Seismic evidence for fluids in fault zones on top of the subducting Cocos Plate beneath Costa Rica

Abstract

S U M M A R YIn the 2005 TICOCAVA explosion seismology study in Costa Rica, we observed crustal turning waves with a dominant frequency of ∼10 Hz on a linear array of short-period seismometers from the Pacific Ocean to the Caribbean Sea. On one of the shot records, from Shot 21 in the backarc of the Cordillera Central, we also observed two seismic phases with an unusually high dominant frequency (∼20 Hz). These two phases were recorded in the forearc region of central Costa Rica and arrived ∼7 s apart and 30-40 s after the detonation of Shot 21. We considered the possibility that these secondary arrivals were produced by a local earthquake that may have happened during the active-source seismic experiment. Such high-frequency phases following Shot 21 were not recorded after Shots 22, 23 and 24, all in the backarc of Costa Rica, which might suggest that they were produced by some other source. However, earthquake dislocation models cannot produce seismic waves of such high frequency with significant amplitude. In addition, we would have expected to see more arrivals from such an earthquake on other seismic stations in central Costa Rica. We therefore investigate whether the high-frequency arrivals may be the result of a deep seismic reflection from the subducting Cocos Plate. The timing of these phases is consistent with a shear wave from Shot 21 that was reflected as a compressional (S×P) and a shear (S×S) wave at the top of the subducting Cocos slab between 35 and 55 km depth. The shift in dominant frequency from ∼10 Hz in the downgoing seismic wave to ∼20 Hz in the reflected waves requires a particular seismic structure at the interface between the subducting slab and the forearc mantle to produce a substantial increase in reflection coefficients with frequency. The spectral amplitude characteristics of the S×P and S×S phases from Shot 21 are consistent with a very high V p /V s ratio of 6 in ∼5 m thick, slab-parallel layers. This result suggests that a system of thin shear zones near the plate interface beneath the forearc is occupied by hydrous fluids under near-lithostatic conditions. The overpressured shear zone probably takes up fluids from the downgoing slab, and it may control the lower limit of the seismogenic zone.

The role of frictional strength on plate coupling at the subduction interface

“…Some earlier geodynamic modeling studies [ van Keken et al , 2002; Conder , 2005; Wada et al , 2008] concluded that a thin weak layer on the plate interface is necessary to obtain a realistic temperature structure in the mantle wedge, and to match heat flow profiles across the convergent margin at the Earth's surface. There is some geophysical evidence for such a thin, weak plate boundary beneath the forearc [ Audet et al , 2009; Van Avendonk et al , 2010], but such observations do not rule out the possibility that some subduction zones have a strong plate interface. A large number of subduction zones have a seismogenic zone dip angle θ > 20° (Figure 2) suggests that the friction angle ϕ is often significantly larger than 1°.…”

“…The in situ friction coefficient of the sediment and serpentinized peridotite is not well constrained from lab experiments. The possible presence of pore fluid would decrease the effective friction coefficient further at the subduction interface at a wide range of depths [e.g., Moore et al , 1995; Wada et al , 2008; Audet et al , 2009; Van Avendonk et al , 2010]. In our geodynamic models we will assume that the friction angle ϕ of the sediments and that of serpentine vary similarly, and we will vary ϕ as a controlling parameter.…”

Geometry and seismic properties of the subducting Cocos plate in central Mexico

“…This suggests that the added pore pressure and the presence of hydrous minerals might be one of key factors to explain the low S wave velocity (or high Poisson's ratio) in the horizontal part of the upper oceanic crust. Also, the friction across the slab interface may be further reduced by high fluid pressures as a result of metamorphic dehydration of the slab [ Van Avendonk et al , 2010]. The influence of the pore pressure on seismic velocities is however expected to be diminish with depth and is unlikely to be significant at lower oceanic crustal depths where the porosity is extremely low [ Christensen , 1984].…”

Crustal structure across the Costa Rican Volcanic Arc

“…[] that relamination of subducted sediments and eroded material from the upper plate is a mechanism for thickening arc crust and recycling continental material. Assuming a constant subduction rate of ~87 mm/yr [ DeMets , ] since ~65 Ma [ Meschede and Frisch , ] with a thickness of sediments subducted ~0.5 km [ Ivandic et al ., ; van Avendonk et al ., ] and density change of ~300 kg/m 3 [ Hacker et al ., ], the total cross‐sectional area of subducted sediments over the life of the MAT can be estimated as ~2500 km 2 . Given the approximate cross‐sectional area of our profile (~4800 km 2 ) and the estimated area of preexisting CLIP across our profile (~2000 to ~3000 km 2 ) from assuming a 12–20 km thickness found in studies of the adjacent Colombian Basin [ Case et al ., ], if all subducted sediment were relaminated beneath the arc, the expected volume would completely fill the available crustal space, leaving no room for new material from arc‐related intrusions.…”