Most of the world?s great earthquakes are shallow events that occur in association with the tectonic process of subduction. Damage and loss of life result not only from the earthquakes themselves, but from the tsunamis they commonly spawn as well. Regions in the U.S. that will certainly experience such events at some time in the future are Alaska and Cascadia (the coastal region extending from northern California to southern Canada). Estimating the ground motions that can be expected from these earthquakes is obviously a matter of considerable importance. Such estimates depend on the width of the subduction megathrust that produces great earthquakes ? the seismogenic zone; the nearer the landward edge of this zone is to the coast, the longer and stronger the ground motion is likely to be. Deep reflection seismic methods may provide a means of locating the downdip limit of the seismogenic zone. This project will test this method by acquiring marine multichannel (MCS) and wide angle reflection data over a large portion of the Alaska-Aleutian subduction zone. The reflection mapping results can then be compared with the earthquake rupture limit defined by existing earthquake aftershock and geodetic data.
Subduction zones are the origin of some of the most profound geohazards on Earth, including the largest earthquakes, destructive tsunamis, landslides and volcanic eruptions. However, many questions remain about the factors that are most important in controlling the magnitude and tsunamigenic potential of earthquakes generated in subduction zones. The size of subduction zone earthquakes is related to the size of the area on the plate boundary that moves during an earthquake. One of the major goals of our project is to use marine seismic imaging to map out variations in properties of the plate boundary and relate them to changes in seismic behavior to elucidate these controls. Our study focuses on the subduction zone offshore of the Alaska Peninsula (see map). This plate boundary exhibits major along-strike variations in the earthquake rupture history and the abundance of seismicity at all depths, making it an excellent place to study subduction processes. The Semidi segment ruptures in great earthquakes every ~50-75 years and appears to be locked at present, while the Shumagin Gap has not ruptured in a great earthquake for at least ~150 years, and may be creeping. In 2011, we collected a marine seismic data during a 37-day cruise aboard the R/V Langseth focused on these areas. Our study has revealed substantial, systematic variations in the characteristics of the plate boundary both along the subduction zone and with depth. We observe a major change in the seismic reflection properties of the plate boundary at larger depths there the subducting slab intersects the upper mantle. These changes may be related to changes in the style of deformation and/or the distribution of fluids, and could be important for controlling the deep limit of great earthquakes. At shallow depths, we can see a coherent layer of sediment subducting; the thickness and depth extent of this layer varies along the subduction zone and may be important for controlling the ability of earthquakes to occur at shallow depths, where they are more likely to be tsunamigenic. We also observe a major fault in the overriding plate, which appears to connect to the plate boundary and to have been recently active. It shares many characteristics with a normal fault offshore Japan that is thought to be important in promoting the tsunami during the 2011 M9.0 earthquake. Subduction zones are also sites of profound water delivery into the Earth; the subducting oceanic plate carries water to depth both in the pores of sediments and in hydrous minerals in the crust and upper mantle. Once it is carried into the earth, water can have major impact on the earthquake behavior, the production of magmas at arcs and Earth’s deep water cycle. We show that the amount of water carried to depth by the subducting oceanic plate can vary over short distances (~10’s km) along the plate boundary, controlled by inherited structures in the subducting oceanic plate, and cause abrupt changes in the abundance of earthquakes at a range of depths. Finally, our study also images dipping structures in the lower crust and upper mantle of oceanic lithosphere away from the subduction zone that was created 55 million years ago. This serendipitous finding provides new insights into magmatic and deformation processes involved in the creation and evolution of the oceanic plate.
