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.

Absence of strain accumulation in the Western Shumagin Segment of the Alaska Subduction Zone

“…The extremal bounds results suggest a way to reconcile the apparently contradictory observation of very small or zero strain accumulation in the forearc with the existence of historical great earthquakes in the Shumagin Islands [Davies et al, 1981] and M-7 earthquakes involving the Shumagin segment in 1917 [Estabrook and Boyd, 1992] and 1993. Even though the simplest interpretation of the GPS results is that the plate interface is freely slipping, we cannot rule out the existence of a small locked region near the trench of width up to 55 km.…”

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

“…Flat-topped ridges at 6-18 m and 40-200 m inland of the storm berm along the northwest coast are the remains of wind-eroded dunes rather than remnants of marine terraces uplifted during prehistoric earthquakes. Because there are no other reports of strong earthquake effects and a regional tsunami was not recorded, the 1880 earthquake probably occurred on an upper plate fault near Chirikof Island, possibly producing the prominent reverse-fault scarp reported in 1962 by Moore (in the Supplemental File [see footnote 1]; Davies et al, 1981;Lander, 1996;e.g., Liberty et al, 2013).…”

“…Radiocarbon ages and core stratigraphy reported here show that the southwest coast of the island has been glaciated, but was ice free by 13.4 ka. Strong earthquakes in the Chirikof Island region, the ground motions and/or tsunamis of which reportedly affected the island, include those in 1788, 1826, 1833, 1847, 1848, 1880, 1938, and 1964( Davies et al, 1981Soloviev, 1990;Lander, 1996 1962, 1963, 2005, and 2013 commented on the rapid erosion of coastal archaeological sites by wind, waves, and artifact hunters, exacerbated through trampling by feral cattle introduced late in the nineteenth century (Keithahn, 1962;Workman, 1966;Chaffin et al, 1983; Saltonstall and Steffian in the Supplemental File [see footnote 1]; Clark in the Supplemental File [see footnote 1]; MacNeil et al, 2007).…”

National Earthquake Hazards Reduction Program; summaries of technical reports, Volume XXIV, prepared by participants in the National Earthquake Hazards Reduction Program

“…Previous great earthquakes that probably ruptured the Shumagin gap occurred in 1788 and 1847 (Davies et al, 1981). The unexplainably long time period between 1847 and 1987 was one of the reasons the earthquake potential for the Shumagins was in question.…”

“…We plan to search local newspapers to see if we can obtain more details about the intensity of shaking. Other earthquakes between 1899 and 1903 which Davies et al (1981) considered possible gap rupturing events have been relocated outside the gap by Boyd and Lerner-Lam (1987) using the added constraint that events must lie along the arc.…”