Landslides and Megathrust Splay Faults Captured by the Late Holocene Sediment Record of Eastern Prince William Sound, Alaska

Abstract

We present new marine seismic-reflection profiles and bathymetric maps to characterize Holocene depositional patterns, submarine landslides, and active faults beneath eastern and central Prince William Sound (PWS), Alaska, which is the eastern rupture patch of the 1964 M w 9.2 earthquake. We show evidence that submarine landslides, many of which are likely earthquake triggered, repeatedly released along the southern margin of Orca Bay in eastern PWS. We document motion on reverse faults during the 1964 Great Alaska earthquake and estimate late Holocene slip rates for these growth faults, which splay from the subduction zone megathrust. Regional bathymetric lineations help define the faults that extend 40-70 km in length, some of which show slip rates as great as 3:75 mm=yr. We infer that faults mapped below eastern PWS connect to faults mapped beneath central PWS and possibly onto the Alaska mainland via an en echelon style of faulting. Moderate (M w > 4) upper-plate earthquakes since 1964 give rise to the possibility that these faults may rupture independently to potentially generate M w 7-8 earthquakes, and that these earthquakes could damage local infrastructure from ground shaking. Submarine landslides, regardless of the source of initiation, could generate local tsunamis to produce large run-ups along nearby shorelines. In a more general sense, the PWS area shows that faults that splay from the underlying plate boundary present proximal, perhaps independent seismic sources within the accretionary prism, creating a broad zone of potential surface rupture that can extend inland 150 km or more from subduction zone trenches.

Submarine Landslide Kinematics Derived From High‐Resolution Imaging in Port Valdez, Alaska

“…The process of ground failure and tsunamigenesis that occurred in Port Valdez is likely similar to events documented throughout the Prince William Sound and Kenai Peninsula region during the 1964 earthquake (Brothers et al, 2016; Finn et al, 2015; Haeussler et al, 2015; Plafker et al, 1969), as well as during some strike‐slip earthquakes in southeast Alaska (Keefer, 1984; Miller, 1960, 1973). Recent geophysical studies have illuminated the locations of submarine landslide debris fields within several coastal fjord settings, characterizing fjord morphology from multibeam bathymetry and subbottom profiling in Port Valdez (Lee et al, 2006, 2007; Parsons et al, 2014; Ryan et al, 2010), Passage Canal near Whittier (Haeussler et al, 2012), Resurrection Bay, near Seward (Haeussler et al, 2007; Lee et al, 2003), and Chenega (Brothers et al, 2016).…”

“…The process of ground failure and tsunamigenesis that occurred in Port Valdez is likely similar to events documented throughout the Prince William Sound and Kenai Peninsula region during the 1964 earthquake (Brothers et al, 2016;Finn et al, 2015;Haeussler et al, 2015;Plafker et al, 1969), as well as during some strike-slip earthquakes in southeast Alaska (Keefer, 1984;Miller, 1960Miller, , 1973. Recent geophysical studies have illuminated the locations of submarine landslide debris fields within several coastal fjord settings, Plafker et al (1969).…”

“…Submarine landslides in coastal fjords and on the continental slope frequently accompany strong ground shaking during large earthquakes. These landslides are important mechanisms for glaciomarine sediment redistribution (Finn et al, 2015;Lee et al, 2003Lee et al, , 2006) and a significant geological hazard that can produce local tsunamis with large run-ups (Hampton et al, 1996;Locat & Lee, 2002;Miller, 1960;Yalçiner et al, 2012). In southern Alaska, tsunamis associated with submarine landslides that were triggered by the 1964 M9.2 "Great Alaska" earthquake devastated several communities throughout Prince William Sound adjacent to the 1964 rupture area (Figure 1) including Whittier, Seward, Chenega, and Valdez (Brothers et al, 2016;Haeussler et al, 2007Haeussler et al, , 2012Lee et al, 2007;Ryan et al, 2010).…”

Quantifying offshore fore‐arc deformation and splay‐fault slip using drowned Pleistocene shorelines, Arauco Bay, Chile

“…The Lachlan fault in New Zealand shares similar kinematics and slip rates with the SMFS (3.0–6.5 m/ka) and has the potential to produce M w 7.6 to 8 earthquakes with a recurrence time of ~1 ka [ Barnes et al , ]. Several splay faults have been mapped in the Prince Williams Sound, Alaska, at ~150 km from the trench slip with slip rates of ~3.7 m/ka and the potential to produce M w 7–8 earthquakes [ Finn et al , ]. In turn, splay faults located at ~50 km from the trench slip at up to ~9 m/ka [ Liberty et al , ] and earthquakes on these structures have an estimated recurrence time of ~790 years, expecting to be reactivated during most megathrust earthquakes.…”

“…In turn, splay faults located at ~50 km from the trench slip at up to ~9 m/ka [ Liberty et al , ] and earthquakes on these structures have an estimated recurrence time of ~790 years, expecting to be reactivated during most megathrust earthquakes. Splay‐fault ruptures in Alaska have been associated with tsunamis [ Liberty et al , ; Finn et al , ; von Huene et al , ]. Liberty et al [] showed that many of these splay faults slipped together during the 1964 Alaska earthquake forming metric scarps on the seafloor and acting as a secondary source for the tsunami.…”