Surface Wave Imaging Of Shallow Refraction Data In Mexico City Basin

A good knowledge of the elastic properties of shallow stratigraphy is fundamental to understand<br>and simulate the strong ground motion characteristics of recent earthquakes recorded by the<br>Mexico City accelerometric array. Accordingly, we are interested in determining shear (S) wave<br>velocities at some sites to correlate them with those of inhabited zones of the city. Despite<br>population density and paved surfaces, it is feasible to gather shallow refraction data and obtain<br>laterally homogeneous subsoil structures at some locations. We recorded Rayleigh and Love<br>surface waves using explosive and sledgehammer sources and low frequency vertical and<br>horizontal geophones, respectively. We focused our analysis in the Texcoco Lake region,<br>northeastern Mexico City basin. This area consists of unconsolidated clay sediments similar to the<br>ones of the lake bed zone in Mexico City, where ground motion amplifications and long durations<br>are commonly observed. We obtained phase velocity dispersion curves from ray parameterfrequency<br>domain analyses and inverted them for vertical distribution of S wave velocity. The<br>initial model was provided by standard first break refraction analyses. We obtained a very good<br>agreement between theoretical and observed dispersion curves for a two-layer model with<br>thicknesses of 18 and 46 m and S-wave velocities of 35 and 85 m/s, respectively, underlain by a<br>halfspace with S-wave of 200 m/s. Results compare well with tilt and cone resistance<br>measurements for the same area, emphasizing the importance of this kind of studies for<br>engineering purposes.