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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS005 Oral Presentation 1911
Maximum Fault Displacements in Extensional Regimes: a Geologic
Perspective
Mrs. Suzanne Hecker
Earthquake Hazards U.S. Geological Survey
Timothy E. Dawson, David P. Schwartz
The characterization of ground motions at very low probabilities of exceedance is important for longlived
critical facilities. As part of an interdisciplinary effort to evaluate the possibility of extreme ground
motions at the proposed high-level nuclear waste repository at Yucca Mountain, Nevada (Basin and
Range Province, USA), we are addressing the issue of earthquake-source capability. Our approach is to
inventory maximum (or largest observed) historical and paleoseismic displacements on normal faults
both regionally and globally. To date, we have identified in the literature earthquake displacements for
144 faults or fault sections in regions of Cordilleran extension in the interior western United States,
including data from seven historical surface ruptures. Most prehistoric ruptures have been studied at
only one or two locations and, consequently, the relation of observed to maximum displacement is
uncertain. However, site-selection practices and preferential preservation of large scarps favor sampling
of above-average displacements. We have found that, with a few potential exceptions, normal faults in
the western United States have produced net coseismic vertical displacements no larger than about 6 m
at a point. We note that the largest historical displacement (5.8 m in the 1915 Pleasant Valley, Nevada,
earthquake) is essentially as large as any identified in the prehistoric record. Interestingly, faults with
large maximum displacements (> 4 m) are twice as common (normalized to total number of faults) in
the Rocky Mountains region, beyond the margin of classic Basin and Range physiography, than in the
rest of the extensional province. Normal faulting in much of the Rocky Mountains is localized along, and
has perhaps reactivated, parts of preexisting thrust faults. We speculate that the youthful, immature
nature of most faults in this relatively unextended region promotes higher static-stress drops.
Keywords: paleoseismology