M 7.0 - Acapulco, Mexico

Tectonic Summary

The September 8, 2021 M7.0 (September 7 locally) earthquake near Acapulco in Guerrero, Mexico, occurred as a result of shallow thrust faulting on or near the plate boundary between the Cocos and North America plates. The depth and focal mechanism solutions of the event are consistent with the earthquake occurring on the subduction zone interface (approximately 20 km deep) between these plates. The earthquake occurred approximately 60 km northeast of the Middle America Trench where the Cocos plate begins its descent into the mantle beneath Mexico. In the region of this earthquake, the Cocos plate moves approximately northeastward at a rate of 65 mm/yr.

While commonly plotted as points on maps, earthquakes the size of the September 8, 2021 event are more appropriately described as slip over a larger fault area. Earthquakes of this size are typically about 40x20 km (length x width).

Historically, several significant earthquakes have occurred along the southern coast of Mexico. In the past 100 years, 17 earthquakes of M7 or larger have occurred within 250 km of the September 8, 2021 earthquake. An M7.0 earthquake occurred on May 11, 1962 in approximately the same location as the September 8, 2021 earthquake. The 1962 earthquake caused four fatalities, extensive regional infrastructure damage, and a local tsunami with a peak recorded amplitude of 2.8 meters. An M7.6 earthquake occurred about 75 km away on July 28, 1957. The September 2021 earthquake occurred at the southeastern end of the Guerrero Gap, a segment of the Middle America subduction zone that is thought to be capable of producing M8 or greater earthquakes. Seismic gaps are regions of faults or plate boundaries that exhibit relative seismic quiescence where previous large earthquakes are known or thought to have occurred. The Guerrero Gap extends ~230 km northwestward from Acapulco along the south Mexico coast. The last large (>M7) earthquake know to have occurred in the Guerrero Gap was in 1911; however, several notable yet smaller earthquakes (M6.1-6.7) have occurred within the region defined by the seismic gap.