1. 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

2. 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.

3. 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 The M9.2 Great Alaska Earthquake and Tsunami of March 28, 1964  Area of crustal deformation: >256,000 km 2  Rupture duration ~4 min.  Major tectonic tsunami and about 20 local submarine and subaerial landslide tsunamis.

4. 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.

5. 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).

6. 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.

7. Numerical Experiments 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

8. 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.

9. 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.

11.  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. Near-Field Results

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13. 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.