An empirical model for estimating the horizontal pseudo-absolute spectral accelerations (PSA) generated by shallow crustal earthquakes was published in 2008 using the recorded earthquake ground motion data collected and documented as part of the original Next Generation Attenuation (NGA) project. A significant number of additional recordings were collected over the past three years, and the 2008 model has been revised using the new data and is presented in this paper. The model was again selected to be simple, and the model parameters were estimated using the expanded database. The revised model incorporates V S30 as an independent variable because, with the expanded database, it was found that V S30 was required to be included as an independent parameter to allow for a reasonably unbiased fit to the recorded data. It is noted that V S30 is not being used to account for nonlinear site response, but strictly to allow for a better fit to the data. These parameters are presented for sites with an average shear wave velocity in the upper 30 m, V S30, for sites with V S30 ≥ 450 m/s. Parameters for sites with V S30 < 450 m/s are not included in this paper. For a site with V S30 = 450 m/s, there is an overall increase in PGA averaging about 50% over a distance of about 100 km using the 2013 model in comparison to the 2008 model. On the other hand, for a site with V S30 = 900 m/s, there is an overall decrease of about 10% using the 2013 model in comparison to the 2008 model.
Application of the microtremor measurements to a site effect study
