HR: 16:15h
AN: T44A-02    [Abstracts]
TI: Seismotectonics of the Blanco Transform Fault Zone
AU: * Braunmiller, J
EM: jochen@seismo.ifg.ethz.ch
AF: Institute of Geophysics, ETH Hoenggerberg, Zurich, 8093 Switzerland
AU: Nabelek, J
EM: nabelek@coas.oregonstate.edu
AF: College of Oceanic and Atmospheric Sciences , Oregon State University Oc. Admin. Bldg. 104, Corvallis, OR 97331 United States
AB: We present the first detailed, long-term seismotectonic study of an entire oceanic transform fault system. The Blanco Transform Fault Zone (BTFZ) offshore Oregon is seismically active along its entire length and close to a dense broadband seismic network. This unique combination allows analysis of the frequent moderate sized earthquakes providing a seismotectonic framework unprecedented in accuracy and detail. We used regional broadband waveforms to determine the seismic moment tensor of 125 $M_{w}$ $\geq$ 3.8 events for the 1994-1998 period. Combined with 28 Harvard-CMTs, available for larger events since 1976, they represent the most complete earthquake source parameter data set for an oceanic transform fault. We relocated 144 larger earthquakes (M $\geq$ 5) from 1964 to 1998, which removes the bias of the routine locations that are consistently northeast of the BTFZ as defined by high-resolution bathymetry. The 350 km-long BTFZ is the Pacific-Juan de Fuca plate boundary between the Gorda and Juan de Fuca ridges. Morphologically, the BTFZ consists of several transform segments separated by extensional step-overs. Relocation of the strike-slip and normal faulting events agrees excellently with bathymetry. Source depths are commonly $\leq$6 km near the Gorda and Juan de Fuca ridges; in BTFZ's central part and along eastern Blanco Ridge depths are $\leq$9 km. Most events have strike-slip mechanisms. We found few, generally small events along short transforms west of Gorda Ridge and between the Cascadia and Surveyor depressions. The 150 km-long Blanco Ridge, east of Cascadia Depression, is the longest transform segment and relocations follow bathymetry closely. BTFZ's largest events ($M_{w}$ = 6.5) are located east of $\sim$$128\deg$W where Gorda Ridge-parallel abyssal hill topography is uplifted. West of $128\deg$W, seismicity is lower, events are slightly shallower and large events nucleating in the eastern part possibly rupture into this western part. Seismic moment release along Blanco Ridge accounts for most of the 5.6 cm/yr plate motion rate. Transform motion dominates along BTFZ's 100 km-long western part from Surveyor Depression to Juan de Fuca Ridge. Relocations are more widely spread than in other areas consistent with its complex morphology. Subtle but consistent slip-vector differences suggest three faults strands are possibly active, one along the north wall of West Blanco Depression (WBD), another that runs obliquely through the WBD to the Juan de Fuca Ridge tip, and a third along WBD's south wall. The complex faulting pattern is possibly related to slight plate motion adjustments. We found no normal faulting events near the basins at BTFZ's western end. The average $291\deg$ slip-vector azimuth of all strike-slip events is parallel to the $290\deg$ Pacific-Juan de Fuca motion direction. T-axes of normal faulting events in the Cascadia Depression are rotated $\sim$$45\deg$ relative to all strike-slip and the normal events near the Gorda and Surveyor depressions. A $45\deg$ rotation is generally observed for transform-ridge systems suggesting Cascadia Depression is a short spreading ridge while Gorda and Surveyor depressions are pull-apart basins. Our detailed analysis of the BTFZ suggests oceanic transform fault zones have complexities comparable to their continental counterparts.
DE: 7220 Oceanic crust
DE: 7230 Seismicity and seismotectonics
DE: 8123 Dynamics, seismotectonics
DE: 8150 Plate boundary--general (3040)
DE: 7215 Earthquake parameters
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting