Shumagin Seismic Gap, Alaska Peninsula: History of great earthquakes, tectonic setting, and evidence for high seismic potential

Abstract

The Shumagin seismic gap, a segment of the plate boundary along the eastern Aleutian arc, has not ruptured during a great earthquake since at least 1899–1903. Because at least 77 years have elapsed since the Shumagin Gap last ruptured in a great earthquake and repeat times for the 1938 rupture zone and part of the Shumagin Gap are estimated to be 50 to 90 years, a high probability exists for a great earthquake to occur within the Shumagin Gap during the next one to two decades. Reconsideration of the rupture zones of the Aleutian earthquakes of 1938, 1946, and 1948 suggests that those events did not break the interplate boundary beneath the Shumagin Islands. Thus, the Shumagin seismic gap extends from the western end of the 1938 rupture zone to the eastern end of that of 1946. These boundaries also coincide with transverse structural features. At least the eastern half of the Shumagin Gap broke in great earthquakes in 1788 and 1847 and possibly in 1898–1903. The Shumagin Gap is probably not the result of aseismic slip; rather, plate motion is accommodated there seismically and episodically and can be expected to produce large earthquakes in the future. Although there is no definitive evidence of long‐term precursors of a possible future earthquake, several observations suggest that the Shumagin Gap is in an advanced stage of the earthquake cycle. Both teleseismic and local network data indicate a near absence of seismic activity (M ≥ 2) above a depth of 30 km along the main thrust zone within the gap; this is in strong contrast to adjacent portions of the arc where seismic activity is scattered across most of the main thrust zone. Two earthquakes with high stress drops (600–900 bars), which occurred at the base of the main thrust zone, may indicate the accumulation of a considerable amount of strain energy within the gap. A possible seismic gap at the eastern end of the aftershock zone of the Aleutian earthquake of 1957 has been identified near Unalaska Island. An earthquake that nucleates in the Shumagin Gap could also rupture the possible Unalaska Gap to the west, the 1938 aftershock zone to the east, or both, with resultant magnitude up to Mw = 9.0. Alternatively, the Shumagin Gap alone, or in one of the above combinations could rupture in a series of very large earthquakes instead of a single great shock. Past Alaska‐Aleutian earthquakes, including those of 1788, 1938, 1946, 1957, 1964, and 1965, have generated very large tsunamis. Future large earthquakes in the Shumagin Islands region could generate wave heights of several tens of meters along shorelines near the rupture areas. The Shumagin Gap is one of two major gaps along the United States portion of the Alaska‐Aleutian plate boundary and is one of the few areas in the United States where processes leading to a great earthquake are likely to be observed within a reasonable span of time.