Earthquakes relocated by joint hypo-center determination define an inclined seismic zone in Guatemala and El Salvador approximately 15 km thick. This zone is segmented by one large change of strike and dip and two smaller changes in dip. The inclined seismic zone marks the upper part of the underthrust slab and the discontinuities represent breaks segmenting the slab. The breaks occur in areas where they had been predicted on the basis of abrupt discontinuities in the volcanic chain.
Quaternary faulting is dominated by transcurrent fault zones. Left-lateral strike-slip fault zones strike transverse to the arc and coincide with the proposed breaks in the underthrust slab. Right-lateral fault zones strike parallel to the arc and approximately coincide with the volcanic chain. Subsidence of at least 500 m has occurred along the strike of the left-lateral and right-lateral fault zones and created a grid of troughs and depressions. The amount of horizontal movement is about 10 km for the transverse and longitudinal fault zones.
These seismic and geologic data suggest a segmented underthrusting process in northern Central America. The wedge of overriding lithosphere and the underthrust slab have transverse structures in the same areas. The left-lateral motion on N30°E-striking transverse faults can be deduced from the segmentation and an increasing rate of plate convergence to the southeast.