EFFECT OF FOCAL DEPTH ON THE PALEOSEISMOLOGY OF REVERSE FAULTS

The best-expressed surface ruptures accompanying historical reverse-fault earthquakes are found in stable continental shields in Australia, India, and Québec, with focal depths of 2-7 km. The 1971 San Fernando, California, 1999 Chi-Chi, Taiwan, and 2001 Susitna Glacier, Alaska, earthquakes, with well-expressed surface faulting, also nucleated at depths much shallower than the brittle-plastic transition (BPT). The shallow depth may be because the maximum principal compressive stress, σ₁, is horizontal, and the minimum, σ₃, is overburden. Toward the surface, σ₃ (overburden) decreases faster than σ₁, and differential stress, σ₁ - σ₃, increases toward failure under the Coulomb-Mohr criterion. Surface expression is extensive because the hypocenter is close to the surface. In contrast, earthquakes nucleating near the BPT, including 1977 Caucete, Argentina, 1989 Loma Prieta, California, 1993 Suusamyr, Kyrgyzstan, 1994 Northridge, California, 2001 Bhuj, India, 2003 Chengkung, Taiwan, and 2004 Niigata-ken-Chuetsu, Japan, earthquakes were poorly expressed at the surface and may not be recorded in paleoseismic trench excavations. Their surface expression is mainly by broad warping and fold-related bending-moment and flexural-slip faulting. The Yakima Fold Belt comprises Washington's example of an active reverse-fault province with no historical surface-rupturing earthquakes and earthquake recurrence intervals as long as tens of thousands of years. Expression of most recent earthquakes is by bending-moment faulting, deformation of terraces of the antecedent Yakima River, and probable accentuation of Yakima folds, similar to earthquakes nucleating near the BPT. The unanswered question in estimating Mw of the next earthquake there is: should the top of the zone of high moment release be near the surface, or would it be at depths of several kilometers, as at Caucete, Northridge, and others nucleating near the BPT?