A network of seismographs operating in the Valle Central of Costa Rica has recorded many small earthquakes near the cities of Cartago and San Jose. This seismicity is similar in many ways to the shallow-focus intraplate seismicity of Central America to the north. The earthquakes occur within tens of kilometers of Quaternary volcanic centers at shallow focal depths. The earthquakes occur predominantly on strike-slip faults, with the nodal plane that would correspond to a left-lateral fault striking approximately east-northeast and the nodal plane that would correspond to a right-lateral fault striking approximately north-northwest. The shocks have a tendency to occur in seismic swarms.
The region of highest seismicity in our study was located southwest of Cartago, about 10 km from the meizoseismal zones of destructive earthquakes of 1841 and 1910. In detail, the recently recorded small earthquakes seem to have occurred on different faults or fault segments than the 1910 earthquakes. The tendency for shallow-focus intraplate earthquakes to occur within kilometers of earthquakes that occurred several decades earlier has been noted elsewhere in Central America. The occurrence of shocks on distinct faults within the overall region of high activity appears similar to the occurrence of earthquakes on different fault strands in Managua, Nicaragua. We discuss the Valle Central seismicity in light of hypotheses proposed for the shocks farther north in Central America. Our data can be interpreted in terms of the hypothesis that shallow-focus intraplate earthquakes in Central America concentrate on zones of strike-slip faults that pass through offsets of the volcanic chain. Our data can also be interpreted in terms of the hypothesis that the earthquakes occur as the response of minor faults to high regional stresses throughout the region surrounding the volcanic chain. Both hypotheses leave some characteristics of the seismicity unexplained, although these characteristics are not crucial evidence against the hypotheses. A third hypothesis, that the Valle Central source regions are different than intraplate source regions northward in Central America and are occurring in a developing transform plate boundary between the Caribbean and Nazca plates, is plausible on the basis of the regional plate tectonic environment, but it is not strongly supported by the local geology of the Valle Central or by the characteristics of seismicity.