AN: T72A-07TI: Seismotectonics of the Explorer Plate RegionAU: Jochen BraunmillerAU: John NabelekAU: Beate LeitnerAF: COAS, OSU, Corvallis, OR 97331EM: jbraunmi@oce.orst.eduAB:             The tectonics of the Explorer plate is poorly understood.    The region is seismically very active, but routine locations    indicate broadly distributed seismicity. We determine source    mechanisms and improved locations of larger events and    show that they outline a rather consistent tectonic picture.        For the period 1994 to 1996, we determined moment-tensor    solutions for 29 earthquakes (M$\geq$4) by inverting    regional broadband seismograms. With additional 26 Harvard    CMT-solutions existing for the study area, they comprise the    most reliable and complete set of earthquake source    parameters for the region to date. All fault-plane    solutions, except one from the Juan de Fuca ridge, are    strike slip and are associated with transform motion along    the plate boundaries or internal deformation of the Explorer    and Pacific plates.        We relocate all large earthquakes occurring between    $47\deg$-$54\deg$N and $135\deg$-$125\deg$W and the years    1964 to 1995 (117 events) using joint epicenter    determination technique applied to teleseismic arrival-time    data. Relocation moves the epicenters on average 25 km    west-southwest. Most epicenters fall on the plate boundaries    delineated by bathymetry; however a few events, particularly    in the Sovanco fracture zone area, remain within the    Explorer and Pacific plates.        On the Revere-Dellwood transform fault, relocated epicenters    narrowly follow the linear bathymetric feature connecting    the Explorer ridge with the Tuzo Wilson sea mounts. The    average slip vector direction (about $330\deg$) is    consistent with the bathymetric trend of about $325\deg$.    Bathymetry of the northern Explorer ridge shows two    prominent structures, the eastern and western rift zone.    Relocated epicenters reveal well-focused activity on both     strands. The southern Explorer ridge and the western Sovanco    fracture zone are relatively aseismic. The Nootka fault zone    is well defined by a narrow band of epicenters;    moment-tensor solutions are consistent with left-lateral    motion.        Relocated epicenters near Sovanco fracture zone's eastern    part are broadly distributed. The fracture zone's average    trend, defined by connecting Juan de Fuca ridge's northern    tip with the southern Explorer ridge, is about $295\deg$.    Average slip vectors, however, are more northerly oriented    (about $310\deg$). Small scale bathymetry shows prominent    uplifted blocks bordered by northwest ($310\deg$) and    northeast ($55\deg$) trending fault scarps consistent with    the slip vectors. This suggests the Sovanco fracture zone is    undergoing clockwise rotation, which is supported by plate    tectonic reconstruction. Current motion probably occurs both    on $310\deg$ trending primary faults and shorter antithetic    strands as indicated by broadly distributed epicenters. A     few rather large (M$\geq$5.5) earthquakes to the north and    south of the fracture zone suggest internal deformation of    the Explorer and Pacific plates accompanying the plate    boundary's reorientation.    SC: TDE: 8150 Plate boundary structuresDE: 7215 Earthquake parametersDE: 7230 SeismicityMN: Fall Meeting 1996