Auxiliary material for
Little late Holocene strain accumulation and release on the Aleutian megathrust below
the Shumagin Islands, Alaska
Robert C. Witter1, Richard W. Briggs2, Simon E. Engelhart3, Guy Gelfenbaum4, Richard
D. Koehler5, and William D. Barnhart2
Geophysical Research Letters, 2014
U.S. Geological Survey, Alaska Science Center, Anchorage, Alaska, USA.
U.S. Geological Survey, Geologic Hazards Science Center, Golden, Colorado, USA.
Department of Geosciences, University of Rhode Island, Woodward Hall, 9 East Alumni
Avenue, Kingston, Rhode Island, USA.
U.S. Geological Survey, Pacific Coastal and Marine Science Center, 400 Natural
Bridges Drive, Santa Cruz, California, USA.
State of Alaska, Division of Geological & Geophysical Surveys, 3354 College Rd.,
Fairbanks, Alaska, USA.
The auxiliary materials include three text files, three tables, and nine figures. The text
files include (1) examples of Russian accounts of strong shaking and marine flooding in
1788 in the Shumagin Island region, (2) an explanation of methods used for modeling
tides at Simeonof Island, and (3) a description of the modeling methods used to estimate
vertical surface displacement generated by megathrust earthquakes. The tables list (1) the
results of Accelerator Mass Spectrometry radiocarbon analyses for dating sediment, (2)
stable carbon isotope data, and (3) estimated fault slip and earthquake magnitude for
megathrust rupture scenarios shown in Figure S8. Nine figures include maps and images
of sites investigated on Simeonof Island, observed and modeled tide data, results of
elastic dislocation modeling, and historical seismicity of the Shumagin gap.
Files included as auxiliary material:
Text S1 (text01.docx) – Historical accounts of earthquakes and tsunamis in 1788.
Text S2 (text02.docx) – Establishing elevations relative to a tidal datum on Simeonof
Text S3 (text03.docx) – Methods for estimating vertical surface displacements from
megathrust slip.
Table S1 (ts01.docx) – Radiocarbon data for samples from surficial sediments on
Simeonof Island. Individual columns are labeled at the top of the table; footnotes provide
further explanation where necessary.
Table S2 (ts02.docx) – Variations in stable carbon isotope (δ13C ) values in sediment
from a core at Boiler Lagoon. Individual columns are labeled at the top of the table;
footnotes provide further explanation where necessary.
Table S3 (ts03.docx) – Estimated megathrust slip amount and moment magnitude for
hypothetical ruptures of the Shumagin Gap. Individual columns are labeled at the top of
the table; footnotes provide further explanation where necessary.
Figure S1 (fs01.pdf) – Oblique rendering of 2009 imagery of Simeonof Island.
Figure S2 (fs02.pdf) – Comparison of water level observations and modeled tides at
Simeonof Island. Methods explained in Text S2.
Figure S3 (fs03.pdf) – Bedrock weathering profiles exposed in bluffs along the coast of
Simeonof Island.
Figure S4 (fs04.pdf) – Image showing hydrologically interconnected lakes on the west
coast of Simeonof Island and a map of surficial deposits and geomorphology of Swan
Lake area.
Figure S5 (fs05.pdf) – Photographs and a map showing lakes and marshes near field
camp on Simeonof Island and a stratigraphic profile of coastal deposits.
Figure S6 (fs06.pdf) – Map of drained lakes near Rita Bay and a stratigraphic profile.
Figure S7 (fs07.pdf) – Map of surficial geology on the northeast side of Simeonof Island
and stratigraphic profile.
Figure S8 (fs08.pdf) – Results of elastic dislocation modeling. Modeling methods
explained in Text S3.
Figure S9 (fs09.pdf) – One hundred years of historical seismicity in the Shumagin Gap
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