Appendix I

The Carrizo Plain segment of the San Andreas fault is noteworthy as an area that clearly displays offset, diverted, and abandoned channels. The channels result from the interaction of strike-slip fault processes, and the geomorphic processes of erosion, transport, and deposition. This interaction produces periodically abandoned channels accompanied by the incision of new channels across the San Andreas fault. Geomorphic features such as scarps, offset stream channels, grabens, and pressure ridges mark the surface trace of the fault.

One of the fastest growing areas of Arizona is the eastern part of the Phoenix Basin near the communities of Chandler, Gilbert, Mesa, and Apache Junction. Much of this development has occurred on the piedmont near the base of the mountains where little of the surficial geology has ever been mapped. Because this area of the Phoenix Basin will continue to grow in the future, there is a need to understand the nature and distribution of surficial deposits.

The smallest geomorphic offsets along a 35 km section of the San Andreas fault in the Carrizo Plain vary from 7 to 10 m. Our three-dimensional excavation of alluvial deposits a few km southeast of Wallace Creek confirms that at least 6.6 to 6.9 m of dextral slip occurred there during the latest large earthquake, in 1857.

Appendix 7.A - Modeling of dynamic soil properties

Have earthquakes strong enough to rupture the ground surface occurred on faults in central Arizona during the recent geologic past? Could such earthquakes happen in the future? If so, where are they most likely to occur? The Seismotectonics and Geophysics Section of the U.S. Bureau of Reclamation has, during the last 6 years, been working on answering these questions

Horseshoe and Bartlett Dams are located In the Transition Zone of central Arizona. Within this province, faults with evidence of Quaternary activity are widely scattered, and selsmiclty Is low In comparison to other parts of the western United States.

This study defines the distribution, grade, and quality of diatomite at the White Cliffs diatomite deposit, Mammoth, Arizona. The deposit is hosted in a lacustrine facies of the Quiburis Formation, a Miocene to Pliocene basin-fill sediment of the lower San Pedro Valley. The lake bed sediments are divided into three informal members, the Redington, White Cliffs, and Gust James. Diatomite is found only in the White Cliffs member, and three potential ore zones are defined.

The contemporary seismicity of the Colorado Plateau based on seismic monitoring in the past 30 yr can be characterized as being of small to moderate magnitude, and contrary to earlier views, of a low to moderate rate of occurrence with earthquakes widely distributed.

A distinctive suite of volcanic, sedimentary, structural, and physiographic characteristics permits differentiation of the post-1 2- to 15- Ma Basin-Range disturbance from earlier mid-Tertiary extensional tectonism in Arizona. Basin-Range volcanism comprises basalts or bimodal basalt-rhyolite suites that are concentrated in several fields in central and northern Arizona; the age of volcanism generally decreases northeast and east onto the Colorado Plateau margin.