Appendix I

After more than a century of research, geologists and other scientists have increased their understanding of the tectonic processes at work in the Wasatch Front and are now beginning to answer the questions about the seismicity and tectonics of the Wasatch Front that Gilbert and others first posed Scientists, engineers, architects, urban planners, and emergency managers are not waiting for a major earthquake disaster to learn that measures must be implemented to mitigate an earthquake's effects.

Site-specific probabilistic seismic hazard assessments (PSHAs) and associated seismic design bases are critically dependent upon the local geological and geotechnical model. For reactor and critical non-reactor facilities on soil or softrock sites exhibiting strain-dependent behavior, estimates of site response can change significantly as site measurements and inferred structure evolve.

This report is a compilation of available data on Quaternary faults in Arizona as of the summer of 1998. These data were compiled as part of a effort to compile data and map information on Quaternary faults throughout the world, which is being overseen by Michael Machette of the U.S. Geological Survey.

This geologic map was produced to compile and reinterpret published geologic information, and present the result of new geologic mapping in the Ray-Superior area. This data set serves as the basis for ongoing efforts to better understand the geologic history of this area, particularly with respect to the distribution and origin of mineral deposits.

Seismic design requirements for critical facilities normally require the development and application of a uniform hazard spectrum having a specified return period. For a soil site, hazard results are often evaluated for bedrock level and soil surface.

The Rio Grande rift has a relatively short and unimpressive record of historical seismicity. However there is abundance evidence of prehistoric (Quaternary) surface faulting associated with large (M>6) earthquakes. This paradox between historical and prehistoric seismicity (paleoseismicity) has important implications for seismic-hazards analyses based primarily on modern seismicity.

The purpose of this study was to gather and analyze previously unused data that led to improved locations for the 1906 (Ms 6.2), 1910 (Ms 6.0) and 1912 (Ms 6.2) northern Arizona earthquakes. Both ground shaking intensity patterns and instrumental data were used. The objective is to provide epicenter locations accurate enough to allow planners to decide what ground acceleration levels are most appropriate for northern Arizona.

Using a database of 655 recordings from 58 earthquakes, empirical response spectral attenuation relations are derived for the average horizontal and vertical component for shallow earthquakes in active tectonic regions. A new feature in this model is the inclusion of a factor to distinguish between ground motions on the hanging wall and footwall of dipping faults.

Map

Both the point- and finite-source stochastic ground motion models represent recent and promising developments in the quantification of strong ground motions for engineering design.