Source mechanism of the tsunamis of 19 and 21 September 1985, in Mexico

Abstract

The major earthquake measuring 8.1 on the Richter scale which struck the west coast of Mexico on Thursday 19 September 1985, generated a small tsunami. A major aftershock on 21 September, with a magnitude of 7.5 also produced a small tsunami. Both tsunamis propagated across the Pacific and were recorded by several tide stations in Central America, Colombia, Ecuador, French Polynesia, Samoa, and Hawaii. No reports of damage were received from any of the stations, and only minor damage due to the first tsunami was reported from the source region.

A survey was made by the International Tsunami Information Center (ITIC) of the coastal area affected, from Manzanillo to Zihuatanejo. Tsunami runup measurements were taken and interviews with local residents in the coastal areas were conducted.

A source mechanism study of the tsunamis was undertaken using seismic and geologic data and empirical relationships. Earthquake and tsunami energies were estimated and the tsunami genertion areas defined.

The earthquake energies were estimated to be 5.61 × 10²⁴ erg for the 19 September event and 9.9 × 10²³ erg for the 21 September event. Tsunami energies were estimated to be 0.7 × 10²⁰ erg for the first event and 0.56 × 10²⁰ erg for the second event. The source area of the first tsunami was determined to be approximately one-half of the earthquake source area, or approximately 7500 km², while the source area of the second tsunami was estimated to be equal to the earthquake area.

The relatively small tsunamis generated by these large earthquakes are attributed to the shallow angle of subduction of the Cocos plate underneath the North American plate for this particular region, and to the small vertical component of crustal displacements. However, the angle of subduction increases further south and local earthquakes from that area have the potential of producing large tsunamis on the west coast of Mexico.