DECEMBER 10, 1993 Hydrological Signatures of Earthquake Strain ROBERT MUIR-WOOD EQE International, Clapton, England GEOFFREY C. P. KING Institut de Physique du Globe, Strasbourg, France The character of the hydrological changesthat follow major earthquakeshas been investigated and found to be dependenton the style of faulting. The most significantresponse is found to accompany major normal fault earthquakes.Increases in spring and river dischargespeak a few days after the earthquake, and typically, excess flow is sustained for a period of 6-12 months. In contrast, hydrological changesaccompanying 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 responsesbut appear to release no more than 10% of the water volume of the same sized normal fault event. For two major normal fault earthquakesin the western United States (those of Hebgen Lake on August 17, 1959, and Borah Peak on October 28, 1983), there is suf•cient river flow information to allow the magnitude and extent of the postseismic discharge to be quantified. The dischargehas been converted to a rainfall equivalent, which is found to exceed 100 mm close to the fault and to remain above 10 mm at distancesgreater than 50 km. The total volume of water released in theseearthquakes was around 0.3 km3 (BorahPeak) and 0.5 km3 (HebgenLake). 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 signatureshave been similar.
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