Figure - uploaded by P. J. Haeussler
Content may be subject to copyright.
Map focused on Prince William Sound showing active faults and crustal deformation during the 1964 earthquake. Topography is a greyscale hillshade. Bathymetry data from NOAA compilation, illumination is from the northwest. Active faults, in red, are from compilation of Plafker et al. (1994) and from our work. HBF, Hanning Bay fault; MSF, Montague Strait fault. Uplift and subsidence contours, in black, are in units of feet as originally mapped by Plafker (1969). Dashed white line is our inferred Last Glacial Maximum (LGM) ice limit. Green lines show the location of the TACT deep seismic profiles discussed in the text. Cross section along line A–A' is shown in Fig. 7. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Source publication
Megathrust splay faults have been identified as important for generating
tsunamis in some subduction zone earthquakes (1946 Nankai, 1964 Alaska,
2004 Sumatra). The larger role of megathrust splay faults in
accretionary prisms is not well known. In Alaska, we have new evidence
that megathrust splay faults are conduits for focused exhumation. In the...
Contexts in source publication
Context 1
... displacements in Prince William Sound reached 20 m at the surface, and slip along the megathrust fault plane has been modeled as up to 40 m ( Christenson and Beck, 1994;Holdahl and Sauber, 1994;Johnson et al., 1996;Ichinose et al., 2007). Plafker (1967Plafker ( , 1969 mapped surface rupture on two megathrust splay faults on Montague Island in southern Prince William Sound (Figs. 2 and 3). The principal fault is the Patton Bay Fault, which he inferred to extend most of the length of Montague Island and farther southwest to offshore of the town of Seward. ...Context 2
... the footwall of the Patton Bay thrust was also uplifted 5 m on Montague Island, which indicates another thrust fault lies beneath the Patton Bay thrust. A 70-km-long scarp associated with this lower fault, which we refer to as the Cape Cleare Fault, is clearly imaged offshore southwest of Montague Island on a compilation of bathymetric surveys ( Fig. 2) and in our new high-resolution seismic reflection data ( Liberty et al., 2013). This fault may constitute a left step-over in the fault system. Collectively, we refer to these thrusts as the Patton Bay megathrust splay fault system. Lastly, Plafker (1969) inferred a separate megathrust splay fault lies beneath Middleton Island, which ...Context 3
... (1969) inferred that a southwestern extension of the Patton Bay megathrust splay fault system caused the tsunami that hit the town of Seward about 30 min after the 1964 earthquake ( Fig. 2; Wilson and Tørum, 1972). Plafker (1969) noted that tsunami arrival times were consistent with a source that is located along strike from the mapped Patton Bay thrust (which we now under- stand to be the Cape Cleare Fault), and were too early to be from a source located farther toward the trench (see also Suleimani et al., 2010). ...Context 4
... collected in 1988. Refraction profiles and interpretations were previously published (Brocher et al., 1994;Fuis et al., 2008), and aspects of the reflection profiles are shown in Liberty et al. (2013). TACT profile B generally trends northwesterly across the strike of structures and shows the d ecollement and two megathrust splay fault systems (Figs. 2, 3A and 3B). One splay fault was imaged beneath Middleton Island, another beneath Wessels Reef. The principal conclusion from the imaging is that the faults clearly branch from the subduction d ecollement at a low angle. Also, the splay faults steepen upward within 2e6 km above the d ecollement. Although the TACT B pro- file 1 does not ...Context 5
... TACT Prince William Sound (PWS) profile reveals under- plating, shortening, and splay fault geometry beneath Montague Straight (Figs. 2, 3C, 3D, 3E). This NEeSW trending profile extends along Montague Strait, and is oriented at a low angle to the strike of the splay faults, which makes the line geometry less than ideal for simple visualization. Nonetheless, there are prominent fault-plane reflections from the Hanning Bay, Patton Bay, and Cape Clear Faults that parallel each other ...Context 6
... thrust fault. Instead, the seismic and bathymetric evidence, discussed below, indicate that it is a normal fault. Moreover, an M w 4.8 normal fault earthquake at a depth of 11 km occurred nearly on the fault trace, on 9 August 2012 (AEIC event id: ak10531690), and it was followed by a M2.8 aftershock at a depth of 4 km (event id: ak10531730) (see Fig. 2). Earthquake hypocenter errors in this area are several kilometers (N. Ruppert, personal comm., 2012), and if this earthquake did not occur on the Montague Strait Fault, then it likely occurred on one of the similar, but smaller, faults ...Context 7
... Montague Strait Fault is one of a number of high-angle active faults between Knight and Montague Islands (Figs. 2 and 5). Some faults are sea-bottom lineations and we interpret other faults to lie where acoustic basement is adjacent to Holocene sed- iments. ...Context 8
... seismic reflection data, and the 9 August 2012 earthquakes, lead us to conclude that there is a region of late PleistoceneeHolocene extension between Montague and Knight Islands. We collected high-resolution seismic data throughout Prince William Sound and normal faulting is limited to this region that extends eastward to Orca Bay near Cordova ( Fig. 2; Finn, 2012). Lastly, the character of faulting, and possibly the Montague Strait Fault, appears to change along strike to the southwest. A high- resolution seismic profile 25 km southwest of Prince William Sound, shows no evidence of normal faulting, and a fault in the along-strike location of the Montague Strait Fault is clearly a ...Context 9
... the world shows considerable variability in the tectonic setting and style of splay faulting, and at least some of these have a life span of several million years. Fig. 7. Schematic cross section from the Alaska-Aleutian trench to the Kenai Peninsula highlighting structures that we infer result in exhumation. Cross section location shown on Fig. 2. Red lines show extent of rupture in the 1964 earthquake based on aftershocks, surface faulting, geodesy, Plafker's (1969) interpretation, and our interpretation of the structure. Underplated region beneath Montague Island is depicted as much thicker than it likely is for clarity. (For interpretation of the references to color in this ...Citations
... The 1964 M w 9.2 Great Alaska Earthquake ruptured >900 km of the Alaska-Aleutian subduction zone (AASZ) and produced a regional and Pacific-wide tsunami that impacted coastlines in Alaska, British Columbia, Hawaii, Oregon, and California (Plafker, 1969). The earthquake triggered a series of reverse faults that connect with the AASZ at seismogenic depths (Plafker, 1965(Plafker, , 1967(Plafker, , 1969Haeussler et al., 2015; Figure 1). These splay faults, collectively known as the Patton Bay splay fault system (herein referred to as the splay fault system) increased uplift of the overriding plate, generated destructive local tsunami waves, and elevated inundation along the coast of the Kenai Peninsula (Lemke, 1967;Suleimani & Freymueller, 2020;Wilson & Tørum, 1972). ...
... MI = Montague Island. (c) Cross section of 1964 CE deformation on the subduction zone and splay faults through the upper plate, modified from Haeussler et al. (2015). Extent of 1964 rupture shown in red. ...
... Gray shaded area shows the Patton Bay splay fault system. (d) Normal (red line) and reverse (red line with teeth) faults of the Patton Bay splay fault system and their estimated onshore and offshore locations from Haeussler et al. (2015). Splay faults are not unique to the AASZ and are known to contribute to the tsunami hazard at subduction zones around the world including Ecuador (Collot et al., 2008), Japan Park et al., 2002), Cascadia (Han et al., 2017), and Chile (Melnick et al., 2012). ...
Coseismic slip on the Patton Bay splay fault system during the 1964 Mw 9.2 Great Alaska Earthquake contributed to local tsunami generation and vertically uplifted shorelines as much as 11 m on Montague Island in Prince William Sound (PWS). Sudden uplift of 3.7–4.3 m caused coastal lagoons along the island's northwestern coast to gradually drain. The resulting change in depositional environment from marine lagoon to freshwater muskeg created a sharp, laterally continuous stratigraphic contact between silt and overlying peat. Here, we characterize the geomorphology, sedimentology, and diatom ecology across the 1964 earthquake contact and three similar prehistoric contacts within the stratigraphy of the Hidden Lagoons locality. We find that the contacts signal instances of abrupt coastal uplift that, within error, overlap the timing of independently constrained megathrust earthquakes in PWS—1964 Common Era, 760–870 yr BP, 2500–2700 yr BP, and 4120–4500 yr BP. Changes in fossil diatom assemblages across the inferred prehistoric earthquake contacts reflect ecological shifts consistent with repeated draining of a lagoon system caused by >3 m of coseismic uplift. Our observations provide evidence for four instances of combined megathrust‐splay fault ruptures that have occurred in the past ∼4,200 years in PWS. The possibility that 1964‐style combined megathrust‐splay fault ruptures may have repeated in the past warrants their consideration in future seismic and tsunami hazards assessments.
... It triggered thousands of snow avalanches and landslides within the Kenai Mountains, the Chugach Mountains, and Prince William Sound (Grantz et al. 1964) and caused tsunamis with run-up heights of up to 67 m along the Chugach coast (Stover and Coffman 1993). Grewingk Fig. 2 Geological map of the Grewingk Lake and Glacier area (modified after Bradley et al. (1999) and Wilson et al. (2015)); mapped faults are based on field observation from this study and geomorphological features, and thrust faults based on Bradley et al. (1999) and Wilson et al. (2015) were approximately located (teeth on upper block) Glacier and Lake was located directly over the rupture zone of this earthquake (Haeussler et al. 2015) and about 260 km away from the epicentre. The Modified Mercalli Intensity (MMI) at the location of the landslide is estimated to be of 7.1 (U.S. Geological Survey 2020). ...
... For example, the Fairweather landslide, which occurred in the Fairweather Range near Glacier Bay National Park and Preserve in 1965, is a major event in the region's landslide history. The slope of the Grewingk landslide sits directly over the rupture zone of the 1964 Great Alaskan earthquake (Haeussler et al. 2015), and a MMI of 7.1 was estimated for the site (U.S. Geological Survey 2020). Hence, situated in a seismically highly active area, with four earthquakes > M5 in the decade preceding the 1967 slope failure, an adverse impact of seismicity on slope stability cannot be ruled out. ...
The relationship between rock-slope failure and glacier retreat is complex, and paraglacial failures often lack clearly identified triggers. To better understand the role of glacier retreat in rock-slope failures, we analysed the processes that led to the October 1967 Grewingk landslide in Kachemak Bay State Park on the Kenai Peninsula, Southcentral Alaska. The rock material collapsed onto the glacier toe and into its pro-glacial lake and produced a tsunami wave that swept the outwash plain. On the day of the failure, rainfall and snowmelt were well within normal ranges, and seismic records show no significant shaking. Three years prior to the 1967 failure, the slope withstood the second largest earthquake ever recorded (Great Alaskan earthquake, M W 9.2). We reassessed the volume of the failure by differencing pre- and post-digital terrain models and found a value of 20–24 × 10 ⁶ m ³ , which is four times smaller than a previous estimate. The back analysis of the Grewingk landslide is based on remote sensing data and field measurements including aerial satellite image analysis, detailed surveying and understanding of the structural geology, a kinematic analysis, and runout modelling. Our research provides an example of a major paraglacial failure that lacks an obvious trigger and points to several geological factors and changing environmental conditions that likely promote such failures. This study further indicates that the Grewingk landslide, pre-conditioned by the geometry of faults and joints, may have reached a critical stability state due to internal processes and the potential combined effects of seismic activity and glacier retreat prior to the collapse.
... expected from surface wave magnitude analysis of the earthquake (e.g., Heidarzadeh, 2011;Kanamori, 1972), such as the 365 Crete, 1946 Nankai, and1964 Alaska earthquakes, have also been linked to splay fault rupture (e.g., Cummins & Kaneda, 2000;Cummins et al., 2001;Chapman et al., 2014;Fan et al., 2017;Haeussler et al., 2015;Hananto et al., 2020;Martin et al., 2019;Shaw et al., 2008;Suleimani & Freymueller, 2020;von Huene et al., 2016). ...
Detailed imaging of accretionary wedges reveals splay fault networks that could pose a significant tsunami hazard. However, the dynamics of multiple splay fault activation during megathrust earthquakes and the consequent effects on tsunami generation are not well understood. We use a 2‐D dynamic rupture model with complex topo‐bathymetry and six curved splay fault geometries constrained from realistic tectonic loading modeled by a geodynamic seismic cycle model with consistent initial stress and strength conditions. We find that all splay faults rupture coseismically. While the largest splay fault slips due to a complex rupture branching process from the megathrust, all other splay faults are activated either top down or bottom up by dynamic stress transfer induced by trapped seismic waves. We ascribe these differences to local non‐optimal fault orientations and variable along‐dip strength excess. Generally, rupture on splay faults is facilitated by their favorable stress orientations and low strength excess as a result of high pore‐fluid pressures. The ensuing tsunami modeled with non‐linear 1‐D shallow water equations consists of one high‐amplitude crest related to rupture on the longest splay fault and a second broader wave packet resulting from slip on the other faults. This results in two episodes of flooding and a larger run‐up distance than the single long‐wavelength (300 km) tsunami sourced by the megathrust‐only rupture. Since splay fault activation is determined by both variable stress and strength conditions and dynamic activation, considering both tectonic and earthquake processes is relevant for understanding tsunamigenesis.
... expected from surface wave magnitude analysis of the earthquake (e.g., Heidarzadeh, 2011;Kanamori, 1972), such as the 365 Crete, 1946 Nankai, and1964 Alaska earthquakes, have also been linked to splay fault rupture (e.g., Cummins & Kaneda, 2000;Cummins et al., 2001;Chapman et al., 2014;Fan et al., 2017;Haeussler et al., 2015;Hananto et al., 2020;Martin et al., 2019;Shaw et al., 2008;Suleimani & Freymueller, 2020;von Huene et al., 2016). ...
Detailed imaging of accretionary wedges reveals splay fault networks that could pose a significant tsunami hazard. However, the dynamics of multiple splay fault activation during megathrust earthquakes and the consequent effects on tsunami generation are not well understood. We use a 2-D dynamic rupture model with complex topo-bathymetry and six curved splay fault geometries constrained from realistic tectonic loading modeled by a geodynamic seismic cycle model with consistent initial stress and strength conditions. We find that all splay faults rupture coseismically. While the largest splay fault slips due to a complex rupture branching process from the megathrust, all other splay faults are activated either top down or bottom up by dynamic stress transfer induced by trapped seismic waves. We ascribe these differences to local non-optimal fault orientations and variable along-dip strength excess. Generally, rupture on splay faults is facilitated by their favorable stress orientations and low strength excess as a result of high pore-fluid pressures. The ensuing tsunami modeled with non-linear 1-D shallow water equations consists of one high-amplitude crest related to rupture on the longest splay fault and a second broader wave packet resulting from slip on the other faults. This results in two episodes of flooding and a larger run-up distance than the single long-wavelength (300 km) tsunami sourced by the megathrust-only rupture. Since splay fault activation is determined by both variable stress and strength conditions and dynamic activation, considering both tectonic and earthquake processes is relevant for understanding tsunamigenesis.
... In this paper, we identify and characterize faults in the region of the Kodiak segment using legacy and new bathymetric, seismic, and potential field ■ TECTONIC SETTING Tsunamigenic splay faults have been imaged within the Gulf of Alaska forearc with seismic and bathymetric data (von Huene et al., 2012;Liberty et al., 2013;Haeussler et al., 2015;Li et al., 2018;Liberty et al., 2019). Similar fault geometries and seafloor uplift patterns presumably span the length of this subduction zone, but differences in plate geometry and subducting structure may give rise to differences in forearc structures and earthquake potential. ...
... Similar fault geometries and seafloor uplift patterns presumably span the length of this subduction zone, but differences in plate geometry and subducting structure may give rise to differences in forearc structures and earthquake potential. From teleseismic receiver function and crustal-scale, active-source seismic data across the Gulf of Alaska, we know that faults splay from the subduction interface where this mega thrust dips to the north between 3°-9° (Moore et al., 1991;Eberhart-Phillips et al., 2006;Liberty et al., 2013;Kim et al., 2014;Haeussler et al., 2015;Bécel et al., 2017, Hayes et al., 2018. ...
... The Kodiak Shelf fault zone (KSfz) and Albatross Banks fault zone (ABfz) have been inferred to control upper-plate fault motions near the Kodiak Islands (Fig. 2;Fisher and von Huene, 1980;von Huene et al., 1980;Moore et al., 1991;. Although no direct evidence has tied the KSfz and ABfz to the mega thrust, we can presume that they splay from this boundary because of their similarity to splay fault structures already imaged on nearby subduction zone segments (e.g., Moore et al., 1991;Liberty et al., 2013;Haeussler et al., 2015;Bécel et al., 2017). mapped the on-land portion of the KSfz, and they named the largest fault the Narrow Cape fault. ...
The Kodiak Islands lie near the southern terminus of the 1964 Great Alaska earthquake rupture area and within the Kodiak subduction zone segment. Both local and trans-Pacific tsunamis were generated during this devastating megathrust event, but the local tsunami source region and the causative faults are poorly understood. We provide an updated view of the tsunami and earthquake hazard for the Kodiak Islands region through tsunami modeling and geophysical data analysis. Using seismic and bathymetric data, we characterize a regionally extensive seafloor lineament related to the Kodiak shelf fault zone, with focused uplift along a 50-km-long portion of the newly named Ugak fault as the most likely source of the local Kodiak Islands tsunami in 1964. We present evidence of Holocene motion along the Albatross Banks fault zone, but we suggest that this fault did not produce a tsunami in 1964. We relate major structural boundaries to active forearc splay faults, where tectonic uplift is collocated with gravity lineations. Differences in interseismic locking, seismicity rates, and potential field signatures argue for different stress conditions at depth near presumed segment boundaries. We find that the Kodiak segment boundaries have a clear geophysical expression and are linked to upper-plate structure and splay faulting. The tsunamigenic fault hazard is higher for the Kodiak shelf fault zone when compared to the nearby Albatross Banks fault zone, suggesting short wave travel paths and little tsunami warning time for nearby communities.
... Thus, the marine limits of these glaciers are estimated almost everywhere on the map (Fig. 1), and in general are inferred to have extended to the edge of the continental shelf, particularly where cross-shelf troughs exist. Minor modifications to these ice limits were inferred by Haeussler et al. (2015) for a region offshore PWS and by Zimmerman et al. (2019) for a region offshore the Alaska Peninsula. ...
... Bedrock bedding, faults, and fractures are clearly visible where not obscured by vegetation, showing the entire landscape was abraded by glaciers during glacial maxima. Finally, given glacial retreat, we assume there was glacial-isostatic Kaufman et al. (2011) with minor modifications from Haeussler et al. (2015). Blue diamonds labeled G, V, and K are locations of prior 14 C dates (see text and Table 1) adjustment (GIA). ...
To understand the timing of deglaciation of the northernmost marine-terminating glaciers of the Cordilleran Ice Sheet (CIS), we obtained 26 ¹⁰ Be surface-exposure ages from glacially scoured bedrock surfaces in Prince William Sound (PWS), Alaska. We sampled six elevation transects between sea level and 620 m and spanning a distance of 14 to 70 km along ice flow paths. Most transect age–elevation patterns could not be explained by a simple model of thinning ice; the patterns provide evidence for lingering ice cover and possible inheritance. A reliable set of 20 ages ranges between 17.4 ± 2.0 and 11.6 ± 2.8 ka and indicates ice receded from northwestern PWS around 14.3 ± 1.6 ka, thinned at a rate of ~120–160 m/ka, and retreated from sea-level sites at 12.9 ± 1.1 ka at a rate of 20 m/yr. The retreat rate likely slowed as glaciers retreated into northern PWS. These results are consistent with the growing body of reported deglacial constraints on collapse of ice sheets along the Alaska margin indicating collapse of the CIS soon after 17 ka. These data are consistent with paleotemperature data indicating that a warming North Pacific Ocean caused catastrophic collapse of this part of the CIS.
... Recent subduction zone earthquakes in Japan and elsewhere have demonstrated the significant earth quake, tsunami, and landslide hazards posed by coseismic or triggered deformation of the upper plate, particularly offshore and above shallow megathrusts. For example, tsunamis produced by upperplate fault rupture and submarine landslide generation during the 1964 Alaska earthquake (Par sons et al., 2014;Haeussler et al., 2015) and shallow megathrust rupture in the 2004 Sumatran (Gulick et al., 2011) and 2011 TohokuOki (Japan) earth quakes (Fujiwara et al., 2011;Sun et al., 2017) were responsible for the majority of fatalities resulting from each earthquake. These events emphasize the need to better understand how the upper plate responds to large shallow megathrust earthquakes in space and time. ...
... Devastating historic tsunamis in Japan (Moore et al., 2007) and Alaska (Haeussler et al., 2015;von Huene et al., 2016) resulted from rupture along megasplay faults in the accretionary wedge. In each of these cases, the proposed tsunamigenic megasplay fault was associated with a distinct mid slope break and/or defined a mechanical boundary, or faultbounded backstop, separating older accre tionary or country rocks from younger accretionary rocks. ...
Studies of recent destructive megathrust earthquakes and tsunamis along subduction margins in Japan, Sumatra, and Chile have linked forearc morphology and structure to megathrust behavior. This connection is based on the idea that spatial variations in the frictional behavior of the mega thrust influence the tectono-morphological evolution of the upper plate. Here we present a comprehensive examination of the tectonic geomorphology, outer wedge taper, and structural vergence along the marine forearc of the Cascadia subduction zone (offshore northwestern North America). The goal is to better understand geologic controls on outer wedge strength and segmentation at spatial scales equivalent to rupture lengths of large earthquakes (≥M 6.7), and to examine potential linkages with shallow megathrust behavior. We use cross-margin profiles, spaced 25 km apart, to characterize along-strike variation in outer wedge width, steepness, and structural vergence (measured between the toe and the outer arc high). The width of the outer wedge varies between 17 and 93 km, and the steepness ranges from 0.9° to 6.5°. Hierarchical cluster analysis of outer wedge width and steepness reveals four distinct regions that also display unique patterns of structural ver-gence and shape of the wedge: Vancouver Island, British Columbia, Canada (average width, linear wedge, seaward and mixed vergence); Washington, USA (higher width, concave wedge, landward and mixed vergence); northern and central Oregon, USA (average width, linear and convex wedge, mixed and seaward vergence); and southern Oregon and northern California, USA (lower width, convex wedge, seaward and mixed vergence). Variability in outer wedge morphology and structure is broadly associated with along-strike megathrust segmentation inferred from differences in oceanic asthenospheric velocities, patterns of episodic tremor and slow slip, GPS models of plate locking, and the distribution of seismicity near the plate interface. In more detail, our results appear to delin-eate the extent, geometry, and lithology of dynamic and static backstops along the margin. Varying backstop configurations along the Cascadia margin are interpreted to represent material-strength contrasts within the wedge that appear to regulate the along-and across-strike taper and structural vergence in the outer wedge. We argue that the morphotectonic variability in the outer wedge may reflect spatial variations in shallow mega-thrust behavior occurring over roughly the last few million years. Comparing outer wedge taper along the Cascadia margin to a global compilation suggests that observations in the global catalog are not accurately representing the range of hetero-geneity within individual margins and highlights the need for detailed margin-wide morphotectonic analyses of subduction zones worldwide.
... Focused and rapid exhumation has taken place around the Montague Island as revealed by young apatite (U-Th)/He ages of <5 Ma, especially in the southwestern island (Arkle et al., 2013;Ferguson et al., 2015;Valentino et al., 2016). Haeussler et al. (2015) suggested that several splay faults there have accommodated the exhumation and rooted into the megathrust where there is probably underplating. In addition, they inferred that the Montague Strait fault between the Knight Island and the Montague Island bounds landward mechanically strong rocks and trenchward mechanically weak rocks based on seismic wide-angle and pre-stack depth migration results. ...
Structural heterogeneities in subduction zones can affect slip behaviors of the megathrust faults and the generation of intraslab earthquakes. In this work we study the 3-D seismic structures (Vp, Vs and Poisson’s ratio) in and around the source zones of the 2018 Anchorage intraslab earthquake (Mw 7.1) and the 1964 Alaska megathrust earthquake (Mw 9.2). The Anchorage earthquake occurred in an anomalous zone within the subducting Y akutat/Pacific plate with higher Poisson’ s ratio than the normal slab. Above the source zone, the overriding North American plate shows low-Vs and high Poisson’s ratio. These features indicate that strong dehydration occurs in the source zone and released fluids ascend into the overlying crust. Two areas with long-term slow slip events in the Upper and Lower Cook Inlet predominantly exhibit high Poisson’s ratio in the lowermost portion of the crust and the cold nose of the mantle wedge, whereas a low Poisson’s ratio zone is revealed between them, suggesting that their segmentation is possibly related to localized slab-releasing fluids. In the Prince William Sound, the rupture of the 1964 Great Alaska earthquake initiated beneath a high-V and high Poisson’s ratio zone of the overlying crust and the large slips occurred beneath a low-Vs and high Poisson’s ratio zone, suggesting that lateral heterogeneities of the overriding plate may have played an important role in the nucleation and rupture processes of the Great Alaska earthquake.
... The U.S. Geological Survey alone has conducted more than 30 marine seismic surveys since 2010 that utilized sparker sound sources across virtually every U.S. continental margin, resulting in more than 50,000 line-km of high-resolution seismic reflection data. Maximizing the data resolution is critical to studies focused on the near-surface processes, such as Quaternary geology, tectonic geomorphology, substrate fluid flow, and submarine landslide generation (e.g., Johnson and Watt 2012;Liberty et al. 2013;Brothers et al. 2013Brothers et al. , 2014Brothers et al. , 2016Brothers et al. , 2017Brothers et al. , 2018aHaeussler et al. 2014Haeussler et al. , 2015Johnson et al. 2014Johnson et al. , 2017aHill et al. 2017;Maier et al. 2017Maier et al. , 2018Beeson et al. 2017;Conrad et al. 2017Conrad et al. , 2018. ...
Sparkers are a type of sound source widely used by the marine seismic community to provide high-resolution imagery of the shallow sub-bottom (i.e., < 1000 m). Although sparkers are relatively simple, inexpensive, and high-frequency (100–2500 Hz) sources, they have several potential pitfalls due to their complicated and unpredictable signature. In this study we quantify the source characteristics of several sparker systems and develop a suite of simple processing approaches for both single channel and multi-channel sparker data. In all cases, the results show improved vertical resolution and reflection coherency. Correcting for small static variations in multi-channel seismic (MCS) data is a critical first step to preserve the broad frequency content during stacking, and to reduce the shot-to-shot variability of outgoing and incoming signals. Application of predictive deconvolution to static-corrected, post-stack traces suppresses short-path multiples and restores the latent high-resolution reflection patterns. However, if shot-to-shot source signatures are recorded directly, pre-stack deterministic deconvolution followed by post-stack predictive deconvolution produces the most robust results. Processing sparker data without broadband techniques results in less confident or completely missed interpretations when compared to the broadband equivalent. If processed correctly, marine sparker data can provide exceptional sub-bottom imagery that rivals other more repeatable marine seismic sources (e.g., high-frequency air-guns).
... Thrust faults that splay from a megathrust within subduction zone accretionary wedges can pose major seismic and tsunami hazards, yet little is known about the spatial and temporal controls on this family of faults. Surface ruptures during subduction zone earthquakes can highlight patterns of coseismic motion (e.g., Fujiwara et al., 2011;Henstock et al., 2006), paleoseismic and geodetic observations can provide estimates of recurrence intervals and patterns of uplift/subsidence (e.g., Atwater & Hemphill-Haley, 1997;Cisternas et al., 2005;Saillard et al., 2017;Shennan et al., 2014;Sieh et al., 2008), and thermochronology measurements can provide regional uplift rates over thousands of earthquake cycles (e.g., Enkelmann et al., 2015;Ferguson et al., 2015;Haeussler et al., 2015). However, detailed slip partitioning and uplift patterns over multiple earthquake cycles remains unknown. ...
... 1994; Bruns, 1983;Kim et al., 2014; Figure 1). The rupture lifted western Montague Island and the adjacent sea floor as much as 12 m along listric thrust faults that splay from a décollement Haeussler et al., 2015; Figure 1). Because little trenchward motion was recorded on Middleton Island in 1964 ( Figure 1), Plafker (1969) concluded that horizontal shortening from this earthquake was accommodated almost entirely along faults that lie on the continental shelf. ...
... Key horizons identified with our new seismic survey represent major changes in Holocene sediment deposition throughout the PWS and Gulf of Alaska regions Haeussler et al., 2015;Liberty et al., 2013). With our data set, we identify three seismic stratigraphic packages above acoustic basement (Figures 3 and S2 to S5). ...
Coseismic slip partitioning and uplift over multiple earthquake cycles is critical to understanding upper-plate fault development. Bathymetric and seismic reflection data from the 1964 Mw9.2 Great Alaska earthquake rupture area reveal sea floor scarps along the tsunamigenic Patton Bay/Cape Cleare/Middleton Island fault system. The faults splay from a megathrust where duplexing and underplating produced rapid exhumation. Trenchward of the duplex region, the faults produce a complex deformation pattern from oblique, south-directed shortening at the Yakutat-Pacific plate boundary. Spatial and temporal fault patterns suggest that Holocene megathrust earthquakes had similar relative motions and thus similar tsunami sources as in 1964. Tsunamis during future earthquakes will likely produce similar run-up patterns and travel times. Splay fault surface expressions thus relate to plate boundary conditions, indicating millennial-scale persistence of this asperity. We suggest structure of the subducted slab directly influences splay fault and tsunami generation landward of the frontal subduction zone prism. Plain Language Summary We identify prominent sea floor scarps that show a similar pattern of tectonic uplift over the past 20 to 30 subduction zone earthquakes in the western Prince William Sound area of Alaska. Our results suggest that plate boundary conditions have been fixed through many earthquake cycles and that subducted plate boundary conditions influence sea floor uplift patterns. We conclude that tsunami patterns observed during the 1964 earthquake will likely repeat to reproduce run-up and travel time observations. Mapping structures along plate boundaries is critical to understanding tsunami sources in subduction zones.
