Appendix I

Rate and distribution of seismic activity are important indicators of the overall state of tectonic stress within a region. In regions characterized by low levels of seismicity, active fault surfaces are rarely visible at the surface, and the analysis of small-magnitude earthquakes at depth may be the most effective way to identify seismic hazard and risk from ambient tectonic activity.

This draft technical memorandum has been prepared to summarize hydrogeologic data and provide an assessment of hydrogeologic conditions and water uses in the vicinity of the Whitford, Silver King, and Happy Camp sites (Near West area) for the RCM Tailings Prefeasibility Study (PFS).

It is important to determine variability in time between earthquakes to constrain uncertainty in probabilistic calculations of rupture potential. Results from our field work since 2005 at the Bidart site in the Carrizo Plain and new radiocarbon dates from archival samples collected for Grant’s 1993 dissertation reveal evidence of six ruptures of the San Andreas fault (SAF) between 1345 and 1857 AD.

Recommendations to the U.S. Geological Survey National Seismic Hazard Mapping Program for the 2014 Update of the National Seismic Hazard Maps

The Frazier Mountain paleoseismic site is located on a poorly understood section of the southern San Andreas fault, mid-way between the well-known Carrizo Plain and Mojave sites of Bidart Fan and Pallett Creek. Emerging paleoseismic evidence indicates that earthquakes along this stretch repeat at a similar pace, with an average interval of - 122 years between AD. 1000 and 1857.

Lake Roberts Dam is located in Grant County in southwestern New Mexico. Tectonically the damsite is within the Southern Basin and Range Province and the Rio Grande rift as defined by Machette (1998). Although the historical seismicity in the region has been low, the site has undoubtedly been shaken by past large prehistoric earthquakes caused by active regional faults and in historical times, as recently as 1887 (Figures 2 to 4).

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The commonly used Uniform Hazard Spectrum (UHS) is shown here to be an unsuitable target for this purpose, as it conservatively implies that large-amplitude spectral values will occur at all periods within a single ground motion. An alternative, termed a Conditional Mean Spectrum (CMS), is presented here.

Paleoseismological data constrain the age, location, and associated magnitude of past surface-rupturing earthquakes; these are critical parameters for developing and testing fault behavior models and characterizing seismic hazard. We present new earthquake evidence and radiocarbon analyses that refine the chronology of the six most recent earthquakes that ruptured the south-central San Andreas fault in the Carrizo Plain (California, United States) at the Bidart Fan site.

Probabilistic seismic hazard analysis (PSHA) has become a fundamental tool is assessing seismic hazards and for estimating seismic design and seismic safety evaluation of ground motions. It is used on a site-specific basis for important and critical facilities and on a national scale for building codes. This report describes a project to test and verify the numerical approaches and software used in PSHA.