Slow slip events in Costa Rica detected by continuous GPS observations, 2002-2011

A network of continuously recording GPS stations has operated in the Nicoya Peninsula of northern Costa Rica since 2002. We processed all available data from this network for the period of 2002-2011 to investigate the occurrence of Slow Slip Events (SSE) on the subduction interface between the Cocos and Caribbean plates. In order to overcome signal masking by high levels of tropospheric noise, we developed a new technique that facilitates detection of transient events in the presence of noise. We identified five significant SSEs during the 2002-2011 period, with event middle times in 2003, 2005, 2007, 2009 and 2011, with an average recurrence interval of 21 +/- 6 months. Time series analysis shows that transient deformation imparts a signature similar to random walk. Removal of the SSEs and regional common mode errors from the time series reduced velocity uncertainty by nearly an order of magnitude. Limited available data for the 2003, 2005 and 2011 events preclude detailed characterization of these events. However, good spatiotemporal coverage of the 2007 and 2009 events suggest that both events had irregular duration and distribution. In the 2007 event, slow slip started in the northwest coastal area and migrated southeastward over a period of similar to 1 month. The 2009 event had a significantly longer event duration and larger surface displacement. Stations in the northwest area observed two separate SSEs in 2008.6 and 2009.4, correlating well with the tremor episodes offshore, indicating a shallow SSE slip patch with shorter recurrence interval. Significant differences between the 2009 and 2007 events lead us to question the simple recurrence interval model for the SSE in Nicoya.