The Oaxaca, Mexico, earthquake of 1931: lithospheric normal faulting in the subducted Cocos plate

Abstract

A study of the great earthquake in Oaxaca, Mexico, on 15 January 1931 shows that it was a normal fault earthquake striking east-west. The lack of evidence of surface faulting near the source region¹ and the focal depth of ∼40 km inferred from body-wave modelling suggest that the earthquake occurred within the subducted slab. The large size of the earthquake (M_s = 8.0) strongly suggests that the rupture broke through the entire lithosphere of the subducted Cocos plate. Other large, normal fault earthquakes in the Japanese², Aleutian³, Peruvian⁴ and Sumba⁵ arcs show lithospheric faulting seawards of the trench or very close to it. The 1931 event, however, occurred farther down-dip in the slab (∼150 km landward of the trench), below the bottom of what appears to be the seismogenic plate interface. We speculate here that large, although infrequent, normal fault earthquakes such as the 1931 Oaxaca earthquake may have a role in decoupling at depth the subducted slab from the upper part of the slab and perhaps from the overriding Mexican continental plate. The epicentral location of the 1858 Michoacan earthquake⁶ (M_s≍7.5) suggests that it may represent another example of lithospheric normal faulting in the Middle American arc. Such earthquakes, for which the periods of recurrence are unknown, constitute an extremely high seismic risk to population centres of Mexico.