Ground Motion in Mexico City During the Intraslab Earthquake of 19 September 2017 (Mw 7.1) Revisited

The intraslab earthquake of 2017 (M(w)7.1) was one of the most destructive earthquakes in the history of Mexico City. Several measures of the ground motion reveal that the 2017 event was unusually energetic at CU, the reference hill-zone site in the city, and, hence, in the entire Valley of Mexico, in the critical frequency range of 0.4 to 1 Hz, e.g., Fourier acceleration spectrum (FAS), peak ground velocity (PGV), and pseudoacceleration response spectrum (Sa), 5% damping, at structural periods of 1 <= T <= 1.8 s (0.55 <= f <= 1 Hz). However, the cause of the large ground motion at CU remains unresolved. The issue merits a careful analysis of all available data. On 7 December 2023, an intraslab M-w 5.8 earthquake occurred in proximity to the 2017 event. We analyze the recordings of the 2017 earthquake separately and of the 2017 and 2023 events together in an attempt to isolate the cause of the anomalous high-frequency radiation. In this context, we take recourse of the 2023 recordings as empirical Green's functions (EGFs). Synthesized Sa for a M(w)7.1 earthquake, using 2023 (M-w 5.8) recordings as EGFs and assuming the same stress drop, Delta sigma, of 3 MPa for both events, are significantly lower than those observed, irrespective of the azimuth. We find that the source was unusually energetic at all azimuths, and the role of rupture directivity in enhancing the ground motion was relatively small. The possibility that the enhanced ground motion in the city in 2017 was due to a particular direction of the incident wavefield on the 3-D structure of the Valley of Mexico may be ruled out since the recordings of the 2023 event, which was nearly collocated with the 2017 earthquake, show nothing anomalous. The simulated Sa using recordings of the 2023 event as EGFs suggest that a postulated intraslab M-w 7.1 earthquake, with source characteristics similar to the 2023 event, at a distance of similar to 130 km from CU, should cause little or no damage in the city. In other words, if the source of the 2017 earthquake had been a scaled-up version of the 2023 event then, quite likely, Mexico City would have been spared the damage and deaths that it suffered.