We present evidence of a tremor and slow slip event occurring offshore of the Nicoya Peninsula, Costa Rica, in August 2008. The temporally coincident location of offshore tremor, very low-frequency earthquakes (VLFEs), slow slip on coastal GPS stations, and a pressure transient in an IODP borehole, all indicate slow slip occurring at shallow depths, near the trench. The periodicity of the tremor during this particular event corresponds to the peak coulomb stress forced by the semi-diurnal ocean tide. In addition to analyzing this event in detail, we will present an updated catalog of tremor activity since 2007, such as described by Walter et al. (2011), and also show VLFE activity during this time period. Results from subduction zones that exhibit different manifestations of tremor and slow slip may provide better insight into the transition from stable sliding to stick-slip motion. The discovery of offshore tremor and slip using land stations at the Nicoya Peninsula is likely only possible due to its proximity to the trench. Such discoveries at the Cascadia margin may also be possible with the installation of seafloor ocean bottom seismometer infrastructure related to the NSF Cascadia Initiative.

Acknoweldgements: This work was supported by NSF awards OCE���0841061 and EAR���0842338 to S.Y.S. and a NASA Earth and Space Science Fellowship to J.I.W.

For further reading: Walter, J. I., S. Y. Schwartz, J. M. Protti, and V. Gonzalez (2011), Persistent tremor of the northern Costa Rica seismogenic zone, Geophys. Res. Lett., 38, L01307, doi:10.1029/2010GL045586. doi: 10.1029/2010GL045586

Keywords: tremor, slow_slip