AN: T72A-05TI: Plate Boundary Deformation Between the Pacific and North    America in the Explorer RegionAU: Corne KreemerAF: Inst Earth Sciences, Utrecht University, The NetherlandsEM: kreemer@geof.ruu.nlAU: Kevin P. FurlongEM: kevin@geodyn.psu.eduAU: William HoltAF: Dept. Earth and Space Sciences, SUNY Stony Brook, Stony Brook,    NYEM: wholt@seism1.ess.sunysb.eduAB:                 The Pacific - North America plate boundary between the    northern end of the Juan de Fuca Ridge and the Queen    Charlotte fault is problematic.  Marine bathymetric    features indicate a transform (Sovanco deformation zone) -    ridge (Explorer Ridge) link in this region which has led to    the defining of a small micro-plate in the region termed    the Explorer plate.  Patterns of seismicity and the    locations of presently active faulting indicate that the    primary zone of deformation is within the Explorer and show    Pacific - North America relative motion.  We have analyzed    the strain information contained in earthquakes (following    the approach of Haines and Holt, 1993) occurring in this    region to determine the present day plate motions and the    plate boundary deformation field.  Using approximately 66    of the larger earthquakes which have occurred in the region    since the 1940s we have determined the following:  (1) the    earthquakes define a strain and displacement field which is    indistinguishable in orientation from the expected Pacific    - North America relative motions in the region spanning the    gap between the northern Juan de Fuca Ridge and the Queen    Charlotte transform; (2) for appropriate assumptions of    elastic moduli, lithospheric thermal structure and depth    extent of seismicity, the earthquake strains in the belt of    seismicity within the Explorer plate during the period of ~    1948 to present represent approximately 25 - 50 \% of the    expected plate boundary strain; (3) the complete predicted    plate boundary strain field could be accounted for by the    occurrence of an additional 1 or 2 moderate sized    earthquakes (M $\approx$ 7.0) in the region; (4) seismicity along    the inferred Nootka fault zone (northern boundary of the    Juan de Fuca plate) does not account for any significant    fraction of expected plate boundary strain, although slip    vectors of earthquakes indicates Juan de Fuca - North    America motion.  These results indicate that the seismicity    maps an existing/developing plate boundary between the    Pacific and North America which serves as the primary locus    of strain accommodation in the region.  This plate boundary    segment is unlikely to experience earthquakes larger than $\approx$        M = 7.0, in contrast to the Queen Charlotte transform which    has experienced substantially larger earthquakes (M $\approx$ 8.0).        The mechanism of strain accommodation along the Nootka    fault zone is unclear and may be closely linked to strain    release along the northern Cascadia zone.      SC: TDE: DE: DE: MN: Fall Meeting 1996