Alaska Megathrust 2: Imaging the megathrust zone and Yakutat/Pacific plate interface in the Alaska subduction zone

Abstract

We image the slab underneath a 450 km long transect of the Alaska subduction zone to investigate (1) the geometry and velocity structure of the downgoing plate and their relationship to slab seismicity and (2) the interplate coupled zone where the great 1964 earthquake (M w 9.2) exhibited the largest amount of rupture. The joint teleseismic migration of two array data sets based on receiver functions (RFs) reveals a prominent, shallow-dipping low-velocity layer at~25-30 km depth in southern Alaska. Modeling of RF amplitudes suggests the existence of a thin layer (V s of~2.1-2.6 km/s) that is~20-40% slower than underlying oceanic crustal velocities, and is sandwiched between the subducted slab and the overriding plate. The observed megathrust layer (with V p /V s of 1.9-2.3) may be due to a thick sediment input from the trench in combination with elevated pore fluid pressure in the channel. Our image also includes an unusually thick low-velocity crust subducting with a~20°dip down to 130 km depth at~200 km inland beneath central Alaska. The unusual nature of this subducted segment results from the subduction of the Yakutat terrane crust. Our imaged western edge of the Yakutat terrane aligns with the western end of a geodetically locked patch with high slip deficit, and coincides with the boundary of aftershock events from the 1964 earthquake. It appears that this sharp change in the nature of the downgoing plate could control the slip distribution of great earthquakes on this plate interface.

Receiver functions using OBS data: promises and limitations from numerical modelling and examples from the Cascadia Initiative

“…In particular, receiver functions have been useful in characterising the variations in the thickness and composition of continental crust (Zandt & Ammon, 1995), the geological processes active at subduction zones as the oceanic plate dives underneath a continent or another oceanic plate (e.g. Bostock et al, 2002;Abers et al, 2009;Kim et al, 2014), in continental transform settings (e.g. Ozacar & Zandt, 2009;Porter et al, 2011;Audet, 2015;Frederiksen et al, 2015) as well as the entanglement and geological transformation of crustal material in large collisional settings (e.g., Nábȇlek et al, 2009).…”

Seismic properties of the Nazca oceanic crust in southern Peruvian subduction system

“…The surface of the Nazca Ridge is imaged to be quite rugged, featuring many seamounts that penetrate the sediment cover (Hampel et al, 2004;Krabbenhoft et al, 2004). The thickness of the sediment layer is thin but variable, and sporadically missing along the margin (from 8 • S to 17 • S) (Krabbenhoft et al, 2004).…”

“…The thickness of the sediment layer is thin but variable, and sporadically missing along the margin (from 8 • S to 17 • S) (Krabbenhoft et al, 2004). The total thickness of the sedimentary section atop the Nazca Ridge at a drill site 1237 (Fig.…”

Earthquake Segment Boundaries and Tsunamigenic Faults of the Kodiak Segment, Alaska-Aleutian Subduction Zone

“…In contrast to the PWS segment, the Kenai and Kodiak segments involve the subduction of only Pacific plate rocks and contains less sediment above incoming plate mafic rocks. Higher density and greater westward age of the subducting oceanic lithosphere result in a steepening of the subduction angle (Plafker et al, 1994;von Huene et al, 1980;Eberhart-Phillips et al, 2006;Kim et al, 2014). The presumed subduction angle for the Kodiak Islands region is about 8 degrees and steepens to the north beneath mainland Alaska (Hayes et al, 2012).…”

“…Many studies have identified and characterized the PWS segment properties through various geophysical and geological investigations (Brocher et al, 1994;Eberhart-Phillips et al, 2006;Liberty et al, 2013;Kim et al, 2014). Similar literature exists for the Semidi segment that spans the region to the southwest of the Kodiak Islands (Johnson and Satake, 1994;Fournier and Freymueller, 2007;Shillington et al, 2015).…”