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

Harm J A Van Avendonk,W Steven Holbrook,Steven Harder,Carlos J Ramírez,Guillermo E Alvarado,Andrew D Bullock,Daniel Lizarralde,Mauricio M Mora

doi:10.1111/j.1365-246x.2010.04552.x

Abstract

In 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 Vp/Vs 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.

Highlights

Some of the largest earthquakes occur at subduction zones, where strain is accumulated and released at the contact between the two converging tectonic plates
If we assume that α = 0.65, as in many other crustal seismic studies (Stachnik et al 2004), and t0∗ = 0.080, which is roughly the average for our observed S×P and shear-to-shear converted wave (S×S) arrivals (Fig. 16), we find that the reflection arrival amplitude at 20 Hz is reduced to 0.75, relative to the amplitude at 10 Hz
We have presented an unusual explosion seismology record from Shot 21 in the TICOCAVA study in central Costa Rica, in which we found crustal refractions Pg and shear turning wave (Sg), with a dominant frequency of ∼10 Hz, followed by two deep seismic phases (S×P and S×S), which both have a dominant frequency of ∼20 Hz

Summary

Introduction

Some of the largest earthquakes occur at subduction zones, where strain is accumulated and released at the contact between the two converging tectonic plates. Seismic studies have shown that these seismogenic zones often do not extend deeper than the base of the forearc crust (DeShon et al 2003; DeShon & Schwartz 2004). Beneath this depth, it has been speculated that hydrous. The transformation from blueschist to eclogite facies in the oceanic crust and the breakdown of serpentinite in the upper mantle of the downgoing plate may be the most important sources of water deep in the subduction zone (Rupke et al 2004; Peacock et al 2005). Fluids released from downgoing slabs into the mantle wedge affect subduction zone processes at a variety of scales. The possible presence of free water here has great implications for the rheology of the megathrust and the conditions that may result in slow-slip events (Shelly et al 2006; Moore & Lockner 2007; Liu & Rice 2007)

Methods

Discussion

Conclusion