Project funded by the National Science Foundation, Tectonics Program
Principal Investigator: John N. Louie
Seismological Laboratory, The University of Nevada, Reno
Co-Investigators: S. John Caskey and Steven G. Wesnousky
Center for Neotectonic Studies, The University of Nevada, Reno
The recognition of active low-angle normal faults (<30 ) and the possibility that they may rupture in large earthquakes has direct implications for seismic hazard analysis. Yet, controversy resides in whether low-angle normal faults can initiate and be active at shallow dip. Our recent geologic observations along the rupture trace of the 16 December 1954 Ms6.8 Dixie Valley earthquake (Figures 1 and 2) indicate a near surface fault dip as low as 25 E along the central portion of the rupture trace. The implication is that the Dixie Valley earthquake may be the first historic example of a large surface rupturing earthquake on a low-angle normal fault. Previous seismological data from the earthquake is unable to confirm or refute this hypothesis because the Dixie Valley event wave forms are contaminated by those from the preceding Fairview Peak (Ms7.2) earthquake, which occurred 4 minutes earlier. Likewise, previous seismic refraction and gravity studies in southern Dixie Valley can not adequately resolve the subsurface geometry of the Dixie Valley fault. The funds requested herein are to conduct a multi-element geophysical study to determine the subsurface geometry of the Dixie Valley fault along the 1954 rupture trace, and hence to test whether the Dixie Valley earthquake provides a true example of a large historical seismic event on a low-angle normal fault. Our approach will employ a combination of high- resolution and medium-resolution seismic reflection profiling to image the fault plane directly. The seismic reflection method will provide controls on both the depth of the fault and the average velocities of the overlying fill, removing the indeterminance associated with the previous seismic refraction and gravity work in southern Dixie Valley. We will also refine the coverage of gravity data in the area, to demonstrate how the existing gravity data can be compatible with our final fault geometry. The study is designed to both further our understanding of the faulting process in an extensional neotectonic setting and to provide results of practical importance to seismic hazard analysis.