Analysis of Seismic Intensity for Inslab and Interplate Earthquakes in Central Mexico

Abstract

This study evaluates the performance of two Ground Motion Prediction Equations (GMPEs) for inslab and interplate earthquakes when applied to the Trans-Mexican Volcanic Belt (TMVB) in central Mexico, addressing the lack of validation of forearc-calibrated models in this backarc region. A retrospective analysis was conducted using instrumental records from six moderate-to-large earthquakes (MW 6.0-7.7) that occurred between 2011 and 2022, independent of the original GMPEs calibrations. Records were obtained from permanent seismological networks. For each event, observed Peak Ground Acceleration (PGAobs) was computed as the quadratic mean of the two horizontal components and compared with the PGA predicted by the corresponding GMPE (PGApred). Spatial contour maps of the PGAobs/PGApred ratio were generated using minimum-curvature interpolation. Stations with critical underestimation were analyzed using the Horizontal-to-Vertical Spectral Ratio (HVSR) technique on earthquake records. The results show that, for interplate earthquakes, the corresponding GMPE systematically underestimates PGA with a median PGAobs/PGApred ratio of 1.73 (95% bootstrap confidence interval: 1.26–2.20; based on 145 records), whereas for inslab events the GMPE overestimates PGA with a median ratio of 0.69 (95% CI: 0.55–0.84; 160 records). Extreme local ratios occur at a few stations, where HVSR analyses reveal amplification factors of 4-6 at frequencies >10 Hz, which contribute to but do not fully explain the observed discrepancies. These findings underscore the need for caution when applying forearc-calibrated GMPEs in the TMVB and have direct implications for seismic hazard assessment and engineering practice in central Mexico.