A magnitude eight earthquake, often called a “great” quake, struck off the southern coast of Mexico last Friday afternoon.
Tuesday, 12th September 2017
A magnitude 8.1 undersea earthquake struck 87 km south-west of Pijijiapan, Mexico at 4:49 p.m. New Zealand time on Friday 8th September. The quake was 70 km deep.
The earthquake caused extensive damage and loss of life in the Oaxaca and Chiapas regions of Mexico, and damage in nearby Guatemala.
Preliminary analysis by the U.S. Geological Survey suggests that the earthquake occurred within the slab of the Cocos tectonic plate which is sliding underneath Central America in this area. Whilst it could have been a megathrust slip, it is thought that the quake resulted from a rupture of an area about 200 km by 50 km within the descending plate.
There have been about eight earthquakes of magnitude 7 or greater within 250 km of the earthquake hypocentre within the last 100 years, the largest of which was a magnitude 7.4 event off the coast of Guatemala in November 2012.
The earthquake generated a tsunami which was measured at 1.75 metres at Chiapas Mexico at 9:49 p.m. New Zealand time on Friday, 5 hours after the quake struck; the largest sea-level displacement recorded for this event. Waves peaked at 72 cm at Acapulco and 69 cm at Huatulco, Mexico, less than two hours after the earthquake. The timing of the Chiapas measurement may be erroneous due to power problems in the area.
The tsunami wave that was projected across the Pacific Ocean was much smaller, being 29 cm at Easter Island, 17 cm at Hawaii and 22 cm at Western Samoa.
GeoNet’s NZ tsunami gauges Satruday 9th September
[click for larger image] New Zealand’s tsunami gauges detected the first arrival of the tsunami at East Cape and Chatham Island at about 8 o’clock the following morning. GeoNet reports that waves of 15-25 cm were detected at some points on New Zealand’s mainland coasts, with peak waves of 35-40 cm recorded at Chatham Island.