The following background information on tsunami observations on coastal sea level stations was provided by the ITIC to participants during training courses in 2006 and 2007. It provides a practical illustration of the tsunami science principles.

1.WAVE HEIGHT AND PERIOD:

Relevant Points:

a. Tsunami are a series of waves that continue for many hours.

b. The 1st wave height may not be the largest.

c. The　largest wave may not occur at the station closest to the earthquake.

d. Tsunami signals arriving at coastal stations are affected by local conditions,such as the
    roughness of seafloor, configuration of the coast (bays,headlands), rate of shoaling or
    shallowing of the seafloor. Consequently, the size of the tsunami can vary greatly over short     distances along a coast.

e. Tsunami wave periods can vary from minutes (5-10) to one hour.

Waveforms for the Tokachi-oki earthquake (M8.0) and tsunami, 26 September 2003

Notes:

1. Record length is about 25 hrs, with tick marks every 5 hours. Amplitude scale of each record aries and normalized. Shown are high-frequency, low-amplitude wind-generated waves (A), tsunami (B), and diurnal tide (C, 1 ~12-hr cycle).
2. Showing of the tides is useful because you can determine if the tsunami will arrive at high tides (which would make it more dangerous), or low tide (less dangerous). Removal of tides is useful for showing the tsunami and measuring its arrival time, amplitude and period.
3. Tsunami warnings are cancelled when signals become small on many stations. In this example, it would be cancelled after about 12 hrs.

2. CHARACTER OF LEADING WAVES ? WHEN IS THE 1st WAVE A RECEDING WAVE? PREDICTIONS BASED ON THEORY VS ACTUAL

Relevant Points:

a. The earthquake rupture determines the initial character of the tsunami wave.

b. In theory, for a subduction zone thrust earthquake, the thrusting plate moves the ocean
    column upward creating a wave crest above and a trough behind. An advancing wave is
    created in the direction of the thrust, and a receding wave in the direction of plate
    subduction.

c. In reality, seafloor topography between the tsunami source and affected coast will modify
    and may complicate the character of the leading wave (so that the prediction from theory
    may be wrong).

The 1st tsunami wave arrival will be an advancing wave (wall of water) toward China, and a receding wave at Luzon, Philippines. With time, the wave character is changed by seafloor topography. The trailing trough catches up to crest in the north (A), but not in the south (A').