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M 7.1 Earthquake Strikes Central Mexico

20 Sep 2017

This maps shows the distribution of different types of earthquakes along the Middle America Trench since 1970. Black stars show the locations of the recent M 8.1 and M 7.1 earthquakes. The yellow rectangle marks the approximate extent of rupture of the M 8.1 event (Data Source: The Global Centroid-Moment-Tensor Project and U.S. Geological Survey)

Another major earthquake struck central Mexico near Mexico City on 20 September 2017 at approximately 2:14 am (Singapore time), just 12 days after the last one. According to the United States Geological Survey (USGS) and the Servicio Sismológico Nacional, Mexico (SSN), this magnitude-7.1 quake occurred at a depth of between 51 and 57 kilometres (km), where the North American Plate moves over the subducting Cocos Plate.  

Based on the preliminary information provided by the USGS and SSN, this M 7.1 quake is likely a normal-faulting event occurring within the subducted slab of the Cocos plate, similar to the M 8.1 intra-slab quake that shook southern Mexico just 12 days before. However, unlike the M 8.1 event that occurred offshore southern Mexico, this M 7.1earthquake occurred inland due to the gentle sloping of the subducting slab beneath central Mexico.  

Although today’s earthquake was not a shallow crustal event – the kind that usually causes major damage – the quake’s relative proximity to Mexico City and the amplification effect from soft sediments deep within the Mexico City Basin caused considerable ground motion. The severe ground shaking, (rated up to VII on the Modified Mercalli Intensity Scale) damaged many buildings in Mexico’s densely populated capital city, located more than 100 km from the earthquake’s hypocentre.

Although intra-slab earthquakes are usually smaller and deeper than megathrust earthquakes – which occur in the shallow portion of the subduction zone interface – they are still a major seismic hazard source. 

Moderate to strong intra-slab earthquakes can cause damage to densely populated cities located several hundred km from megathrust faults in countries like Mexico, Japan, Myanmar, and Indonesia. While these quakes usually originate from deep (more than 50 km) within the ground, they are still capable of producing a moderate degree of surface ground motion, which can damage low-quality buildings and put people at risk