RAPID/Collaborative Research: Dynamic Site Characterization Following Mw 7.1 Puebla Earthquake for Development of a Refined 3D Shallow Crust Velocity Model of the Mexico City Basin

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The 2017 Mw 7.1 Puebla-Mexico City Earthquake caused numerous building collapses, ground failures, and over 300 casualties. Documented field evidence revealed damage distribution patterns that have been recognized in published studies of previous earthquakes in the region, specifically that the complex regional and local geology, hydrology and geotechnical conditions play a decisive role in shaping the ground shaking characteristics of the clay sediments in Mexico City. While ground motion recordings showed clear evidence of one-dimensional (1D) site amplification in the lake zone of the basin, the uneven damage distribution and ground motion variability in areas of reportedly similar deposit depths and structural characteristics in the transition zone suggest that three-dimensional (3D) site effects played a significant role in the observed damage. The documented evidence of the role of site effects in Mexico City combined with the high density instrumentation and decades of site characterization research provide an excellent opportunity for engineers and earth scientists to study in real scale the coupling of 3D basin effects and 1D local (shallow) site response, and the interaction between hydrological conditions and seismic amplification. Advances in our understanding of these phenomena can be used to develop better prediction models for seismically active regions, including U.S. metropolitan areas such as Los Angeles, San Francisco and Seattle. The overarching goal of this Grant for Rapid Response Research (RAPID) project is to combine instrumentation and monitoring, ata collection and analysis, and numerical modeling to better characterize regional basin and local site effects during earthquakes. The rapid response framework maximizes the project resources by aligning its goal with ongoing and upcoming activities of Mexican researchers and engineers to refine the geological and geotechnical site characterization of the basin, and map the evolution of dynamic geotechnical properties with time to the rapidly changing hydrologic conditions in the area. This award addresses the NSF mission to promote the progress of science and to advance the national health, prosperity, and welfare. On a technical level, this project focuses on conducting dynamic site characterization of strong motion stations and other key places in Mexico City using active source and ambient wavefield surface wave methods along with horizontal-to-vertical spectral ratios. The goal of conducting these measurements is to develop seismic shear wave velocity profiles and estimate site periods that can lead to improved microzonation maps for the city and regional hazard assessment. Combining field measurements with the spatial variability of shallow deposits and geological information of the deeper sediments from Mexican research databases, a 3D shallow crust velocity model of Mexico City will be developed. This model is essential for physics based ground motion simulations, given the known influence that small-scale lateral heterogeneities in very soft materials have on ground motions, especially in cases of complex resonance interaction between the deeper geologic formations, the shallow sediments, and the structural vibration characteristics of buildings. A better understanding of these phenomena contributes by extension to improved seismic hazard estimates not only for Mexico City, but also for U.S. cities that lie on similar sedimentary basins.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.