1. Strain Release Along the Northern Costa Rica Seismogenic Zone
Susan Y. Schwartz
Department of Earth and Planetary Sciences
UC Santa Cruz

2. Modes of Strain Release

3. 1950 M~7.7

4. Costa Rica Subduction Zone-Instrumenting the Plate Boundary with a Seismic, GPS and Fluid Flow Network
Collaborators:
Tim Dixon, LeRoy Dorman, Kevin Brown, Marino Protti, Victor Gonzales, Heather DeShon, Edmundo Norabuena, Andy Newman, Sue Bilek, Ernst Flueh

6. EPR interface seismicity: 17-28 km
CNS interface seismicity:12-26 km shallower dip

7.
% Locked

8. Plate boundary earthquakes begin deeper than start of seismogenic zone
Up-dip limit of seismogenic zone is defined by shallowest geodetic locking and is consistent with models of thermal control ( o C)
Plate boundary earthquakes begin deeper than start of seismogenic zone
Plate boundary earthquakes terminate shallower than continental Moho and 350 C isotherm
Thermal Modeling by Spinelli and Saffer ( 2004)
300o C isotherm from Harris and Wang (2002)

9. Implications
Locked portion of plate boundary is accumulating strain to be released in next large earthquake
Something is weakening the plate boundary to allow the transition from locked with no seismicity to unlocked with seismicity at ~250o C

10. What causes the onset of microseismicity at km depth where modeled temperatures on the plate interface are ~ oC
Stable sliding
Fault zone weakening by increased pore-fluid pressure from low grade metamorphic reations in basalt
Stick-slip

15. Fluid flow excursions caused by 3 episodes of slow slip on the plate interface
Brown et al. (2005)

16. 2003 Geodetically observed slow slip event
Interseismic Strain Accumulation
2003 Geodetically observed slow slip event
Slow Earthquake Signal

18. Episodic Aseismic Slip - Locates at frictional transitions between stable sliding and stick slip behavior
% Locked

19. Global Distribution of Episodic Aseismic Slip

20. Modified from Obara and Hirose [2005]
Modified from Dragert and Rogers [2004]

21. Northeast Japan strain accumulation, asperity and afterslip patterns
2005 Nias Earthquake Afterslip
DIFFERENT FRICTIONAL PROPERTIES CONTROL FAST VS. SLOW SLIP
Velocity Weakening Velocity Weakening/ Strengthening Transition
Strain Accumulation Slip- Abundant Microseismicity
Coseismic Slip- Asperity Afterslip
Slow Slip Events

22. Different frictional properties control fast vs. slow slip
No observations of slow slip in strongly coupled regions exist

23. Hot or warm subduc-tion zones have deep slow slip
Cascadia
SW Japan
Mexico
Deep slow slip may require frictional transition at shallow depth (low pressure) or fluids generated from dehydration reactions (baslate-eclogite).
Cooler subduction zones or with thin overriding crust have shallow slow slip
NE Japan (afterslip)
Boso Japan
Costa Rica

24. New GPS/Seismic/Tilt Nicoya Network for Detection of Slow Slip Events
Collaborators:
Tim Dixon, Kim Psencik (UM), Marino Protti, Victor Gonzales (OVSICORI-UNA)
Technical Support:
Dan Sampson (UCSC), Jacob Sklar & Freddy Blume (UNAVCO)
SFB574 Borehole Seismic:
Ernst Flueh, Wolfgang Rabbel, Martin Thorwart & Nilay Dinc

25. CONCLUSIONS:
Strain along the northern Costa Rica plate boundary is released in large earthquakes and slow slip.
Spatial separation exists between these two modes of strain release with strain presently accumulating just offshore the Nicoya Peninsula and slow slip occurring in regions up and down-dip of this.
A dense network of cGPS, seismic and tilt stations has been installed to improve our understanding of slow slip at this plate boundary.