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Pacific Coastal & Marine Science Center

Tsunamis & Earthquakes

USGS PCMSC Tsunami and Earthquake Studies

 

Descriptive Model of the July 17, 1998 Papua New Guinea Tsunami:
Assumptions and Specifications for Animations

Hydrodynamic Model:

  • A linearized form of the shallow-water wave equations was used to simulate the tsunami. Because non-linear terms were not included, the computations are probably only accurate for water depths deeper than 50 m.
  • The computations were performed on a 3.7 km grid at a time interval of 10 seconds. Although the model is stable, the nearshore evolution of the tsunami is not accurately portrayed because of the large grid size.
  • The model does not compute runup or inundation.

Earthquake Source Model:

  • Because only the location, depth, and seismic moment of the event are known at this time, certain assumptions must be made to determine the rupture area and slip.
  • Slip on the vertical plane is assumed.
  • An empirical relationship is used to determine the rupture area from the seismic moment (Wells and Coppersmith, 1994). An assumed 3:2 aspect ratio (length:width) yields the dimensions of the rupture zone.
  • Using the assumed rupture area (above) and a shear modulus of 30 GPa results in an average slip of 2.15 m.
  • Assuming uniform slip typically underestimates the tsunami that is produced (Geist, 1998). Therefore, an arbitrary and symmetric slip distribution is assumed that averages to 2.15 m.
  • Because this was a very shallow event, the rupture extends to the seafloor.
  • A 10 second rise time is assumed, making this a nearly impulsive event with respect to tsunami propagation speeds.
  • Because of the steep bathymetric gradient, errors in the epicenter location result in significant changes to the tsunami time history. The Harvard centroid location is used.

 

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