HR: 15:10h
AN: NS23A-07 INVITED    [Abstracts]
TI: Shallow Dip of Two Great Basin Normal Faults Demonstrated by Shallow Seismic Reflection With Refraction Tomography
AU: * Louie, J N
EM: louie@seismo.unr.edu
AF: Nevada Seismological Laboratory, University of Nevada 174, Reno, NV 89557, United States
AU: Pullammanappallil, S
EM: satish@optimsoftware.com
AF: Optim Inc., University of Nevada 174, Reno, NV 89557, United States
AB: Whether normal earthquake faulting can ever occur on planes dipping as shallowly as 30° is a subject of debate. We have investigated suspect low-angle normal faults in the Great Basin with a combination of two near- surface seismic imaging techniques. Shallow seismic reflection imaging combined with first-arrival travel-time tomography has proved effective in detailing normal-fault geometries from the surface to about 1 km depth on two major Great Basin normal faults. Additional constraints provided by gravity surveys leave no doubt that the basin- forming normal faulting occurred and continues to occur on fault planes dipping less than 40°. The 1954 Dixie Valley earthquake ruptured the southern segment of the Dixie Valley fault in central Nevada. Abundant geologic and paleoseismic evidence points to a 30° dip for the rupture. A smooth, straight fault-plane reflection extends at that dip from within 2 m of the surface to 0.5 km depth in a series of seismic reflection images. Below 0.5 km the fault plane is concealed below a strong capping basalt reflection, though we observe basin stratigraphy terminating against the fault in a rollover geometry. Computing a velocity tomography section coincident with the reflection section, from first arrivals on the reflection records with SeisOpt^{\textregistered} @2D^TM (\copyright Optim, 2007), shows that the basin-bottom velocity contrast is coincident with the 30°-dipping reflection and stratigraphic terminations. Although the northern Dixie Valley fault certainly dips steeply at the geothermal plant 45 km north, the southern Dixie Valley 1954 rupture must be shallow-dipping. At the West Ruby Mts. range-front fault in eastern Nevada, a smaller-scale program similarly combining shallow reflection with refraction tomography constrained the geometry of that fault to 100 m depth. Superimposing the reflection image on the tomographic section allows easy identification of basin-bottom, fault, and basin- stratigraphic reflections. Classic tilted gravity-slide blocks 20-30 m wide riding on a detachment are clear in the combined image. This geometry suggests progressive abandonment of slide blocks updip, with the most recent, prehistoric scarp the most downdip of a series of scarps.
DE: 0530 Data presentation and visualization
DE: 0935 Seismic methods (3025, 7294)
DE: 7270 Tomography (6982, 8180)
DE: 8002 Continental neotectonics (8107)
DE: 8109 Continental tectonics: extensional (0905)
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting