Technical Studies

Earthquake Ground Motions with 2% Probability of Exceedence in 50 Years

Boston

The table below provides detailed information on the records generated for Boston having probabilities of exceedence of 2% in 50 years. These acceleration time histories have been derived from historical recordings or from physical simulations and have been altered so that their mean response spectrum matches the 1997 NEHRP design spectrum, modified from soil type of S_B-S_C to soil type S_D and having a hazard specified by the 1997 USGS maps.

There are 20 different horizontal components (10 2-component records). Each original historical ground motion pair has typically been altered in the following way:

• The two horizontal components originally provided were resolved into fault-normal and fault-parallel orientations.
• The records are adjusted in the frequency domain to have characteristics appropriate for NEHRP S_O soil sites.
• The records are amplitude scaled so that the average of the two horizontal spectra match the target spectrum.
• The individual components were rotated 45 degrees away from the fault-normal fault-parallel orientations. This rotation reduces the large differences that exist between the fault-normal and fault-parallel components of near fault ground motions at periods longer than about 0.5 seconds. This rotation reduces the large differences that exist between the fault-normal and fault-parallel components of near fault ground motions at periods longer than about 0.5 seconds.

Additional details can be found from the main Ground Motions page. This page also provides a link to a draft report summarizing all of the SAC Phase 2 ground motion studies.

To see the acceleration time history of any of the ground motions listed in the table below, just click on the ground motion name.