Focal Mechanism Solutions and Tectonics of the Middle America ARC

Fault plane solutions of 67 earthquakes along the Middle America arc were determined. These focal mechanisms delineate tectonic activity characteristics of a subduction zone. Focal mechanisms of shallow events beneath the trench axis indicate normal faulting resulting from initial flexure of the downgoing plate. Shallow focus earthquakes along the inclined seismic zone yield focal mechanisms which indicate underthrusting on fault planes dipping beneath the coast. The tensional axes of the focal mechanism solutions of the intermediate focus earthquakes are aligned down the dip of the seismic zone and indicate that the subducted portion of the plate is under extensional stress. The strike-slip solutions of nine events along the arc possess one nodal plane which strikes approximately perpendicular to the trend of the arc. The locations of these nine events correlate very well with transverse features mapped by Stoiber and Carr (1973) from discontinuities and offsets in the volcanic chain and dip changes in the inclined seismic zone. These discontinuities are marked by more and larger magnitude earthquakes than the areas between them. An aftershock sequence in eastern Guatemala was spatially bounded by two transverse breaks. The data suggest that the underthrust plate is broken into segments 100-300 km wide which are bounded by tear faults and which are subducted with dips and strikes which may differ from those of adjacent segments.