Uplift and subsidence reveal a nonpersistent megathrust rupture boundary (Sitkinak Island, Alaska)

Briggs, Richard W.; Engelhart, Simon E.; Nelson, Alan R.; Dura, Tina; Kemp, Andrew C.; Haeussler, Peter J.; Corbett, D. Reide; Angster, Stephen J.; Bradley, Lee‐Ann

doi:10.1002/2014gl059380

Cited by 61 publications

(77 citation statements)

References 32 publications

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“…On the south coast of Sitkinak Island, only 125 km east of Chirikof Island, Briggs et al (2014a) identified uplift and subsidence during 5 great earthquakes and 6 possible tsunami beds dating from the past 3000 yr. The youngest of the beds, probably deposited by the 1788 tsunami, can be traced continuously 1.3 km inland.…”

Section: Tsunami Recurrencementioning

confidence: 98%

“…11). Bed correlations at the Drained Lake site show that bed D rises higher than the beds of other probable tsunami deposits, reaching almost 15 m ( (Lander, 1996;Briggs et al, 2014a).…”

Section: Historical Tsunamismentioning

confidence: 99%

“…Five older sand beds with similar characteristics extend >0.8 km inland. Applying the methods that we used to construct the age distributions of Figure 11 to the ages reported by Briggs et al (2014a), we calculate age distributions for the five older possible tsunami deposits on Sitkinak Island (Fig. 12).…”

Section: Tsunami Recurrencementioning

confidence: 99%

“…Sitkinak Island Briggs et al, 2014a 1788 slip that splay off the megathrust where it is most strongly coupled. Although each splay fault may rupture independently of the megathrust, following similar conclusions about analogous subduction zones (e.g., Melnick et al, 2006;Strasser et al, 2009) Liberty et al (2013) argued that most splay faults slip during megathrust earthquakes and that the location of the area of high seismic coupling has changed little during the Holocene.…”

Section: Kodiak Islandmentioning

confidence: 99%

“…The variable source characteristics and tsunami-generating potential of megathrust ruptures in the region (Ryan et al, 2012b;Briggs et al, 2014aBriggs et al, , 2014bShennan et al, 2014aShennan et al, , 2014bWitter et al, 2014) suggest that Southwest Anchorage has been exposed to a large range in inundation heights that may only loosely correlate with tsunami volume, rupture extent, and earthquake magnitude. For this reason, forecasting future great earthquakes in the eastern Aleutian arc accompanied by southwarddirected high tsunamis will require carefully reconstructed histories of rupture lengths and inundation extents for the past 3-5 k.y.…”

Section: Kodiak Islandmentioning

confidence: 99%

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Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

et al. 2015

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Despite the role of the Alaska-Aleutian megathrust as the source of some of the largest earthquakes and tsunamis, the history of its pre-twentieth century tsunamis is largely unknown west of the rupture zone of the great (magnitude, M 9.2) 1964 earthquake. Stratigraphy in core transects at two boggy lowland sites on Chirikof Island's southwest coast preserves tsunami deposits dating from the postglacial to the twentieth century. In a 500-m-long basin 13-15 m above sea level and 400 m from the sea, 4 of 10 sandy to silty beds in a 3-5-m-thick sequence of freshwater peat were probably deposited by tsunamis. The freshwater peat sequence beneath a gently sloping alluvial fan 2 km to the east, 5-15 m above sea level and 550 m from the sea, contains 20 sandy to silty beds deposited since 3.5 ka; at least 13 were probably deposited by tsunamis. Although most of the sandy beds have consistent thicknesses (over distances of 10-265 m), sharp lower contacts, good sorting, and/or upward fining typical of tsunami deposits, the beds contain abundant freshwater diatoms, very few brackish-water diatoms, and no marine diatoms. Apparently, tsunamis traveling inland over low dunes and boggy lowland entrained largely freshwater diatoms. Abundant fragmented diatoms, and lake species in some sandy beds not found in host peat, were probably transported by tsunamis to elevations of >10 m at the eastern site. Single-aliquot regeneration optically stimulated luminescence dating of the third youngest bed is consistent with its having been deposited by the tsunami recorded at Russian hunting outposts in 1788, and with the second youngest bed being deposited by a tsunami during an upper plate earthquake in 1880. We infer from stratigraphy, 14 C-dated peat deposition rates, and unpublished analyses of the island's history that the 1938 tsunami may locally have reached an elevation of >10 m. As this is the first record of Aleutian tsunamis extending throughout the Holocene, we cannot estimate source earthquake locations or magnitudes for most tsunami-deposited beds. We infer that no more than 3 of the 23 possible tsunamis beds at both sites were deposited following upper plate faulting or submarine landslides independent of megathrust earthquakes. If so, the Semidi segment of the Alaska-Aleutian megathrust near Chirikof Island probably sent high tsunamis southward every 180-270 yr for at least the past 3500 yr.

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Section: Tsunami Recurrencementioning

confidence: 98%

“…11). Bed correlations at the Drained Lake site show that bed D rises higher than the beds of other probable tsunami deposits, reaching almost 15 m ( (Lander, 1996;Briggs et al, 2014a).…”

Section: Historical Tsunamismentioning

confidence: 99%

Section: Tsunami Recurrencementioning

confidence: 99%

Section: Kodiak Islandmentioning

confidence: 99%

Section: Kodiak Islandmentioning

confidence: 99%

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Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

et al. 2015

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A possible transoceanic tsunami directed toward the U.S. west coast from the Semidi segment, Alaska convergent margin

Huene

Miller

Dartnell

2016

Geochem Geophys Geosyst

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The Semidi segment of the Alaska convergent margin appears capable of generating a giant tsunami like the one produced along the nearby Unimak segment in 1946. Reprocessed legacy seismic reflection data and a compilation of multibeam bathymetric surveys reveal structures that could generate such a tsunami. A 200 km long ridge or escarpment with crests >1 km high is the surface expression of an active out‐of‐sequence fault zone, recently referred to as a splay fault. Such faults are potentially tsunamigenic. This type of fault zone separates the relatively rigid rock of the margin framework from the anelastic accreted sediment prism. Seafloor relief of the ridge exceeds that of similar age accretionary prism ridges indicating preferential slip along the splay fault zone. The greater slip may derive from Quaternary subduction of the Patton Murray hot spot ridge that extends 200 km toward the east across the north Pacific. Estimates of tsunami repeat times from paleotsunami studies indicate that the Semidi segment could be near the end of its current inter‐seismic cycle. GPS records from Chirikof Island at the shelf edge indicate 90% locking of plate interface faults. An earthquake in the shallow Semidi subduction zone could generate a tsunami that will inundate the US west coast more than the 1946 and 1964 earthquakes because the Semidi continental slope azimuth directs a tsunami southeastward.

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Downdip variations in seismic reflection character: Implications for fault structure and seismogenic behavior in the Alaska subduction zone

Shillington

Bécel

et al. 2015

JGR Solid Earth

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Seismic reflection data collected offshore of Alaska Peninsula across the western edge of the Semidi segment show distinctive variations in reflection characteristics of the megathrust fault with depth, suggesting changes in structure that may relate to seismic behavior. From the trench to ~40 km landward, two parallel reflections are observed, which we interpret as the top and bottom of the subducted sediment section. From ~50 to 95 km from the trench, the plate interface appears as a thin (<400 ms) reflection band. Deeper and farther landward, the plate interface transitions to a thicker (1–1.5 s) package of reflections, where it appears to intersect the fore‐arc mantle wedge based on our preferred interpretation of the continental Moho. Synthetic waveform modeling suggests that the thin reflection band is best explained by a single ~100 to 250 m thick low‐velocity zone, whereas the thick reflection band requires a 3 to 5 km thick zone of thin layers. The thin reflection band is located at the center of the 1938 Mw 8.2 Semidi earthquake rupture zone that now experiences little interplate seismicity. The thick reflection band starts at the downdip edge of the rupture zone, correlates with a dipping band of seismicity, and projects to the location of tremor at greater depth. We interpret the thin reflection band as a compacted sediment layer and/or localized shear zone. The thick reflection band could be caused by a wide deformation zone with branching faults and/or fluid‐rich layers, representing a broad transition from stick‐slip sliding to slow slip and tremor.

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Uplift and subsidence reveal a nonpersistent megathrust rupture boundary (Sitkinak Island, Alaska)

Abstract: We report stratigraphic evidence of land-level change and tsunami inundation along the

Cited by 61 publications

References 32 publications

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

A possible transoceanic tsunami directed toward the U.S. west coast from the Semidi segment, Alaska convergent margin

Downdip variations in seismic reflection character: Implications for fault structure and seismogenic behavior in the Alaska subduction zone

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