An intraslab subduction zone earthquake of moment magnitude 7.1 occurred on September 19, 2017 approximately 60 km southwest of Puebla, Mexico, and 120 km southeast of Mexico City, Mexico.
This earthquake occurred at a depth of 57 km as a normal faulting mechanism near the point of maximum curvature of the Cocos plate, which is being subducted beneath the North American plate. The event was recorded by over 80 strong ground motion instruments located in Mexico, and produced strong ground motions that exceeded an intensity level VII in Mexico City and Puebla according to the Modified Mercalli Index (MMI). Ground motions recorded on soft rock showed a much higher frequency content in this event than was recorded previously in the 1985 Michoacan earthquake that ravaged Mexico City. The rock ground motions appeared to resonate in transition zone soils (Zone II) and lake zone soils (Zone IIIb) in the western half of Mexico City, causing large horizontal spectral accelerations at periods between 0.8 and 1.5 seconds and resulting in significant damage to many structures between five to eight stories in height. The event led to the collapse of more than 40 buildings in Mexico City, and thousands of collapsed dwellings in the southern states of Puebla and Morelos. As would be expected, unreinforced masonry and adobe structures did not perform well in this earthquake, particularly in regions nearest to the epicenter.
Immediately following the September 19 event, a joint geotechnical engineering reconnaissance effort was organized between the Universidad Nacional Autónoma de México (UNAM) and the U.S. National Science Foundation (NSF)-sponsored Geotechnical Extreme Events Reconnaissance Association (GEER). Two UNAM-GEER teams of researchers were sent to the region to investigate and document the effects from the earthquake: an advance team (from September 24 to September 30) and a main team (from September 29 to October 6).
The GEER report from this event is available hele.
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