A model for estimating average horizontal values of pseudo-absolute spectral accelerations generated by crustal earthquakes is developed as part of this study. The NGA-West2 Project significantly expanded the data base of motions recorded during earthquakes and offered the opportunity to re-examine and update the NGA relationships published by the NGA developers in 2008. The new data set comprised 21,539 recordings obtained during earthquakes with magnitudes ranging from 3 to 7.9, recorded at distance ranging from 0.2 km to well over 300 km, and for recording stations with Vs30 ranging from 100 to 2000 m/sec. These data spanned the large magnitude range (M = 4.5 to 7.9) and the small magnitude range.
This study concentrated on the use of the large magnitude free field data set recorded at distances less than 175 km; this data set consists of 7120 recordings recorded at sites with Vs30 ranging from 100 to 2000 m/sec. These were further segregated into three Vs30 bins. One bin was for data recorded at sites with Vs30 = 100 to 211 m/sec, which constitute "soft soil sites." The second bin comprised the data recorded at sites with 211 m/sec < Vs30 < 450 m/sec, which exhibit moderate to strong nonlinear characteristics, especially for sites with Vs30 less than about 300 m/sec (~1000 fps).
The third bin was for data recorded at sites with Vs30 = 450 to 2000 m/sec. These sites may be reasonably designated as "quasi-linear site s" because they appear to show very weak nonlinearity, especially for sites with Vs30 exceeding about 600 m/sec (~2000 fps). The model presented in this report covers only sites with this range of Vs30.
The use of this model should be limited to M ≥ 5, to distances less than about 150 km, and to Vs30 ≥ 450 m/sec. For sites with Vs30 > 1200 m/sec, the PSA values calculated using Vs30 = 1200 m/sec are used.
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