Appendix I

California’s 35 million people live among some of the most active earthquake faults in the United States. Public safety demands credible assessments of the earthquake hazard to maintain appropriate building codes for safe construction and earthquake insurance for loss protection. Seismic hazard analysis begins with an earthquake rupture forecast—a model of probabilities that earthquakes of specified magnitudes, locations, and faulting types will occur during a specified time interval.

We combine geodetic, geologic, and seismic information to estimate frequencies of damaging earthquakes in three types of seismotectonic zones.

Because of increased public interest and concern about expected losses from future earthquakes in California, the National Earthquake Prediction Evaluation Council recommended that the probability of occurrence of large (magnitude 7 or greater) earthquakes in California be evaluated. In response to this recommendation, the U.S. Geological Survey formed the Working Group on California Earthquake Probabilities.

Data describing the locations, slip rates, and lengths of Quaternary faults are the primary basis in this work for constructing maps that characterize seismic hazard in California. The expected seismic moment Me0 and the strength of ground shaking resulting from the entire rupture of each mapped fault (or fault segment) are estimated using empirical relations between seismic moment M0, rupture length, source to site distance, and strong ground motions.

Source parameters for historical earthquakes worldwide are compiled to develop a series of empirical relationships among moment magnitude (M), surface rupture length, subsurface rupture length, downdip rupture width, rupture area, and maximum and average displacement per event. The resulting data base is a significant update of previous compilations and includes the additional source parameters of seismic moment, moment magnitude, subsurface rupture length, downdip rupture width, and average surface displacement.

Recent requirements of seismic risk estimation have led to a re-evaluation of historical earthquake records and statistical methods in many countries, with a view to optimizing the use of the available information. Whatever approach is chosen to quantify risk, the basic information is earthquake catalogs from which a recurrence relation is derived. Its most widely used form is still the Gutenberg-Richter loglinear relation, log N = a - bm, perhaps with some modification at larger magnitudes.

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).

This report presents the results of probabilistic seismic hazard analyses for use in screening/scoping-level dam safety and/or risk assessments of Granite Reef Diversion and Theodore Roosevelt Dams in southern Arizona. The purpose of these evaluations is to estimate the levels of ground motions, which will be exceeded at specified annual frequencies (or return periods), at the damsites.

The San Andreas fault zone has been a very significant source of major California earthquakes. From 1812 to 1906 it generated four major earthquakes of M ~7 or larger in two pairs on two major portions of the fault. A pair of major earthquakes occurred on the central to southern region, where the 1857 faulting overlapped the 1812 earthquake faulting. A pair of major earthquakes occurred on the northern region, where the 1906 faulting overlapped the 1838 earthquake faulting.

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.


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