Subduction zones worldwide exhibit remarkable variations in seismic activity and slip behavior along strike and down dip. Active-source seismic data can provide essential constraints on the properties of the plate boundary, the distribution of fluids and hydration, and deformation in the overriding and subducting plate, all of which can be used to assess possible contributions to this variability. Here I describe results from a marine active-source seismic study of the subduction zone offshore Alaska.
MCS reflection and wide-angle seismic data were collected offshore from the Alaska Peninsula in the summer of 2011 aboard the R/V Langseth during the Alaska Langseth Experiment to Understand the megaThrust (ALEUT) program. This region encompasses the full spectrum of coupling: 1) the weakly coupled Shumagin Gap; 2) the Semidi segment, which last ruptured in the 1938 M8.2 event, appears to be locked at present, and 3) the western Kodiak asperity, which marked the western extent of the 1964 M9.2 rupture and also appears to be locked. It also exhibits substantial variations in seismicity. ALEUT data reveal changes along-strike in incoming sediment thickness and plate structure and along-strike and downdip variations in megathrust reflection characteristics.
Remarkable variations in the style and amount of bending faulting and hydration in the subducting oceanic plate are observed along strike, which appear to be controlled by the local relationship between the orientations of pre-existing structures in the incoming oceanic plate and the subduction zone. Significantly more bending faulting and hydration are observed in the Shumagin Gap, where pre-existing structures are favorably aligned, than the Semidi segment. The thickness of sediment on the incoming plate also changes along strike. Over 1 km of sediment is observed on the incoming oceanic plate in the Semidi segment prior to subduction, and a relatively thick and continuous layer interpreted as subducted sediment can be imaged at the plate boundary here up to ~50 km from the trench. In the Shumagin Gap, where the incoming sediment section is half as thick and more pervasively faulted at the outer rise, a subducting sediment layer is also observed but it is thinner, less continuous and is not observed to continue as far from the trench. These changes in bending faulting, hydration and sediment thickness correlate with variations in interplate and intermediate depth intraslab seismicity. Although the Semidi segment is capable of producing great earthquakes, the comparatively thick sediment here may contribute to the relative paucity of interplate seismicity compared with adjacent segments. More intermediate depth seismicity is observed in the Shumagin Gap, where there is more hydration and bending faulting in the subducting plate.
Downdip variations in the characteristics of the plate boundary are also observed; a simple and bright reflection is generally observed at depths of ~12-25 km, ~40-100 km from the trench, within the center of the estimated locked zone. The character changes where the megathrust appears to intersect the forearc mantle wedge to a wide (~2 km thick), bright band of reflections and may arise from a change in deformation style, distribution of fluids, and/or plate boundary properties. Although the overall patterns in reflection characteristics are consistent between profiles across different segments, this transition in reflection characteristics occurs at larger distances from the trench within the Semidi segment than in the Shumagin Gap.
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