Appendix I

Several methods for evaluating the effect of local soil conditions on ground response during earthquakes are presently available. Most of these methods are based on the assumption that the main responses in a soil deposit are caused by the upward propagation of shear waves from the underlying rock formation.

Changes since 1855 in reported section-line lengths and positions of survey monuments that span the San Andreas fault (SAF) were used to measure displacement interpreted to be from the 1857 Fort Tejon earthquake in south-central California. In 1855–1856 James E. Freeman established township and range lines across the SAF between Rancho Cholame and the northern Carrizo Plain. At least 26 1-mile sections lines spanned the SAF in the area between present-day California Highways 46 and 58. Each section line was marked by monuments at the midpoint and endpoints.

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

The 2008 U.S. Geological Survey (USGS) National Seismic Hazard Maps display earthquake ground motions for various probability levels across the United States and are applied in seismic provisions of building codes, insurance rate structures, risk assessments, and other public policy.