Susan Bilek University of Michigan 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 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 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

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 Subducting features ~300-350 km2 1983 Rupture area ~3000 km2 Ruptured lateral extent of Cocos Ridge

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

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