The Mw 7.1 Puebla, Mexico Earthquake of 19 September 2017 – the anniversary of the Mw 8.0 Michoacan earthquake of 1985.

By Paul Somerville, Risk Frontiers

As reported by the USGS, the September 19, 2017, Mw 7.1 Puebla earthquake in Central Mexico occurred as the result of faulting within the subducted Cocos plate at a depth of approximately 50 km and about 120 km southeast of Mexico City. At least 220 people were killed at Mexico City, 74 in Morelos, 45 in Puebla, 13 in Estado de Mexico, 6 in Guerrero and 4 in Oaxaca. At least 6,000 people were injured. At least 44 buildings collapsed and many others were damaged at Mexico City. Many other buildings were damaged or destroyed in the surrounding area. Significant damage occurred to the electrical grid in Estado de Mexico, Guerrero, Mexico City, Morelos, Oaxaca, Puebla and Tlaxcala.

This earthquake occurred on the anniversary of the devastating Mw 8.0 Michoacan earthquake of 19 September 1985, which caused extensive damage to Mexico City and the surrounding region. That event occurred as the result of thrust faulting on the plate interface between the Cocos and North America plates, about 450 km to the west of the September 19, 2017 earthquake.

Most of Mexico City is founded on a clay-filled lake. The clay has a resonant period of 1 to 2 seconds and has very unusual properties – it is very elastic (has low damping), which allows a very large resonance to build up due to the trapping of energy within this shallow sedimentary basin (Figures 1 and 2).  This resonance caused the collapse of buildings, especially ones having natural periods of 1 to 2 seconds, and generated a seiche in Lake Chapultepec (part of the original lake that has not been filled in) seen in a widely viewed video, in which the waves have a period of about 2 seconds. https://www.youtube.com/watch?v=vfTaHOoC2rs

The 1 to 2 second resonance of the lakebed can also be set up by marching soldiers.  This occurred exactly 33 years earlier to the day, when I was on holiday in Mexico City.  It was September 19, Independence Day, and the soldiers were marching down Reforma Avenue.  I was standing on the roof of my ten story hotel, which was swaying noticeably.  One year to the day later, at 07:17 am on 19 September 1985, the Mw 8.0 Michoacan earthquake occurred.  I doubt that my hotel  survived the earthquake.

After the 1985 earthquake I spoke with my colleague, Lloyd Cluff, who had been at a meeting with Mexican government officials on the day of the earthquake to discuss seismic issues for nuclear power plants.  The meeting was held on the edge of Mexico City outside the lakebed area (blue area of Figure 1).  After he returned to his hotel that evening he turned on the TV and saw photos of a disastrous earthquake.  It took him some time to recognise the scene of the disaster as Mexico City.  No one at the meeting had known that it had occurred early that morning in Mexico City, because the shaking outside the lakebed area had been so weak.

Figure 1. Response spectral accelerations at 1 second period in Mexico City from the Mw 7.1 Puebla, Mexico earthquake of 9 September 2017. The largest ground motions, shown in the red and yellow colours, occurred in the parts of the city founded on the lakebed. Source: UNAM.
Figure 2. Locations of damaged buildings in Mexico City from the 2017 Puebla earthquake showing correlation with the western part of the ground motion map in Figure 1.