ALEUT project:

Alaska Langseth Experiment to Understand the megaThrust

 

The largest earthquakes, with magnitudes greater than 8-9, occur on subduction zone megathrusts. Tsunamis generated by the M9.2 1964 Alaska megathrust earthquake caused the greatest loss of life in Alaska, and subsidence, liquefaction, other ground failure, and strong ground motions caused extensive damage to Anchorage, containing most of Alaska’s infrastructure.  The single greatest uncertainty in probabilistic seismic hazards maps for megathrust events is the downdip limit of the region of maximum coseismic slip. This experiment will provide a critical test of a recent hypothesis that would, when tested and verified, allow for the direct mapping of the downdip end of the locked zone that ruptures during megathrust earthquakes. The study is focussed  on the 1938 rupture area that is likely well along its cycle of strain buildup to the next large earthquake.


During July 2011, the R/V Langseth, towing two 8km long streamers, was used to image reflections of seismic energy off of the interface between the two plates along a series of lines across the subduction zone west of Kodiak Island. Two profiles of seismic velocity were obtained using seismic travel time observations from 20 ocean bottom seismometers along two of the reflection lines. The experiment is lead by Donna Shillington, Mladen Medimovic (Dalhousie) and Spahr Webb. Anne Becel and graduate students Jiyao Li and Harold Kuehn  have joined the project. Several papers on results from the project are in press currently.



Schematic diagram of the great earthquake cycle adapted from Hyndman and Wang (1995). (a) Between events part of the megathrust becomes locked (strongly coupled) causing elastic deformation in the overlying continental crust. (b) Stresses build until the megathrust ruptures in a great or large earthquake. Locked, transition, slow slip and rupture zones in (a) and (b) are marked by dashed lines of different width and thickness. The main focus of our study is to test a recent hypothesis that the location of the downdip (landward) limit of the seismogenic portion of the megathrust can be imaged using reflection seismology.