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# 1615.2 Site-Specific Procedure for Determining Ground Motion Accelerations

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A site-specific study shall account for the regional seismicity and geology; the expected recurrence rates and maximum magnitudes of events on known faults and source zones; the location of the site with respect to these; near source effects if any and the characteristics of subsurface site conditions.

Where site-specific procedures are used as required or permitted by Section 1615, the maximum considered earthquake ground motion shall be taken as that motion represented by an acceleration response spectrum having a 2-percent probability of exceedance within a 50-year period. The maximum considered earthquake spectral response acceleration at any period,

*S*, shall be taken from the 2-percent probability of exceedance within a 50-year period spectrum._{aM}**Exception**: Where the spectral response ordinates at 0.2 second or 1 second for a 5-percent damped spectrum having a 2-percent probability of exceedance within a 50-year period exceed the corresponding ordinates of the deterministic limit of Section 1615.2.2, the maximum considered earthquake ground motion spectrum shall be taken as the lesser of the probabilistic maximum considered earthquake ground motion or the deterministic maximum considered earthquake ground motion spectrum of Section 16 15.2.3, but shall not be taken as less than the deterministic limit ground motion of Section 1615.2.2.

The deterministic limit for the maximum considered earthquake ground motion shall be the response spectrum determined in accordance with Figure 1615.2.2, where site coefficients,

*F*and_{a}*F*, are determined in accordance with Section 1615.1.2, with the value of the mapped short-period spectral response acceleration, SS, taken as 1.5g and the value of the mapped spectral response acceleration at 1 second, S_{v}_{1}, taken as 0.6g.The deterministic maximum considered earthquake ground motion response spectrum shall be calculated as 150 percent of the median spectral response accelerations,

*S*, at all periods resulting from a characteristic earthquake on any known active fault within the region._{aM}Where site-specific procedures are used to determine the maximum considered earthquake ground motion response spectrum, the design spectral response acceleration, Sa, at any period shall be determined from Equation 16-48:

and shall be greater than or equal to 80 percent of the design spectral response acceleration, Sa, determined by the general response spectrum in Section 1615.1.4.

*S*= 2/3

_{a}*S*

_{aM}**(Equation 16-48)**

and shall be greater than or equal to 80 percent of the design spectral response acceleration, Sa, determined by the general response spectrum in Section 1615.1.4.

Where the site-specific procedure is used to determine the design ground motion in accordance with Section 16 15.2.4, the parameter SDS shall be taken as the spectral acceleration,

*S*, obtained from the site-specific spectra at a period of 0.2 second, except that it shall not be taken as less than 90 percent of the peak spectral acceleration,_{a}*S*, at any period. The parameter_{a}*shall be taken as the greater of the spectral acceleration, Sa, at a period of 1 second or two times the spectral acceleration,Sa, at a period of 2 seconds. The parameters*_{SDI}_{SMS}^{and}*shall be taken as 1.5 times*_{SMI}*and*_{SDS}*S*, respectively. The values so obtained shall not be taken as less than 80 percent of the values obtained from the general procedures of Section 1615.1._{DI}