Seismic rupture and arc segmentation within the Shumagin Islands Seismic Gap, Alaska

Abstract

The locations of teleseismically well‐recorded earthquakes suggest that up to 80% of the Shumagin Gap has ruptured seismically during the last 90 years. An earthquake in 1917 (M_s=7.9) nucleated near the gap's eastern edge and appears to have ruptured as much as a 200 km long section of the gap, and an event in 1948 (M_s=7.5) appears to have ruptured the deepest portion of the main thrust‐zone (> 30 km) in the central section of the gap. The rupture extents of these earthquakes suggest that the gap can be divided into two or three smaller arc segments. These observations do not preclude the possibility of failure of the entire region in a single great earthquake (M_w=8.5) in the near future. The two historically documented recurrence intervals for the eastern portion of the gap are 59 and 70 years. If the average recurrence interval for large earthquakes near the Shumagin Islands is 65 years, the conditional probability of a large earthquake rupturing this portion of the arc in the next 20 years is between 62 and 96%.