Interpretation of magnetic maps of the northern Gulf of Alaska, with emphasis on the source of the Slope Anomaly

Griscom, Andrew; Sauer, Peter E.

doi:10.3133/ofr90348

Cited by 19 publications

(32 citation statements)

References 11 publications

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“…This gradual transition contrasts with the Yakutat/Pacific plate boundary along the Transition fault beneath the eastern Gulf of Alaska, where an abrupt change in crustal thickness is observed [Christeson et al, 2010] [31] Between positions 30 km and 90 km on the TACT profile, we observe additional reflectors below the megathrust that form a lens-shaped zone of reflective rocks ( Figure 5). This region is coincident with the prominent magnetic high [Griscom and Sauer, 1990;Figure 1] and is consistent with a low seismic velocity (6.4-6.6 km/s) zone beneath the megathrust in the Brocher et al [1994] refraction model ( Figure 5). We identify three megathrust splay faults emerging from the décollement surface along this~4 km-thick, 60 km-wide lens-shaped zone, all with apparent dips above the décollement of~20°-30°.…”

Section: Deep Splay Fault Geometry From Seismic Imagesmentioning

confidence: 49%

“…(Figure 3), land surface [Plafker, 1969], and on the megathrust (Figure 4). Note the north-south alignment of maximum fault uplifts at the outer limit of inner wedge deformation (as defined by Wang and Hu [2006]) and the trailing edge of the subducted Yakutat terrane [from Griscom and Sauer, 1990].…”

Section: Deep Splay Fault Geometry From Seismic Imagesmentioning

confidence: 99%

“…West of Montague Island, the subducting Yakutat slab is absent and the Pacific Plate subducts directly beneath the North American plate [e.g., Brocher et al, 1994;EberhartPhillips et al, 2006]. The PWS asperity, defined as a region of high moment release [e.g., Lay et al, 1982;Scholz and Campos, 2012], was centered beneath the southwest end of Montague Island near a prominent magnetic high that defines the western boundary of the subducted Yakutat terrane (Figure 1) [Bruns, 1983;Griscom and Sauer, 1990;Brocher et al;1994;Johnson et al, 1996;Zweck et al, 2002;Eberhart-Phillips et al, 2006].…”

Section: Introductionmentioning

confidence: 99%

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Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

et al. 2013

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[1] High-resolution sparker and crustal-scale air gun seismic reflection data, coupled with repeat bathymetric surveys, document a region of repeated coseismic uplift on the portion of the Alaska subduction zone that ruptured in 1964. This area defines the western limit of Prince William Sound. Differencing of vintage and modern bathymetric surveys shows that the region of greatest uplift related to the 1964 Great Alaska earthquake was focused along a series of subparallel faults beneath Prince William Sound and the adjacent Gulf of Alaska shelf. Bathymetric differencing indicates that 12 m of coseismic uplift occurred along two faults that reached the seafloor as submarine terraces on the Cape Cleare bank southwest of Montague Island. Sparker seismic reflection data provide cumulative Holocene slip estimates as high as 9 mm/yr along a series of splay thrust faults within both the inner wedge and transition zone of the accretionary prism. Crustal seismic data show that these megathrust splay faults root separately into the subduction zone décollement. Splay fault divergence from this megathrust correlates with changes in midcrustal seismic velocity and magnetic susceptibility values, best explained by duplexing of the subducted Yakutat terrane rocks above Pacific plate rocks along the trailing edge of the Yakutat terrane. Although each splay fault is capable of independent motion, we conclude that the identified splay faults rupture in a similar pattern during successive megathrust earthquakes and that the region of greatest seismic coupling has remained consistent throughout the Holocene.

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Section: Deep Splay Fault Geometry From Seismic Imagesmentioning

confidence: 49%

Section: Deep Splay Fault Geometry From Seismic Imagesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

et al. 2013

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“…Modeling of magnetic data provides compelling evidence that the trailing edge of the Yakutat terrane, marked by the slope magnetic anomaly (SMA), lies between Montague and Middleton Islands ( Fig. 1; Griscom and Sauer, 1990). Seismic tomography data show an increase in subduction angle near the southwestern extent of the Yakutat plate along the western margin of PWS (Eberhart-Phillips et al, 2006).…”

Section: Tectonic Setting and Earthquake Historymentioning

confidence: 99%

“…Noted waterways include Hinchinbrook Entrance (HE) and the Canoe Passage (CP). The slope magnetic anomaly (SMA) defines the trailing and southwestern edge of the subducted Yakutat block (Griscom and Sauer, 1990). port communities of Whittier, Cordova, and Seward, and essentially destroyed the towns of Valdez and Chenega (Fig.…”

Section: The 1964 Great Alaska Earthquakementioning

confidence: 99%

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

Finn¹,

Liberty²,

Haeussler³

et al. 2015

Bulletin of the Seismological Society of America

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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.

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Alaska megathrust 1: Seismicity 43 years after the great 1964 Alaska megathrust earthquake

et al. 2013

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[1] The largest moment release during the 1964 Mw 9.2 Alaska earthquake was on the portion of the megathrust under the eastern Kenai Peninsula and the Prince William Sound. The area is currently locked geodetically and corresponds to where the Yakutat terrane is subducting. In 2006-2009, a seismic array consisting of 34 broadband seismometers was deployed in the region. An automatic algorithm was used to detect 12,563 local earthquakes using 13 months of data from the experiment. Of these, 9427 good quality earthquakes could be relocated in a joint inversion for hypocenters and velocity structure. They were then relocated by double difference to generate a final catalog of 8308 hypocenters. These microearthquakes delineate a deeper steeply dipping Wadati-Benioff zone contiguous with an 11 km wide seismic zone dipping at 3°between 20 km and 40 km depth along the 1964 earthquake's rupture zone. Focal mechanisms do not show interplate thrust faulting events, but mostly normal faulting events with T axes generally parallel the slab dip direction, indicating them to be intraslab seismicity. The shallow narrow band of seismicity lies within the subducting Yakutat terrane and right below the thrust zone. Possibly, thrust faulting has not yet resumed yet in this early phase of the earthquake cycle. Thick subducted sediments overlying the Yakutat terrane could also form a large strong contact zone on a relatively smooth plate boundary, which does not favor seismic sliding on small patches but ruptures homogenously in great earthquakes.

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Interpretation of magnetic maps of the northern Gulf of Alaska, with emphasis on the source of the Slope Anomaly

Cited by 19 publications

References 11 publications

Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

Megathrust splay faults at the focus of the Prince William Sound asperity, Alaska

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

Alaska megathrust 1: Seismicity 43 years after the great 1964 Alaska megathrust earthquake

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