Geophysical confirmation of low-angle normal slip on the historically active Dixie Valley fault, Nevada

摘要

The December 16, 1954, Dixie Valley earthquake (M-s=6.8) followed the nearby Fairview Peak earthquake (M-s=7.2) by 4 min, 20 s. Waveforms from the Fairview Peak event contaminate those from the Dixie Valley event, making accurate fault plane solutions impossible. A recent geologic study of surface rupture characteristics in southern Dixie Valley suggests that the Dixie Valley fault is low angle (<30) along a significant portion of the 1954 rupture. To extend these observations into the subsurface, we conducted a seismic reflection and gravity experiment. Our results show that a portion of the Dixie Valley ruptures occurred along a fault dipping 25 degrees to 30 degrees. As such, the Dixie Valley event may represent the first large, low-angle normal earthquake on land recorded historically. Our high-resolution seismic reflection profile images the rupture plane from 5 to 50 m depth. Medium-resolution reflections, as well as refraction velocities, show a smoothly dipping fault plane from 50 to 500 m depth. Stratigraphic truncations and rollovers in the hanging wall show a slightly listric fault to 2 km depth. Gravity profiles conservatively constrain maximum basin depth and define overall geometry. Extension along the low-angle section may have occurred in two phases during the Cenozoic. Current fault motion postdates a 13 to 15 Ma basalt, imaged in the hanging wall, and inherits from a fault formed during an earlier extensional pulse, concentrated at 24.2 to 24.4 Ma. The earlier extension suggests extraordinary slip rates as high as 18 mm/yr, resulting in the formation of the low-angle fault break. Sections of the Dixie Valley fault where there is no evidence for current low-angle slip correlate well with areas where no pre-15 Ma slip has been documented. [References: 34]