Day 5. Early this morning our team spent some time planning our work for the next week and reviewing maps of the city’s infrastructure. The cooperation and collaboration was astounding. Seven engineers worked furiously to collect specific information and then brought it all together at the end of the work session. Amazing teamwork!
Once the maps and plans were assembled, we set out on foot to investigate building performance in the Reforma area of Mexico City. This very dense urban area was hit particularly hard by the earthquake because it is situated near the edge of Lake Texcoco’s ancient lake bed. The soft lake sediments that underlie most of Mexico City typically amplify earthquake shaking and damage. See the Map of Mexico City to the right.
Buildings in Mexico City constructed before 1985 represent a unique research opportunity. Many of these buildings were seismically retrofitted after the devastating M8.0 Mexico City Earthquake that occurred exactly 32 years to the day before last week’s M7.1 Central Mexico Earthquake (September 19, 2017). As David Gonzalez mentioned in his blog post on Day 1, the 2017 earthquake happened shortly after residents participated in an earthquake drill on the anniversary of the deadly 1985 earthquake. Many of the 1985 earthquake building seismic retrofits are unique structural systems consisting of braces, frames, or even diagonal cables applied like an “exoskeleton” on the exterior of these older buildings. This recent earthquake is a test of the suitability and earthquake performance of these seismic retrofit systems, as well as the design standards on which they were based. These buildings are real-world case studies that allow the engineering community to evaluate the seismic performance of these unique seismic retrofit design approaches.
However, it is important to note several key differences in duration and the type of shaking between this recent 2017 M7.1 and the 1985 M8.0 earthquakes. The 1985 M8.0 earthquake was a subduction zone type of earthquake (similar to expected earthquake types on the Cascadia Subduction Zone in the Pacific Northwest), with powerful strong ground shaking that lasted for several minutes! Whereas the 2017 M7.1 earthquake was an intraplate deep earthquake (similar to the 2001 Nisqually Earthquake in the Pacific Northwest) with less powerful shaking and a duration of less than 30 seconds. So, considering that long duration subduction zone earthquakes are still likely in this part of Mexico, this recent earthquake was a good test for these seismically retrofitted buildings, but not the ultimate test relative to the code-level earthquake ground motions that the retrofits were designed to resist. Therefore, it’s important for the engineering community to remind the public, building owners, and policy makers of this so that they don’t get a false sense of security by thinking these retrofitted buildings have now been through the “big one,” because they have not. In general, the unique “exoskeleton” seismic retrofit design approaches that we observed appeared to have performed well. It is especially inspiring to see the creativity that our fellow Mexican engineers and architectural colleagues utilized in these innovative designs.
Tomorrow we are heading toward the epicenter near Puebla, Mexico, where the ground motions were even more powerful than here in Mexico City.
Stay tuned for daily updates from our team… and if you have questions about our efforts, please comment below and we will respond with an answer.