Mexico City Earthquake of 11 May 2023 (Mw3.2)

On 11 May 2023 a local earthquake in Mexico City was felt very strongly in Mixcoac, San Angel, and Coyoacán. The event was part of a seismic sequence that had begun about 6 months earlier. Peak Ground Acceleration (PGA) at the closest station (distance ~ 1 km), located in the hill zone, was ~ 0.18 g. Although the response spectrum at short periods at this station exceeded the design spectrum specified in the Mexico City´s Building Code, no structural damage was reported. Moment tensor inversion of bandpass filtered (0.08 – 0.24 Hz) displacement records yields M0 = 6.8x1013 N-m (Mw 3.2), H = 0.7 km, and the likely fault plane characterized by φ = 2700, δ = 760,On 11 May 2023 a local earthquake in Mexico City was felt very strongly in Mixcoac, San Angel, and Coyoacán. The event was part of a seismic sequence that had begun about 6 months earlier. Peak Ground Acceleration (PGA) at the closest station (distance ~ 1 km), located in the hill zone, was ~ 0.18 g. Although the response spectrum at short periods at this station exceeded the design spectrum specified in the Mexico City´s Building Code, no structural damage was reported. Moment tensor inversion of bandpass filtered (0.08 – 0.24 Hz) displacement records yields M0 = 6.8×1013 N-m (Mw 3.2), H = 0.7 km, and the likely fault plane characterized by φ = 2700, δ = 760, λ = -750. These source characteristics are very similar to those estimated for the 17 July 2019 earthquake which occurred during a swarm-like seismic activity about 5 km to the north. Spectral analysis of recordings at 19 sites in the hill zone, 14 in the transition zone, and 41 in the lake-bed zone reveals great variability of the ground motion within each of the zones. Estimated stress drop, Δσ, is 0.5 MPa. A large disparity is found between the observed source spectrum and theoretical source spectrum; their ratio provides an estimation of the amplification of seismic waves as they travel through the layers of decreasing velocity at shallower depth. We denote this ratio as the site effect. Predicted PGA and PGV for an Mw 3.2 earthquake, computed using stochastic technique (Boore 1983, 2003), assuming a Brune ω-2 source, Δσ = 0.5 MPa and including the site effect, are in reasonable agreement with the observations. Expected PGA and PGV at the epicenter of a postulated Mw 5 earthquake are 0.6 g and 60 cm/s at a generic hill-zone site; the expected values are twice as large in the lake-bed zone. These predictions should, however, be taken with caution as they are based on several approximations.