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Non-technical summary: A collaboration of geophysicists from nine institutions (including Fred Pollitz from the USGS), led by Edmundo Norabuena of the University of Miami, has produced a comprehensive seismotectonic interpretation of the tectonic convergence zone off Costa Rica. This zone produces large earthquakes and results from subduction of the oceanic Cocos plate beneath central America, which is considered part of the Caribbean plate. Data used for the study includes locations of microseismicity as well as Global Positioning System (GPS) data collected around the Nicoya and Osa peninsulas in southern and northern Costa Rica, respectively, which allows crustal deformation to be measured very close to the shallow part of the tectonic convergence zone. The data were collected as part of a large international project - The Costa Rica Seismogenic Zone Experiment. Interpretation of the GPS data by Norabuena et al., (2004) suggests the existence of two locked patches centered at 14±2 and 39± 6km depth. Interplate microseismicity is concentrated in the more freely slipping region located between the locked patches. Therefore if this pattern continues, then the locked portions of the plate are expected to contribute to significant seismic moment release (i.e. rupture with larger slip) when the next major earthquake occurs, in comparison with the regions of elevated microseismic activity. Other observations highlighted in the study are that the Nicoya fore-arc block translates northwest at an average rate of 8±3mm/yr, which is likely related to oblique convergence between the Cocos and Caribbean plates. In the Osa region to the south convergence is orthogonal to the trench as a result of this slip partitioning. Norabuena et al., (2004) found that the inferred position of the locked patches approximately coincides with the rupture areas of past large earthquakes (i.e., M7.7 in 1950 and M7.0 in 1978), lending support to the notion that areas that lock during one seismic scycle are likely to lock again in the next seismic cycle and continue to produce large earthquakes.

Figure 1. Location map with major physiographic and plate tectonic features of the study area. GN is Golfo Nicoya, GO is Golfo Osa, FC is Fila Costena, QP is Quepos Plateau. NPDB is North Panama Deformed Belt, part of the northern boundary of the Panama block (grey band). Volcanoes mentioned in text are Arenal (A), Tenorio (T) and Miravalles (M). Stars mark location of major earthquakes since 1980. White boxes outline areas shown in Figure 3a,b. Line S-S’ shows location of seismic line 101 [Christeson et al., 1999; Sallarès et al., 2001] used in Figure 3c. Click on image for the whole pdf article or on the link above.