Valley of Mexico site response from non‐reference site approach

We present a model for Valley of Mexico site response that is referenced to a 30 m time‐averaged shear wave velocity ( V S 30 ) of 760 m/s. This reference condition ensures compatibility with regional ground motion models (GMM) for Central America and Mexico (CAM), which were developed from an expanded version of the NGA‐Subduction (NGA‐Sub) database as described in a companion paper. The site response model is derived using ground motion data from 89 sites within the Valley of Mexico and uses V S 30 and soft soil depth as independent variables, the former of which was evaluated using shear wave velocity from literature for 56 sites (10 co‐located with ground motion stations). As a consequence of the unique Lake Texcoco geology, the model predicts far greater levels of site response than those provided by global ergodic models, and notably, larger site response than prior models for the Valley of Mexico that had utilized a reference site approach using a medium‐stiff soil site as the reference site. Linear amplification increases with decreasing V S 30 in a period‐dependent manner that captures strong resonance effects associated with variable depths of soft lacustrine soils. Site period is found to not appreciably improve predictions, due to strong negative correlation with V S 30 . Differential depth (depth minus V S 30 ‐conditioned mean depth) produces repeatable trends of reduced short period amplification and increased long period amplification for especially deep sites, the effects of which are modeled. We identify nonlinear site response for high‐frequency ground motions using variable‐amplitude reference site ground motions. The provided models improve upon current practice for Mexico because the reference GMM applies across CAM, V S 30 and soft soil depth are used in lieu of site classes, and nonlinearity is incorporated.