Slow Slip and Tremor in the Northern Costa Rica Seismogenic Zone

Slow Slip and Tremor in the Northern Costa Rica Seismogenic Zone Locations of tremor episodes and Low Frequency Earthquakes (LFEs) compared with the 2007 slow slip distribution.
Several episodes of slow slip and tremor are believed to have occurred at the plate boundary of northern Costa Rica between 2000 and 2008. The evidence for these events varies and consists of: 1) correlated fluid flow excursions and seismic tremor recorded on ocean bottom instruments in 2000 [Brown et al., 2005]; 2) offsets in continuous GPS data in September 2003; 3) offsets in GPS data accompanied by seismic tremor in May 2007 [Outerbridge et al., 2010]; and 4) strong prolonged seismic tremor in August 2008. Modeling of the 2000 event suggested that slip occurred at shallow depth, between the surface and ~15 kilometers. The much better constrained slip distribution of the 2007 event consisted of 2 patches, the stronger centered at ~30 km depth, near the down dip transition from stick-slip to stable sliding, and the weaker patch located at ~6 km depth at the up dip edge of the shallow frictional transition. Tremor locations for the 2008 episode locate offshore at depths of between 6-10 km . The 2003 event was recorded on too few instruments to be modeled. These results are significant in that they are the first to suggest that slow slip occurs at the up dip transition from stick-slip to stable sliding; locations of slow slip in other environments have been limited to the down dip frictional transition.
</p><p>Due to the relatively small surface displacements (1-2 cm) associated with Costa Rica slow slip events, the coincident occurrence of seismic tremor is important for their detection and study. Similar to tremor observations in southwest Japan, Costa Rica tremor consists of swarms of low-frequency earthquakes that occur as repetitive stick-slip motion on the plate interface [Brown et al., 2009], contains very low frequency earthquakes, with dominant energy between 20-50 s, and appears to be tidally modulated.
</p><p>Brown, J. R., G. C. Beroza, S. Ide, K. Ohta, D. R. Shelly, S. Y. Schwartz, W. Rabbel, M. Thorwart, and H. Kao (2009), Deep low-frequency earth- quakes in tremor localize to the plate interface in multiple subduction zones, Geophys. Res. Lett., 36, L19306.
</p><p>Brown, K. M., M. D. Tryona, H. R. DeShon, L. M. Dorman, and S. Y. Schwartz, (2005), Transient fluid pulsing and seismic tremor: Evidence of episodic creep at the updip edge of the seismogenic zone, Costa Rica, Earth Planet. Sci. Lett., 238, 189-203.
</p><p>Outerbridge, K. C., T. H. Dixon, S. Y. Schwartz, J. I. Walter, M. Protti, V. Gonzalez, J. Biggs, M.M. Thorwart, and W. Rabbel (2010), A tremor and slip event on the Cocos-Caribbean Subduction zone as measured by a GPS and seismic network on the Nicoya Peninsula, Costa Rica, J. Geophys. Res., in press.
</p><p>Acknowledgements: This research was supported by several grants from NSF’s MARGINS and Instrumentation and Facilities programs including OCE- 0841061 and OCE-0841091 along with EAR-0842338, EAR-0506463, EAR-0502488, EAR-0842137, EAR-0502221 and EAR-0506382.</p>


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