Download Alaska Tsunami Mapping

Near-Field Modeling of the
1964 Alaska Tsunami:
A Source Function Study
Elena Suleimani, Natalia Ruppert, Dmitry
Nicolsky, and Roger Hansen
Alaska Earthquake Information Center
Geophysical Institute
University of Alaska Fairbanks
XXIV International Tsunami Symposium
Novosibirsk, July 2009
Motivation
 Most coastal communities in Alaska were
affected by the 1964 tsunami. For the purposes
of tsunami inundation mapping, it can be
considered as a credible worst case scenario for
a number of communities.
 This event is an excellent field benchmark for
numerical modeling studies, since effects of the
tsunami are well documented. However, details
of co-seismic slip distribution are very crucial
for the near-field modeling and analysis.
 Existing source functions of the 1964 tsunami
allow to use this event as a validation scenario
for inundation modeling and mapping of Alaska
coastal communities.
The M9.2 Great Alaska Earthquake and
Tsunami of March 28, 1964
 Area of crustal deformation:
>256,000 km2
 Rupture duration ~4 min.
 Major tectonic tsunami and
about 20 local submarine and
subaerial landslide tsunamis.
Tsunami damage:
 Alaska: 106 deaths and $84 M
 British Columbia: $10 M
 Oregon: 4 deaths and $0.7 M
 California: 12 deaths and $17 M
Source Function by Johnson et al. (1996)
 Joint inversion of the far-field
tsunami waveforms (23 tidal
stations) and geodetic data (vertical
displacements and horizontal
vectors).
 The source model consisted of 17
subfaults plus one subfault
representing the Patton Bay fault
(splay fault).
 Results support division of the
1964 rupture zone into the Kodiak
and PWS blocks. Kodiak asperity is
constrained entirely by the tsunami
data.
Source Function by Ichinose et al. (2007)
 Combined least squares inversion
of teleseismic P waves, tsunami
records (9 tidal stations) and
geodetic leveling survey
observations.
 Multiple time window kinematic
rupture model based on Green’s
function technique.
 Source model consists of 85
subfaults of 50x50 km, and 10
subfaults of 20x20 km representing
the Patton Bay fault.
 Three regions of major seismic
moment release (slip more than
twice the average).
Source Function by Suito et al. (in review)
 A 3-D viscoelastic model was
developed together with afterslip
model to study postsiesmic
deformations of the 1964
earthquake.
 The model uses realistic geometry
including an elastic slab with very
low dip angle.
 The model extends the Montague
Island splay fault farther along the
coast of Kenai Peninsula, and, as a
result, slip on megathrust in this
region is smaller.
Numerical Experiments
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•
•
Model propagation of tsunami waves using
different source models.
Compare results in the far field (Northern
Pacific).
Use higher resolution grids in the Gulf of
Alaska (near-field) and compare to
observations.
G. Plafker (1969), Tectonics, USGS Prof. paper 543-I
Distribution of Maximum Amplitudes
 All sources result in strong
directivity of energy radiation towards
west coast of the US and Canada,
although with slightly different angles.
 Coastal areas of southern Alaska,
BC, Washington and Oregon show
amplitude enhancement in all runs.
Far-Field Results
 Calculations were performed on a
2 arc-min grid of Northern Pacific.
 Arrival times agree very well with
tide gauge records.
 Amplitudes are generally
underestimated, but increasing grid
resolution around tide gauges results
in better fit to data.
Near-Field Results
 Kodiak and Prince William Sound
grids of 8 arc-sec resolution (125m x
245m).
 Distribution of maximum amplitudes
after an 8-hour model run.
 Results are different for all 3 source
models.
up
up
down
up
Conclusions
• We modeled the 1964 Alaska tsunami using 3 different source
functions and compared results in the far and near fields.
• The far-field tsunami waveforms produced by all models are very
similar, indicating that the far-field results are not very sensitive to
fine details of the slip distribution.
• The near-field modeling results are very different for all 3 models
and neither one matches the observations well.
• More work is needed to decompose source functions and to relate
different segments of slip to the near-field observations.
• Lack of good bathymetry data for Alaska coast makes these
modeling attempts of near-field tsunami effects difficult.