HR: 10:50h
AN: T22E-03    [Abstracts]
TI: Complex Tectonics of an Oceanic Transform Fault: Segmentation of the Blanco Transform Fault Zone from Earthquake Analysis
AU: * Braunmiller, J
EM: jochen@sed.ethz.ch
AF: Oregon State University, College of Oceanic and Atmospheric Sciences 104 COAS Admin. Bldg., Corvallis, )R 97331-5503,
AU: Nabelek, J
EM: nabelek@coas.oregonstate.edu
AF: Oregon State University, College of Oceanic and Atmospheric Sciences 104 COAS Admin. Bldg., Corvallis, )R 97331-5503,
AB: The Blanco Transform Fault Zone (BTFZ), northeast Pacific Ocean, forms the ~350 km-long Pacific-Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Near-by broadband seismic networks provide the unique framework for a detailed, long-term seismotectonic study of an entire oceanic transform fault (OTF) system. We use regional waveforms to determine 125 earthquake source parameters; combined with 28 Harvard moment-tensors, they represent the largest waveform derived OTF source parameter dataset. Joint epicenter determination removes the northeasterly routine location bias. Projecting seismicity onto the BTFZ, we map along-fault seismic slip rate variations. Earthquake source parameters and morphology indicate several transform segments separated by extensional step-overs. The eastern segment from Gorda Ridge to Gorda Depression is a pull-apart basin. The longest transform (~150 km) following Blanco Ridge from the Gorda to Cascadia depression is seismically very active, fully coupled, has a wider seismic zone (~9 km) than other transform segments and accommodates the largest (M_w 6.4-6.5) BTFZ earthquakes. Interpretation of Cascadia Depression as spreading ridge is supported by plate motion parallel normal faulting T-axes. Spreading is currently tectonic; 9 km deep earthquakes indicate a deep source for intermittent intrusives and rapid post- emplacement cooling. A short transform connects to the pull-apart Surveyor Depression. Widely spread seismicity along the western BTFZ reflects complex morphology indicating ongoing plate boundary reorganization along short, narrow-width sub-parallel faults that limit earthquake size to M_w ≤ 6.2. Seismic coupling is low in extensional (≤15%) compared to transform areas (35-100%) implying different mechanical properties. Centroid depth variations are consistent with seismic slip cut-off near 600°C.
DE: 3040 Plate tectonics (8150, 8155, 8157, 8158)
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8150 Plate boundary: general (3040)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting