IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS002 Oral Presentation 1761
Numerical studies of multiple submarine slope failures and tsunamis near
Seward, Alaska, during the M9.2 1964 earthquake.
Mrs. Elena Suleimani
Geophysical Institute University of Alaska Fairbanks IASPEI
Peter Haeussler, Keith Labay, Roger Hansen
We are creating tsunami inundation maps for Seward, Alaska, in the scope of the National Tsunami
Hazard Mitigation Program. Tsunami potential from tectonic and submarine landslide sources must be
evaluated in this case for comprehensive mapping of areas at risk for inundation. Seward is a
community located at the head of Resurrection Bay, in southern Alaska, which was hit hard by both
tectonic and landslide-generated tsunami waves during the 1964 earthquake. Resurrection Bay is a
glacial fjord fed by several rivers and creeks draining nearby glaciers and depositing sediments into the
bay at a high rate. Sediment accumulation on the steep underwater slopes contributes to the landslide
tsunami hazard in the Resurrection Bay. We constructed a 5-m grid of combined topography and
bathymetry for the northern part of Resurrection Bay. The data is of exceptional quality and includes (1)
a 2006 LIDAR survey of the entire area of interest, (2) a 2001 multi-beam survey of the bathymetry of
all of Resurrection Bay, and a (3) 2006 survey of the Seward harbor and surrounding areas. Gaps
between the LIDAR and multi-beam surveys were minimal, with the exception of a shallow tidal area at
the head of the bay. Where gaps exist, interpolation was used to create a smooth transition between
the surveys. All pre-1964 (1905 to 1961) bathymetric surveys from NOAA smooth sheets were digitized,
with corrections applied for coseismic subsidence, post-seismic uplift, sea level rise, and rounding
errors. We then compared these to a 2001 NOAA multi-beam survey to assess the location and size of
submarine slides. More than 100 million m3 of sediment moved during the 1964 earthquake, with much
of it flowing about 10 km to the south into a bathtub-shaped depression in the fiord bottom. There were
four major slides in the bay with volumes in excess of 14 million m3. A slide along the Seward
waterfront, which is likely responsible for most of the initial damage, has a volume of about 19 million
m3, and left behind a blocky lag deposit. To reconstruct the sequence of waves observed at Seward on
March 27, 1964, we model tsunami waves caused by superposition of the local landslide-generated
tsunamis and the major tectonic tsunami, which arrived about 30 minutes after the start of the
earthquake. We use a three-dimensional numerical model of an incompressible viscous slide with full
interaction between the slide and surface waves to simulate Seward slope failures and associated
tsunami waves. The long-wave approximation is used for both water waves and slides. The equations of
motion and continuity for the slide and for surface waves are solved simultaneously using an explicit
finite-difference scheme.
Keywords: tsunami, landslide, modeling