1787 New Spain earthquake
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| Date | 28 March 1787 |
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| Magnitude | 8.6 Mw |
| Epicenter | 16°30′N 98°30′W / 16.5°N 98.5°WCoordinates: 16°30′N 98°30′W / 16.5°N 98.5°W |
| Type | Thrust |
| Areas affected | New Spain |
| Total damage | Severe |
| Tsunami | Yes |
| Casualties | 11 killed in the tsunami, total unknown |
The 1787 New Spain earthquake, also known as the San Sixto earthquake, occurred on 28 March at 11:30 local time (17:30 UTC). It caused a large tsunami that affected the coast of the Puebla Intendancy (includes the current state of Guerrero) and the Oaxaca Intendancy (currently Oaxaca) in Southwestern Mexico. With an estimated magnitude of 8.6 on the moment magnitude scale it was more powerful than any instrumentally recorded Mexican earthquake.
Southwestern Mexico lies above the convergent boundary where the Cocos Plate is being subducted below the North American Plate at a rate of 6.4 cm/yr. The dip of the subducting slab is about 15° as defined by focal mechanisms and earthquake hypocenters. Seismicity in this area is characterised by regular megathrust earthquakes along the plate interface.
The earthquake was reported as lasting between 6 and 7 minutes. It was followed by three strong aftershocks on 29 March, 30 March, and 3 April, all of which were probably of magnitude 7 or more, based on estimates of the reported intensities in the city of Oaxaca. It was felt over a wide area from Valladolid to Tehuantepec along the coast and as far inland as Tulancingo. The magnitude of this earthquake has been estimated from contemporary reports of seismic intensity. Analysis of other subduction interface earthquakes along this margin suggest that the extent of shaking of VIII or greater on the Mercalli intensity scale matches well with the rupture extent for each event. This suggests that the 1787 rupture extended for about 450 km along the coast, providing an estimate of magnitude 8.6, based on scaling against other similar earthquakes.
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