Felt intensity versus instrumental ground motion: A difference between California and Eastern North America?


We examine the question of whether a single ground-motion intensity correlation equation (GMICE) is applicable to both eastern North America (ENA; intraplate) and California (interplate). We initially address this issue with the datasets from previous studies and separate them into central United States (CUS), California, and Canada datasets. We then add data from the 2008 M 5.2 Mt. Carmel, Illinois, and the 2005 M 4.7 Riviere du Loup earthquakes to the CUS and Canada datasets, respectively. For each dataset, the median of ground-motion values at each modified Mercalli intensity (MMI) level and their 95% confidence limits are calculated for peak ground velocity (PGV), peak ground acceleration (PGA), and spectral acceleration (SA) at 0.3, 1.0, and 2.0 s. The California median value is relatively higher than the CUS and Canada for PGV and SA at 1.0 and 2.0 s. Based on the median values and the associated uncertainty, we combine all datasets for PGA and 0.3-s SA in order to determine a GMICE. The CUS and Canada datasets are combined separately from California for PGV, 1.0-s SA, and 2.0-s SA. ENA earthquakes have a different spectral shape than those in California, particularly at intermediate periods (~1:0 s) where ENA spectral amplitudes are reduced, which may explain the difference we see at longer periods. Finally, a log-linear-fit of MMI to the median values of ground motion is used to determine GMICE.

Publication Title

Bulletin of the Seismological Society of America