Yakataga Gap, Alaska: Seismic History and Earthquake Potential

Abstract

A 250-kilometer-long seismic gap in southern Alaska, which is situated along the boundary between the North American and Pacific plates, ruptured in two great earthquakes in 1899. Within the gap, earthquakes of moderate size form a ring of activity around a region of very low seismicity. The number of shocks of magnitude 6 or larger in this ring appears to have increased significantly since the 1958 earthquake, which occurred on the adjacent part of the plate boundary. This space-time pattern is similar to long-term patterns that preceded several large earthquakes in Japan. A shock of magnitude 7.7 on 28 February 1979 ruptured only a small part of the seismic gap. The remaining part, which already may have stored sufficient strain to generate a great shock, warrants intensive study to evaluate its potential for such an event.

Gravity anomalies, crustal structure, and seismicity at subduction zones: 2. Interrelationships between fore‐arc structure and seismogenic behavior

“…East of the Beringian margin, the transition to ocean‐continent subduction is characterized in the landward half of the fore arc by a region of positive residual free‐air gravity anomalies that is well correlated with the extensively studied Shumagin Islands seismic gap [ Sykes , ; Kelleher et al ., ; McCann et al ., ; Davies et al ., ; Boyd and Jacob , ]. This gap is likely associated with the transition in overthrusting plate structure and is defined by the absence of slip in the 1938 Mw 8.2 earthquake [ Johnson et al ., ] and 1957 Mw 8.7 Andreanof Islands earthquake [ Johnson et al ., ].…”

Deformation in the Yakataga Seismic Gap, southern Alaska, 1980–1986

“…The Yakataga seismic gap (south coast of Alaska between 140øW and 144øW, Figure 1), the site of collision between the north-northwestward moving Yakutat block and the North American plate, is thought to have a high potential for a major thrust earthquake within the next 20 years [McCann et al, 1980' Jacob, 1984. The gap apparently last ruptured in a sequence of great earthquakes in September 1899.…”

“…vations in the preseismic interval might then be extrapolated to suggest that the Chugach-St. Elias fault or one of the subparallel imbricate thrusts represents the greatest seismic hazard in the Yakataga seismic gap. The previous rupture of the Yakataga seismic gap (September 3, 1899[McCann et al, 1980]) is not well enough described that the causative fault can be identified. The coast at Cape Yakataga (near station Furr inFigure 4) was uplifted about 1 m at the time of that earthquake[Tarr and Martin, 1912], but no significant tsunami was reported[Cox and Pararas-Carayannis, 1976].…”

Strain accumulation in the Yakataga Seismic Gap, southern Alaska

“…The Yakataga seismic gap (Figure 1), described in detail by McCann et al [1980], has been identified as a region with a high potential for a great earthquake within the next two decades [Jacob, 1984]. The gap is bounded on the west by the rupture area of the 1964 Alaska earthquake (Mw -9.2) and on the east by the 1958 earthquake (Mw = 7.7) on the Fairweather transform fault (Figure 1).…”

“…The St. Elias earthquake (Mw = 7.6) ruptured the eastern part of the Yakataga gap in February 1979 (Figure 1). At least part of the gap was previously ruptured in the sequence of great earthquakes that occurred near Yakutat in September 1899 [McCann et al, 1980;Thatcher and Plafker, 1977].…”