Abstract

We have analyzed seismograms of the 3 June 1932 (Ms = 8.2) and the 18 June 1932 (Ms = 7.8) Jalisco earthquakes and their aftershocks at Manzanillo (MNZ), Guadalajara (GUM), and Tacubaya (TAC). The aftershocks locations, the first motions at MNZ, and the isoseismic maps of the two main shocks strongly suggest that: (a) the 3 June 1932 earthquake initiated NW of but close to MNZ and propagated NW for an estimated length of rupture of 220 km; (b) the 18 June 1932 earthquake nucleated SW of MNZ (offshore) and perhaps ruptured a length of about 60 km; and (c) the width of rupture was approximately 80 km. There is no evidence of events occurring SE of MNZ even up to 1 1/2 yr after the first main shock. A gap of about 60 km remains between the aftershock areas of the 1932 Jalisco and the 1973 Colima earthquakes whose seismic potential is unknown. Although the boundary between the Rivera and Cocos plates is uncertain, there is little doubt that the 1932 earthquakes broke the shallow part of the Rivera subduction zone. Our results may help in quantifying seismic potential of tectonically similar areas such as the Juan de Fuca subduction zone in the NW United States.

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