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Newman, AV, Schwartz SY, Gonzalez V, DeShon HR, Protti JM, Dorman LM.  2002.  Along-strike variability in the seismogenic zone below Nicoya Peninsula, Costa Rica. Geophysical Research Letters. 29   10.1029/2002gl015409   AbstractWebsite

[1] At the subduction zone in northwestern Costa Rica, the seismogenic zone lies directly beneath the Nicoya Peninsula, allowing for near source seismic studies of earthquake activity. We located 650 earthquakes along the seismogenic plate interface using a dense seismic network in the vicinity of the Nicoya Peninsula. Using these data we constrained the updip limit of the seismogenic zone there and found a transition in depth, 10 km in the south to 20 km in the north, that occurs where the subducting oceanic crust changes from warmer Cocos-Nazca Spreading center (CNS) origin to colder East Pacific Rise (EPR) origin. We argue that the temperature of the incoming oceanic crust controls the seismogenic updip limit beneath Nicoya, Costa Rica; subducting colder oceanic crust deepens the seismogenic updip limit.

Nolet, G, Dorman LM.  1996.  Waveform analysis of Scholte modes in ocean sediment layers. Geophysical Journal International. 125:385-396.   10.1111/j.1365-246X.1996.tb00006.x   AbstractWebsite

In an effort to determine the characteristics of seismic noise on the ocean bottom and its relationship to the structure of the sea-floor, we have adapted the method of nonlinear waveform fitting to accommodate multidimensional models (shear velocity beta and shear damping Q(s)), and have applied it to invert several records of interface waves (Scholte 1958) from the THUMPER experiment off southern California. Waveform fitting is a very powerful tool to determine the S velocity in the top few metres of the sediment. Starting from beta = 30 m s(-1) at the top clay layer, the S velocity increases with a gradient of 2.8 m s(-1) m(-1) over the first 150 m of sediment. A theoretical estimation of the source strength gives coherent estimates of Q(s) as a function of depth for distances between 400 and 1070 m from the source. The Q(s) models are characterized by very low values (10-20) in the top three metres, but by values in excess of 100 below that level. The results confirm the identification of the noise as harmonics of interface waves. In the area of this experiment, the largest noise amplitudes belong to the fundamental mode and penetrate to a depth of about 20 m into the sediment. The overtone energy can be appreciable too, and is noticeable to about 80 m depth. The Q(s) structure confirms the strong influence that the sea-floor structure has on the noise spectrum. The high attenuation at frequencies above 3-4 Hz suppresses noise propagation and produces low noise at higher frequencies. (Similarly, high attenuation in the asthenosphere suppresses noise propagation below 0.1 Hz.)

Norabuena, E, Dixon TH, Schwartz S, DeShon H, Newman A, Protti M, Gonzalez V, Dorman L, Flueh ER, Lundgren P, Pollitz F, Sampson D.  2004.  Geodetic and seismic constraints on some seismogenic zone processes in Costa Rica. Journal of Geophysical Research-Solid Earth. 109   10.1029/2003jb002931   AbstractWebsite

[1] New seismic and geodetic data from Costa Rica provide insight into seismogenic zone processes in Central America, where the Cocos and Caribbean plates converge. Seismic data are from combined land and ocean bottom deployments in the Nicoya peninsula in northern Costa Rica and near the Osa peninsula in southern Costa Rica. In Nicoya, inversion of GPS data suggests two locked patches centered at 14 +/- 2 and 39 +/- 6 km depth. Interplate microseismicity is concentrated in the more freely slipping intermediate zone, suggesting that small interseismic earthquakes may not accurately outline the updip limit of the seismogenic zone, the rupture zone for future large earthquakes, at least over the short (similar to 1 year) observation period. We also estimate northwest motion of a coastal "sliver block'' at 8 +/- 3 mm/yr, probably related to oblique convergence. In the Osa region to the south, convergence is orthogonal to the trench. Cocos-Caribbean relative motion is partitioned here, with similar to 8 cm/yr on the Cocos-Panama block boundary ( including a component of permanent shortening across the Fila Costena fold and thrust belt) and similar to 1 cm/yr on the Panama block - Caribbean boundary. The GPS data suggest that the Cocos plate - Panama block boundary is completely locked from similar to 10 - 50 km depth. This large locked zone, as well as associated forearc and back-arc deformation, may be related to subduction of the shallow Cocos Ridge and/or younger lithosphere compared to Nicoya, with consequent higher coupling and compressive stress in the direction of plate convergence.