GREAT SUBDUCTION ZONE EARTHQUAKES AND RAPID FOREARC BASIN FORMATION AND SEDIMENT FILLING--EXAMPLE OF THE ATKA FOREARC BASIN, ANDREANOF SECTOR OF THE ALEUTIAN ISLANDS

During the past 50 years two great Aleutian megathrust earthquakes (1957 Mw8.7; 1986 Mw8.0) nucleated below the deeply submerged (4-5 km) Atka Basin sector (~174 degs W) of the laterally continuous deep water (3000-4000 m) Aleutian forearc basin. During these events the highest co-seismic slip (i.e., moment release) occurred beneath the structural low of Atka Basin. The giant1957 rupture launched a large transoceanic tsunami.

Seismic data, dredging, and DSDP drilling document that Atka Basin is a young, rapidly subsiding structural depression underlain by thinned arc crust. The basin is bowl-shaped in structural profile, un-bound by high-angle faults with significant throw, but fronted to seaward by Hawley Ridge, a rapidly rising, outer forearc antiformal high that in turn is fronted by a major right-lateral shear zone paralleling the Aleutian Trench. Atka Basin is filled with 2-3 km of late Cenozoic deposits that involve arc-shed turbiditic and mass wasting deposits. GPS studies attest that the underlying subduction zone is effectively locked beneath the deepening Atka Basin and rising Hawley Ridge, but unlocked to the east where the laterally continuous Aleutian forearc basin is weakly formed or is inverting and large megathrust earthquakes have not been recorded.

It is not clear why the Atka Basin area is the locus of rapid subsidence and great earthquakes. But the coincidence of low-angle underthrusting of a rough seafloor, focused interplate coupling, rapid basal subduction erosion, and a structurally strong sector of arc massif, are likely involved. Basin formation and filling also seems to be facilitated by the insertion of a thick (>1.5 km), laterally continuous section of trench sediment into the subduction channel separating the arc massif from the underthrusting Pacific plate.

Repeated nucleation of high magnitude earthquakes appears to facilitate active forearc basin subsidence. These great strain-releasing events have been posited (Wells et al., 2003) to hasten basal subduction erosion and, in positive feedback, continue forearc basin deepening. The implication of the Atka Basin setting is that high magnitude megathrust earthquakes can be importantly involved in the evolution and location of forearc basins.