Strong along-arc variations in attenuation in the mantle wedge beneath Costa Rica and Nicaragua

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Geochemistry Geophysics Geosystems
Journal Date: 
Oct 9
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Attenuation structure in the Central American subduction zone was imaged using local events recorded by the Tomography Under Costa Rica and Nicaragua array, a 20-month-long deployment (July 2004 until March 2006) of 48 seismometers that spanned the fore-arc, arc, and back-arc regions of Nicaragua and Costa Rica. P and S waveforms were inverted separately for the corner frequency and moment of each event and for the path-averaged attenuation operator (t*) of each event-station pair, assuming attenuation is slightly frequency-dependent (proportional to = 0.27). Then, tomographic inversions were performed for S and P attenuation (Q(S)(-1) and Q(P)(-1)). Since P wave amplitudes reflect both shear and the bulk moduli, tomographic inversions were also performed to determine shear and bulk attenuation (Q(S)(-1) and Q(kappa)(-1)), the loss of energy per cycle owing to shearing and uniform compression, respectively. Damping and other inversion tomographic parameters were systematically varied. As is typical in subduction zone attenuation studies, a less attenuating slab, upper plate, and wedge corner and a more attenuating mantle wedge were imaged. In addition, first-order differences between the mantles beneath Nicaragua and Costa Rica were observed. The slab in Nicaragua is more attenuating than the slab in Costa Rica. A larger zone of higher shear attenuation also characterizes the Nicaraguan mantle wedge. Within the wedge, maximum attenuation values at 1 Hz correspond to Qs = 38-73 beneath Nicaragua and Qs = 62-84 beneath Costa Rica, and average values are Qs = 76-78 and Qs = 84-88, respectively. Attenuation variations correlate with along-arc trends in geochemical indicators that suggest that melting beneath Nicaragua occurs at more hydrated conditions, and possibly to greater extents and depths, relative to northern Costa Rica. Shear attenuation dominates over bulk attenuation in the well-resolved regions of the wedge. The more extensive zones of greater shear attenuation observed in the Nicaraguan wedge could be explained by higher temperatures and/or greater hydration, but comparison with petrological data suggests that hydration variations play a larger role. Average wedge attenuation values are comparable to estimates for the Andes and Japan, greater than those for Alaska, and less than those for Tonga-Lau.


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Doi 10.1029/2008gc002040