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Hydrologic control of forearc strength and seismicity in the Costa Rican subduction zone


Subduction zones can exhibit variable seismic behaviour, ranging from great earthquakes to slow slip1. This variability may be linked to fault frictional properties2, and the rheology and structure of the upper plate3. The subduction zone beneath the Nicoya Peninsula, Costa Rica, is characterized by strong variations in fault-slip behaviour and a lateral change in the origin of the subducting plate4,5,6,7. In the northwest, the plate interface is locked8,9, and experiences large, infrequent earthquakes8,9, and the subducting plate is formed at the East Pacific Rise. In contrast, in the southeast, slow-slip events occur frequently10 and the subducting plate is formed at the Cocos–Nazca spreading centre. Here we use seismic receiver-function data to analyse the structure of the subduction zone beneath the Nicoya Peninsula. We find extremely high P–S seismic-velocity ratios within the entire subducting oceanic crust that we interpret as high pore-fluid pressure11,12. Velocity ratios in the overriding continental crust, however, change from lower values in the northwest to higher ones in the southeast, indicating a disparity in fluid accumulation. We infer that this disparity is caused by a higher supply of fluid from the subducting slab in the southeast, owing to the permeability structure of oceanic crust formed at the Cocos–Nazca spreading centre. We suggest that the spatial gradient in fluid content influences upper-plate strength and controls the segmentation of seismogenic behaviour in this subduction zone.

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Figure 1: Tectonic setting and location of broadband seismic stations along the Nicoya Peninsula, Costa Rica.
Figure 2: Seismic structure within the subduction zone forearc.
Figure 3: Along-strike variations and northwest–southeast regionalization of Vp/Vs along the Nicoya Peninsula.
Figure 4: Schematic interpretation of results.

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This work is supported by the Natural Science and Engineering Research Council through a Discovery Grant to P.A. and the National Science Foundation through grants OCE-0841061 and EAR-0842338 to S.Y.S.

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P.A. performed data processing and inversion. P.A. and S.Y.S. wrote the paper.

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Correspondence to Pascal Audet.

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The authors declare no competing financial interests.

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Audet, P., Schwartz, S. Hydrologic control of forearc strength and seismicity in the Costa Rican subduction zone. Nature Geosci 6, 852–855 (2013).

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