Opposing gradients of permanent strain in the aseismic zone and elastic strain across the seismogenic zone of the Kodiak shelf and slope, Alaska

An industry-acquired seismic reflection line from the Aleutian Are to the A laska Trench was prestack depth-migrated and further constrained with wide- angle ocean bottom hydrophone seismic velocity data and swath mapping. From the seismic reflection image, individual thrust slices, the backstop, and the forearc basin were resolved sufficiently well to balance the section. T ectonic shortening and age estimates provided rates of permanent contractio n in five discrete segments over similar to 150 km during the past 3 Myr. A bout 70% of the plate convergence is measured as contraction across the sim ilar to 40-km-wide Neogene accretionary prism which composes less than 10% of the forearc. Permanent contraction is minimal above the 1964 Alaska eart hquake aftershock zone indicating dominently elastic strain. Since elastic strain is poorly recorded in the geological record, it was approximated fro m the historical record of the 1964 Alaska earthquake. The 1964 coseismic v ertical deformation displays an inverse pattern to the permanent strain, co nsistent with the earthquake cycle concept. Elastic deformation is dominant beneath the shelf and upper slope, rapid permanent deformation is concentr ated beneath the lower slope, and a transition between them occurs beneath the middle slope.