Large earthquakes in the Tehuantepec subduction zone: evidence of a locked plate interface and large-scale deformation of the slab

Abstract

The Tehuantepec subduction zone was identified as a seismic gap by several authors. The presence of three large earthquakes at the turn of the twentieth century, in temporal and spatial proximity, poses questions on the extent of the seismic gap and on the type of tectonic deformation in this region. On 19 April 1902, a large earthquake occurred in Guatemala. The macroseismic data and recent relocations suggest that it is not a subduction event but an intraplate earthquake within the subducted Cocos plate. The recent relocations and the macroseismic data of the 23 September 1902 earthquake, which is frequently interpreted as an interplate subduction event, show that it is an intermediate-depth earthquake located inland in central Mexico. The 14 January 1903 earthquake, located in the Gulf of Tehuantepec, may be interpreted as an intraplate event, similar to the 8 September 2017 earthquake (Mw 8.2) or a subduction earthquake. Thus, there is no clear evidence of large subduction earthquakes (Mw > 7) in Tehuantepec at least in the last 250 years. Geodetic data suggest that the subduction zone is highly coupled and, therefore, a considerable amount of plate motion has not been released. The length of the presumed seismic gap is equivalent to a Mw ~ 8.5 earthquake. In contrast, the September 1902 (Mw 7.8), 1903 (Mw 7.4), 1931 (Mw 7.8), 1999 (Mw 7.5) and the great 8 September 2017 events (Mw 8.2) are all down-dip tensional earthquakes in the slab, reflecting a complex state of stress in the downgoing Cocos plate. These frequent and large tensional earthquakes may reflect a large-scale deformation of the slab that locked at the plate interface is under tensional stress under its own gravitational weight. As a result, seismic hazard in the Isthmus of Tehuantepec stems from both subduction and intraplate earthquakes.