Our project will focus on the subduction zone off southern Alaska, which produces large and destructive earthquakes. The Good Friday earthquake in 1964 was the second largest earthquake ever recorded (magnitude 9.2). The primary purpose of our study is to use marine seismic reflection and refraction data to: (1) estimate the size of the seismogenic zone, the portion of the fault that controls the magnitude of earthquakes, off southern Alaska, and (2) determine how the properties of the seismogenic zone change downdip and along the subduction zone, such that some areas produce large earthquakes and others do not. In particular, we will concentrate on the Semidi segment, which last ruptured in a great earthquake (magnitude 8.3) in 1938. This area has a recurrence interval of ~50-75 years, and thus might be due to produce another big earthquake relatively soon. We will acquire data using the R/V Marcus G. Langseth in July-August 2011.