Steep subduction geometry of the Rivera Plate beneath the Jalisco Block in western Mexico

The morphology of the Rivera plate subducted beneath the Jalisco block in western Mexico is determined from accurately located hypocenters of locally recorded microearthquakes, and from earthquakes with magnitude m b ≥4.5 recorded at teleseismic distances. The hypocenters of these latter earthquakes are relocated, and for five of them the focal depth is constrained by the inversion of long‐period body waves. The Wadati‐Benioff zone inferred from these data indicates a steep subduction of the Rivera plate that resembles the geometry of subduction of the Cocos plate beneath Central America. It is, however, very different from the shallower and almost subhorizontal subduction of the Cocos plate observed in southern Mexico, southeast of this region. The Rivera plate is comparable to the Juan de Fuca plate in terms of the small areal extent, young seafloor age, low relative velocity, and low teleseismic activity in the subduction zone. This study shows that the dip of both the Juan de Fuca and Rivera plates are similar once they are decoupled from the overriding continental crust. The downgoing Rivera plate initially starts with a dip of ∼10° down to a depth of 20 km and then increases gradually to a constant dip of ∼50° below a depth of 40 km. Intermediate‐depth seismicity is low in this zone associated with the subduction of the slow (2 cm/yr) and young (9 m.y.) Rivera plate. The maximum depth extent of earthquakes observed in the Rivera subduction zone is about 130 km. The andesitic, calc‐alkaline Colima volcano appears to be directly related to the subduction of the Rivera plate. To the NW of this volcano, the observed Quaternary volcanism in the Jalisco block, which is parallel to the trench, may also be explained by the subduction of the Rivera plate.