Seismic rupture associated with subduction of the Cocos Ridge

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Abstract
The April 3,1983, Costa Rica earthquake (Mw = 7.4, Ms = 7.3) occurred where the aseismic Cocos Ridge has been partially subducting beneath the Caribbean plate. The epicenter is at the eastern edge of the 70 × 110 km² initial aftershock area, which later shows considerable expansion. P wave directivity analysis indicates that the rupture front propagated westward 63 km from the epicenter to where the major moment release was truncated. The source time history of the 1941 event (Ms = 7.5) which occurred in the same epicentral area appears to be similar to the 1983 event but with more complexity and a slower initiation of moment release. The low stress drop of the 1983 event, 2–3 bar, and low ratio of seismic slip to tectonic slip, 0.15 for the 1983 event, suggest that the coupling between the descending slab and the overlying plate is weak. Yet, a recurrence time for large events (mag > 7.0) in this zone of about 40 years is implied by the 1904, 1941, and 1983 events, suggesting that the Cocos Ridge causes increased partial coupling compared with that in the adjacent subduction zone. The slow source rupture process may reflect the nature of the contact area between the Costa Rica arc and the subducting plate in the vicinity of the bouyant Cocos Ridge. The normal faulting event of July 1, 1979, (mb = 5.3) is consistent with present‐day uplift of the Burica Peninsula, which previous studies suggest is related to the subduction of the Cocos Ridge. The Burica and Osa peninsulas may behave as detached blocks which influence the distribution of partial coupling reflected in the aftershock expansion pattern.