Control of Seafloor Roughness on Earthquake Rupture Behavior: Costa Rica
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Control of Seafloor Roughness on Earthquake Rupture Behavior:
Costa Rica
Susan Bilek
University of Michigan
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Detailed examination of Costa Rica subduction zone
-- Large earthquake rupture and seismicity limits
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Asperity Model for Large Earthquake Rupture
After Lay et al., 1982
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CRSEIZE Experiment
PIs:
Schwartz (UCSC),
Protti (UNA-OVSICORI),
Dorman (USCD),
Dixon (Miami),
Lundgren (JPL)
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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)
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Surface Wave AnalysisAug 20, 1999 Mw 6.9
Earthquake
RSTF durations, azimuths related to true duration, rupture direction
Search over all azimuths, find most linear fit between duration & directivity parameter best rupture azimuth
Empirical Green’s function deconvolution method using surface waves -- remove path and instrument effects, leave information about source
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P wave inversion results - 1999 Osa Earthquake
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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
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1990
1999
1983
Rupture area ~600 km2
seamounts ~300-350 km2
Rupture area ~350 km2
Subducting features ~300-350 km2
Rupture area ~3000 km2
Ruptured lateral extent of Cocos Ridge
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Locations of Highest Quality Events
DeShon et al., in review
Aftershock locations concentrate near regions of moment release
Significant diversity in focal mechanism of aftershocks
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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