HR: 09:45h
AN: S41D-08    [Abstracts]
TI: Transient Fluid Pulsing and Seismic Tremor-like Seismic Noise: Episodic Creep and/or Fluid Expulsion at the Updip Edge of the Seismogenic Zone, Costa Rica
AU: * Brown, K M
EM: kmbrown@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, La Jolla, CA 92093 United States
AU: Tryon, M D
EM: mtryon@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, La Jolla, CA 92093 United States
AU: DeShon, H R
EM: hdeshon@es.ucsc.edu
AF: University of California Santa Cruz, Dept. of Earth Science, Santa Cruz, CA 95064 United States
AU: Dorman, L M
EM: ldorman@ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Dr, La Jolla, CA 92093 United States
AU: Schwartz, S Y
EM: sschwartz@earthsci.ucsc.edu
AF: University of California Santa Cruz, Dept. of Earth Science, Santa Cruz, CA 95064 United States
AB: We propose that we have recorded coupled ultra-slow "nearly silent" ruptures and associated subsurface tremor related to fracture flow events originating from the stable/unstable slip transition zone near or at the up dip limit of the seismogenic zone in the Cost Rica subduction system. Long-term measurements of fluid flow were made across the Nicoya Peninsula, Costa Rica, convergent margin utilizing osmotically driven fluid flow meters that are designed to quantify both inflow and outflow rates on the order of $\sim$10$^{-5}$ cm/d to 3 cm/d. The instruments are capable of measuring volumetric strain rates as low as 10$^{-7} - 10$^{-8}$ d$^{-1}$. Significant transience in flow was observed through the surface of the forearc. Three periods of correlated flow signals are seen on three instruments located in the out-of-sequence-thrust region separated by $\sim$30 km. Seismic noise amplitude recorded on collocated ocean bottom seismometers (OBS) increases during the three correlated flow events. The seismic noise has a number of frequency characteristics similar to both volcanic and non-volcanic tremor (non-double coupled seismic sources) suggestive of flow though fracture systems and associated generation of oscillatory instabilities. We show one model where tremor is related to non-linear effects associated with changes in the fluid momentumat fracture constriction points similar to the "knocking of pipes" in old buildings. The momentum changes transmit pressure impulses to the elasticlly damped fracture walls. Elevated flow rates cause the induced oscillations to become unstable leading to sustained and elevated seismic noise. We propose either a series of somewhat complex rupture dislocations and/or pulses of high pressure fluids along the decollement that generate accelerated flow episodes through the fracture networks. These are recorded both as increased seismic noise, and by the fluid flow meters at the surface as direct complex strain-related pulses of outflow or indirectly by incipient liquefaction events caused by the action of the noise on shallow poorly consolidated sediments. The nearly silent strain and flow events may be coupled and migrate across the toe of the wedge at probably 100s m to several kms a day with along strike dimensions of $>$30km. The flow measurement systems appear more sensitive to these signals at the toe of the wedge than the OBSs are to the primary low amplitude seismic noise.
DE: 7230 Seismicity and seismotectonics
DE: 7209 Earthquake dynamics and mechanics
DE: 3025 Marine seismics (0935)
DE: 3094 Instruments and techniques
DE: 3220 Nonlinear dynamics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting