1. Susan Bilek University of Michigan
Control of Seafloor Roughness on Earthquake Rupture Behavior: Costa Rica
Susan Bilek
University of Michigan

2. Detailed examination of Costa Rica subduction zone
-- Large earthquake rupture and seismicity limits

3. Asperity Model for Large Earthquake Rupture
After Lay et al., 1982

4. CRSEIZE Experiment PIs: Schwartz (UCSC), Protti (UNA-OVSICORI),
Dorman (USCD),
Dixon (Miami),
Lundgren (JPL)

5. Analysis of Large Earthquake Rupture - Aug 20, 1999 Mw 6
Analysis of Large Earthquake Rupture - Aug 20, 1999 Mw 6.9 Underthrusting Earthquake
One of the largest earthquakes in this segment of margin
Occurred shortly before temporary network deployment -- opportunity for recording aftershocks
Occurred in region of disrupted subducted crust (Quepos Plateau)

7. Surface Wave Analysis Aug 20, 1999 Mw 6.9 Earthquake
Empirical Green’s function deconvolution method using surface waves -- remove path and instrument effects, leave information about source
RSTF durations, azimuths related to true duration, rupture direction
Search over all azimuths, find most linear fit between duration & directivity parameter best rupture azimuth

8. P wave inversion results - 1999 Osa Earthquake

9. Rupture Process of 1999 Mw 6.9 Costa Rica Earthquake
Generally simple rupture
Simple source time function
Fairly concentrated moment release on fault plane
Some evidence for minor rupture directivity to SE
Short rupture duration

12. 1990 1999 1983 Rupture area ~600 km2 seamounts ~300-350 km2
Subducting features ~ km2
1983
Rupture area ~3000 km2
Ruptured lateral extent of Cocos Ridge

13. Locations of Highest Quality Events
Aftershock locations concentrate near regions of moment release
Significant diversity in focal mechanism of aftershocks
DeShon et al., in review

14. Summary
Variable rupture processes observed for shallow seismogenic zone earthquakes in Costa Rica
Comparison with detailed bathymetry suggests heterogeneous Cocos Plate topography influences large earthquake rupture