5.2 Evidence of Rupture Across a Heterogeneous Plate Interface
The simple plate interface observed in our Kodiak study compared to the more complicated plate interface structure beneath the Kenai Peninsula supports other evidence that the 1964 earthquake ruptured across distinctive asperities. During the 1964 event, the northern Kenai asperity slipped an average of 18 m, while Kodiak slipped an average of ~10 m (Johnson et al., 1996) Major earthquakes in the Kenai area have a recurrence interval of 700-800 years (Wesson et al., 1999) and the plate interface is strongly locked (Zweck et al., 2002). In Kodiak, major earthquake recurrence interval is 60 years (Nishenko & Jacob, 1990) hile the southern end of Kodiak interface appears strongly locked (S. Li & Freymueller, 2018), locking decreases to the north. Subduction geometry in the Kenai segment is controlled by subduction of the Yakutat microplate, a thick, buoyant oceanic plateau (Christeson et al., 2010) and a thick, subducting sediment package (Y. Kim et al., 2014; Worthington et al., 2012). The plate interface here dips shallowly at ~3-4 degrees. In Kodiak, the Pacific plate subducts beneath North America at ~8 degrees, and incoming plate structure includes ~2.5 km-thick sediments from the distal Surveyor Fan (Reece et al., 2011) and the Kodiak-Bowie seamount chain (Fig. 1a).
In their study of Kodiak region seismicity between 1964 and 2001, Doser et al. (2002) found that, while most earthquakes occur within the downgoing plate, several events beneath southern Kodiak Island have depths and thrust faulting mechanisms consistent with seismicity on the interface. Detailed seismicity studies on the Kenai Peninsula using the MOOS array show a well-defined seismic zone concentrated in the down-going plate, just below the plate boundary, that parallels the megathrust zone and is dominated by normal faulting mechanisms (J. Li et al., 2013). In contrast to observations in the Kodiak region, active thrusting and seismicity on the plate interface itself was absent (J. Li et al., 2013), possibly related to thick sediment subduction that forms a smooth interface between the North American and Yakutat plates. Shallow seismicity in the upper plate is spread out across the Kenai Peninsula without clear lineation and focal mechanisms are thrust or strike-slip.
Large megathrust e at other subduction zones, such as the 1700 M 9.0 Cascadia (Wang et al., 2013), 2011 M 9.0 Tohoku-Oki (Wei et al., 2012), 2004 M 9.2 Sumatra (Chlieh et al., 2007), and the 2011 M 8.8 in Chile (Lorito et al., 2011) events encompassed patches of slip rates different from the ambient slip rates within their rupture extents. The ubiquity of heterogeneous coseismic slip during large earthquakes further illustrates that the Great Alaska earthquake entraining multiple major segments during rupture is not unique to the Alaska subduction zone.