Dual-Frequency Coherence as Signature of Fluid Flow Noise Observed in Costa Rica Subduction Zone

In the CRSEIZE experiment of 1999-2000 (DeShon and others, 2006), an array of 14 "ONR" OBSs were deployed in an array across the forearc of the subduction zone west of Costa Rica. The station spacing was about 15 km and the array was about 35km by 60km. Half the sensors were PMD broadband electrochemical seismometers and half were 1-Hz inertial sensors. At three sites near an out-of-sequence thrust, correlations were observed between signals on collocated fluid flow sensors and seismic noise in the 1-10Hz range (Brown and others, 2005). It appears that this correlated noise is statistically different from the usual noise. In particular, there exists a pattern revealed by dual-frequency coherence calculations. This indicates a harmonic relationship between frequency multiples of about 0.4 Hz. A possible cause for this pattern is the type of fluid flow regime proposed by Julian, 1994. This proposed cause of this fluid flow is creep-induced strain change.