Deadly intraslab Mexico earthquake of 19 September 2017 (M (sub w) 7.1); ground motion and damage pattern in Mexico City

In the wake of death and destruction left by the 2017 earthquake in Mexico City, it is natural to ask whether the event was unexpected and anomalous. Although such an intraslab earthquake (M (sub w) 7.1; depth=57 km; epicentral distance = 114 km from the city) was considered likely, the recordings in the city during the last 54 yrs reveal that the ground motion during the 2017 earthquake was anomalously large in the critical frequency range to the city (0.4-1 Hz). The intraslab earthquakes occur closer to Mexico City, at greater depth, and involve higher stress drop than their interplate counterparts. Consequently, the ground motion is relatively enriched at high frequencies as compared with that during interplate earthquakes, which is dominated by lower frequency waves (f<0.5 Hz). This explains the observed difference in the damage pattern during the 2017 and the disastrous interplate earthquake of 1985 (M (sub w) 8.0).