Specifications include, but are not limited to: B2.1 The seismic ground shaking hazard shall be characterized using an acceleration response spectrum determined in accordance with the site-specific procedure of Article 3.4.3 Guide Specification. The sitespecific procedure shall consist of a site-specific hazard analysis, a site-specific ground motion response analysis, or both. A site-specific hazard analysis consists of either a deterministic seismic hazard analysis (DSHA) or a probabilistic seismic hazard analysis (PSHA). A DSHA involves evaluating the seismic hazard at a site for an earthquake of a specific magnitude occurring at a specific location, considering the attenuation of the ground motions with distance. The DSHA is usually conducted without regard for the likelihood of occurrence. The product of the DSHA is an estimate of ground motion parameters at a site for each potential source. The PSHA involves evaluation of the probability of seismic shaking considering all possible sources. The USGS conducted a PSHA in the development of the USGS/AASHTO Seismic Hazard Maps. A PSHA consists of completing numerous deterministic seismic hazard analyses for all feasible combinations of earthquake magnitude, source-to-site distance, and seismic activity for each earthquake source zone located in the vicinity of the site. The result of a PSHA is a relationship of the mean annual rate of exceedance of the ground motion parameter of interest with each potential seismic source considered. See Kramer (1996) for further discussions of the types and methods used to conduct DSHAs and PSHAs. A site-specific ground response analysis is used to determine the influence of local ground conditions on the design ground motions. The analysis is generally based on the assumption of a vertically propagating shear wave though more complex analyses can be conducted if warranted. A site-specific ground motion response analysis is typically used to evaluate the influence of “non-standard” soil profiles on ground response to the seismic hazard level. Site-specific ground motion response analyses may also be used to assess the effects of pore-water pressure buildup on ground response, vertical motions resulting from compression wave propagation, laterally non-uniform soil conditions, incoherence, and the spatial variation of ground motions. If the probabilistic seismic hazard analysis (PSHA) is used, the site-specific analysis shall be conducted in a manner to generate a uniform-hazard acceleration response spectrum considering a seven percent probability of exceedance in 75 yr for spectral values over the entire period range of interest. This analysis shall establish the following: The contributing seismic sources, An upper-bound earthquake magnitude for each source zone, Median attenuation relations for acceleration response spectral values and their associated standard deviations, A magnitude-recurrence relation for each source zone, and A magnitude fault-rupture length or source area relation for each contributing fault or source area. If the deterministic seismic hazard analysis (DSHA) method is used, the analysis shall establish all the items listed above, except the magnitude-recurrence relation for each seismic source. The site-specific deterministic spectrum at the ground surface, adjusted by the site coefficients in Article 3.4.2.3, shall be no less than the seven percent probability of exceedance in 75 yr response spectrum determined using the general procedure in Articles 3.4.1 and 3.4.2 in the region of 0.5TF to 2TF of the spectrum, where TF is the bridge fundamental period. The same would also apply to the free field ground acceleration As. Where use of a deterministic spectrum is appropriate, the spectrum shall be either: The envelope of median spectra calculated for characteristic maximum magnitude earthquakes on known active faults, or The deterministic spectra for each fault and, in the absence of a clearly controlling spectrum, each spectrum should be used. Uncertainties in source modeling and parameter values shall be taken into consideration in the PSHA and DSHA. Detailed documentation of seismic hazard analysis shall be provided and shall be peer reviewed as appropriate (Article C3.4.1). For sites located within 6 mi of an active surface or shallow fault, as depicted in the USGS Active Fault Map, near-fault effects on ground motions should be considered to determine if these could significantly influence the bridge response.
