Deadly Intraslab Mexico Earthquake of 19 September 2017 (Mw 7.1): Ground Motion and Damage Pattern in Mexico City

Abstract

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 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 w 8.0). Electronic Supplement: Figures showing spectral ratios, peak ground acceleration (PGA) and peak ground velocity (PGV) as function of distance R, comparison of observed response spectra SA, and predicted median and 1 s SA from a site-specific ground-motion prediction equation (GMPE) model at CU, plot of accelerograms, Fourier acceleration spectra, and SA at SCT of interplate 1985 M w 8.0 and intraslab 2017 M w 7.1 earthquakes, accelerographic stations in Mexico City, which recorded the 2017 M w 7.1 earthquake, SA of the 2017 earthquake at sites in and near Condesa and Roma colonies, and basis for the estimation of exceedance rate of PGA at CU in Mexico City from intraslab earthquakes, and tables providing a list of 20 (interplate and intraslab) earthquakes with largest recorded PGA at CU in the 1964-2017 period, significant pre-1975 intraslab earthquakes, and a comparison of observed PGA and PGV at CU during the 2017 M w 7.1 earthquake with predictions from GMPE.

The September 19th, 2017 Puebla, Mexico earthquake

“…Intraslab earthquakes, at close distances are likely to trigger different seismic features compared to earthquakes originating from the subduction zone. Singh et al (2018) found that intraslab earthquakes can lead to higher PGA than interplate earthquakes with similar magnitude [3]. This is confirmed by a comparison of the ground motion records from the 1985 and 2017 earthquakes which indicates that interplate and intraslab earthquakes trigger different response in the Mexico City soil.…”

“…Seismic demands in Mexico City depend mainly on soil conditions rather than being related of the size, depth, epicentral distance, and location of the source [3]. Figure 5 shows a map of Mexico City with response spectra for recording stations located in different zones (hill, transition zone, and soft soil), clearly indicating the influence of the site conditions.…”

“…The affected areas were localised in the metropolitan area of Mexico City (21.2 million people), Puebla (1.6 million people), and Morelos (2 million people). The death toll was 369, out of which 228 were in Mexico City [3]. In the urban area, 77 buildings partially or totally collapsed, and 8,000 buildings experienced different levels of damage ranging from severe structural damage requiring demolition to damage to infill wall only [4].…”

Nucleation and Kinematic Rupture of the 2017 Mw 8.2 Tehuantepec Earthquake

“…This earthquake is ranked as the strongest intraplate event ever recorded in Mexico with a moment magnitude of M 8.2, and the third largest intraplate event known globally (Wiens et al, 1998;Yue et al, 2012). Eleven days after the event, another intraslab earthquake ruptured near Mexico City (e.g., Melgar, Pérez-Campos, et al, 2018) causing widespread damage in Mexico's capital, where nearly~300 people died and~45 building collapsed due to basin amplification (Çelebi et al, 2018;Cruz-Atienza et al, 2016;Galvis et al, 2017;Sahakian et al, 2018;Singh et al, 2018). The Mexican earthquake early warning system delivered an alert to major cities in central Mexico within a few seconds after the earthquake, sirens in Mexico City went off 92 s before the arrival of the surface waves which arrived near midnight 7 September 2017 local time (Suárez et al, 2018).…”

“…The Mexican earthquake early warning system delivered an alert to major cities in central Mexico within a few seconds after the earthquake, sirens in Mexico City went off 92 s before the arrival of the surface waves which arrived near midnight 7 September 2017 local time (Suárez et al, 2018). Eleven days after the event, another intraslab earthquake ruptured near Mexico City (e.g., Melgar, Pérez-Campos, et al, 2018) causing widespread damage in Mexico's capital, where nearly~300 people died and~45 building collapsed due to basin amplification (Çelebi et al, 2018;Cruz-Atienza et al, 2016;Galvis et al, 2017;Sahakian et al, 2018;Singh et al, 2018). This event coincidentally occurred on the 32nd anniversary of the great 1985 M 8.1 Michoacan earthquake, only 2 hr after the annual national earthquake drill.…”