HR: 11:35h
AN: T41E-06    [PDF]
TI: Transient Fluid Pulsing and Noise in the Costa Rican Subduction Zone: Nearly Silent Slip Events?
AU: * Brown, K M
EM: kmbrown@ucsd.edu
AF: 1. Scripps Institution of Scripps Institution of Oceanography, University of California,, San Diego, CA 92093 United States
AU: DeShon, H
EM: hdeshon@emerald.ucsc.edu
AF: 2 Inst. of Tectonics and Earth Sciences, University of California,, Santa Cruz, CA 95064 United States
AU: Tryon, M
EM: mtryon@ucsd.edu
AF: 1. Scripps Institution of Scripps Institution of Oceanography, University of California,, San Diego, CA 92093 United States
AU: Dorman, L
EM: ldorman@ucsd.edu
AF: 1. Scripps Institution of Scripps Institution of Oceanography, University of California,, San Diego, CA 92093 United States
AU: Schwartz, S
EM: susan@emerald.ucsc.edu
AF: 2 Inst. of Tectonics and Earth Sciences, University of California,, Santa Cruz, CA 95064 United States
AB: Long-term measurements of fluid flow were made across the Costa Rica (Pacific) convergent margin utilizing a recently developed osmotically driven Chemical and Aqueous Transport (CAT) meter designed to quantify both inflow and outflow rates on the order of ~0.01 to ~1500cm/yr. As part of the study 14 ocean bottom seismometer (OBS)/CAT meter combinations and another 7 autonomous CAT meters were deployed across the Costa Rica margin northwest of the Nicoya and Osa Peninsulas for respective periods of about 180 and 75 days. Significant transience in flow was observed both on the incoming plate and through the surface of the forearc. A series of pulsed flow events on the incoming plate suggest significant repeated transient episodes of volumetric dilation that could relate to dilational hardening and the onset of instability prior to a Mw 6.4 normal faulting event in the outer rise region of the trench. Three periods of correlated flow signals are also seen on the trench slope of the subduction zone. These elevated flow events are seen between several instruments located in the out-of-sequence thrust (OOST) zone over along-margin strike distances of aprox. 30 km. The causes of the transience are still being investigated but seismic "noise" on the local OBS instruments also picks up during three correlated flow events at one site (i.e. there are three associated periods of elevate seismic noise). The noise was originally a broad band signal and appears to be transmitted as S-waves who's frequency content is locally modified by a soft substrate under the instrument location. The grouping of three flow events, and three elevated seismic "noise" events argues against coincedence. The flow signals do not correlated with instruments further up the trench slope or on the incoming plate. The toe or OOST structure therefore seems to be highly sensitive to what we postulated are episodic tectonic volumetric strains and associated hydrologic pulsing. We suspect the strain events are associated with the propagation of slow lateral rupture along the subduction thrust to the toe of the wedge. Our current working hypothesis is that a component of the plate boundary slip at shallower depths is occurring via slow nearly silent rupture processes that occur discontinuously on a critically stressed Nicoya segment.
DE: 7209 Earthquake dynamics and mechanics
DE: 7230 Seismicity and seismotectonics
DE: 8105 Continental margins and sedimentary basins
DE: 8123 Dynamics, seismotectonics
DE: 8150 Plate boundary--general (3040)
SC: Tectonophysics [T]
MN: 2003 Fall Meeting