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Boyd, T.M., 1988

Seismic studies of the Aleutian arc

Bibliographic Reference

Boyd, T.M., 1988, Seismic studies of the Aleutian arc: New York City, New York, Columbia University, Teachers College, Ph.D. dissertation, 260 p., illust., maps.


The structure of the descending oceanic lithosphere, and the nature of seismicity associated with convergent plate margins is studied in a series of reports based on observations derived from the Aleutian Island Arc. The first portion of this dissertation contains a group of studies whose primary goal is to better constrain seismic source parameters for the accurate determination of seismic hazard estimates. Seventeen earthquakes with surface-wave magnitudes above 6.8 that were recorded between 1898 and 1917 are located using P, S, and S-P arrival times with the a priori constraint that the final location must be near the geographically defined arc. The arc-proximity constraint provides tectonically meaningful locations which fit the travel-time observations as well as the unconstrained locations. Of the four events suggested as being possible candidates for rupturing either the Shumagin or Unalaska Seismic Gaps, none relocate into these arc segments. One other large event ($M\sb{s}$ = 7.9), however, did rupture the Shumagin Gap in 1917. The documentation of this event provides strong evidence that up to 70% of the Shumagin Gap has failed seismically during the last 90 years. Source parameters of the recent Atka Islands Earthquake (1986; $M\sb{s}$ = 7.7) indicate that bilateral rupture along a 145 km long portion of the arc occurred with an average slip of about 237 cm. Although this event occurred only 29 years after the great 1957 earthquake ($M\sb{w}$ = 8.6), the amount of slip released is consistent with that accumulated since 1957. The second portion of this dissertation considers the constraints place on the subduction zone geometry by seismic observations. The structure of the subducting Aleutian slab is estimated by modeling travel-time residuals and by constructing a kinematic model of slab flow. P-wave travel-time residuals require slab penetration to depths of at least 300 km below the deepest seismicity along the entire arc. Our best estimate of the thermal coefficient of velocity (P-wave), $-$0.5 m/s/$\sp\circ$K, is consistent with that obtained from other travel-time studies and from laboratory experiments. Slab flow is modeled, assuming the slab's geometry is known, by approximating the slab as a thin Newtonian sheet subducted into a less viscous mantle. We require the flow field to have a minimum dissipation power. The maximum depth of seismicity along the entire arc coincides with a constant fraction of mantle's solidus temperature, consistent with the hypothesis that seismicity terminates with the initiation of high-temperature, steady-state creep. The in-plane strain-rates calculated from the flow model are highest (10$\sp{-15}$/s) in the Central Aleutians, and their orientations are consistent with the orientations of source mechanism P and T axes.

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