Relocation and waveform modeling of the 1924 Orotina, Costa Rica, earthquake (MS 7.0)

We have determined the location and focal mechanism for the 1924 Orotina earthquake using two different relocation algorithms developed for the analysis of historic earthquakes and forward waveform modeling of teleseismic body waves. Our relocation results suggest that the 1924 earthquake occurred similar to 50 km southeast of Orotina near the Pacific coast. The heavy damage reported in the Orotina region, coupled with a low population density to the southeast, had led previous researchers to believe that the earthquake occurred near Orotina, possibly on the Tarcoles or Bijagual faults. Our waveform modeling results indicate rupture on a thrust fault at a depth (25 +/- 5 km) near the plate interface with M-w 6.4-6.7. These are inconsistent with slip on any mapped surface faults located near Orotina and with the mechanism of the nearby 2004 M-w 6.4 Damas earthquake that indicates transtensional deformation in the upper crust. This suggests that the 1924 event may be one of a sequence of events (1924, 1990, 1999) generated by subduction of the central Costa Rica seamount belt. (C) 2009 Elsevier B.V. All rights reserved.