In the Basin and Range extensional province, the earthquake risk may be low, but the mechanisms of Neogene tectonics are controversial. Active-source survey records there show reflections from normal- and oblique-slip faults.  Simulated-annealing velocity optimization allows a prestack Kirchhoff migration to reconstruct these fault-plane reflections into true-dip sections.  A unique image from Death Valley shows the listric geometry of a normal fault.  The Dixie Valley fault plane, which ruptured in 1954, dips at only 28 degrees in 1998 survey results.  Basin and Range extension may well follow more thin-skinned models.

• Fault reflectivity and imaging
• Crustal imaging, southern California, using earthquakes as sources
• Imaging a listric normal fault, Death Valley
• A seismogenic normal fault with shallow dip, Dixie Valley

I hope to convince you first that my asymptotic scattering assumptions are valid for the geometric imaging results I want to obtain. In addition, I have come to believe that an acoustic approach may well help my imaging summations focus on tectonic structures, and lessen the interfering images of non-tectonic geologic structure.

We will then look at very new and controversial images of fault structure through the whole crust on the northern margin of the Los Angeles basin, where the huge population and economy is at risk of destruction by moderate, or magnitude 6.5, earthquakes. Any suggestion of new faults, or geometric description of known faults, can help the hazard assessments enormously. My images below the 1994 Northridge sequence suggest a master thrust that is not really shallowly-dipping, supporting what can be called thick-skinned tectonic models and somewhat less seismic hazard. Yet I use aftershock seismograms recorded on the seismic monitoring network, instead of the usual active sources, so the generation and interpretation of the images in the presence of all the artifacts of poor ray coverage is problematic.

Next I will shift to my analysis of a normal fault in the Basin and Range extensional province, using standard reflection survey data recoded by the COCORP consortium in Death Valley. I will use this story to explain additional validity checks on my images. Twelve percent of the Pacific to North America plate motion extends into this moountainous plateau, which has undergone, according to various geologists, anywhere from twenty to several hundred percent extension in the Cenozoic. I will show you the first ever image of a basement-involved listric normal fault, which suggests more thin-skinned tectonic models.

I will finish with a report on a new survey I did in March earlier this year, that shows a seismogenic normal fault to have an astonishingly low dip - just twenty-eight degrees. This fault broke through Dixie Valley in 1954, and mapping of the spectacular historic rupture supports the low dip angle. Our observations challenge tectonophysicists to explain how a normal fault may rupture at such low angle, and support the concept of low-angle detachment even in the classic central Basin and Range.

I can't forget the debt I have to my students Sathish Pullammanappallil, Sergio Chavez-Perez, and Robert Abbott, who actually performed much of this work; neither can I get away without acknowledging my co-investigators John Caskey and Steve Wesnousky for Dixie Valley.