Control of Seafloor Roughness on Earthquake Rupture Behavior:

Costa Rica

Susan Bilek

University of Michigan

Detailed examination of Costa Rica subduction zone

-- Large earthquake rupture and seismicity limits

Asperity Model for Large Earthquake Rupture

After Lay et al., 1982

CRSEIZE Experiment

PIs:

Schwartz (UCSC),

Protti (UNA-OVSICORI),

Dorman (USCD),

Dixon (Miami),

Lundgren (JPL)

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)

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

P wave inversion results - 1999 Osa Earthquake

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

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

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

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