Alaska Earthquake and Tsunami, I — Pararas-Carayannis 
307 
and spreading across the Sound (Plafker and 
Mayo, 1965). This wave was caused by slump- 
ing of the glacial deltas in Port Valdez which 
had been shaken loose by the force of the 
earthquake. 
TSUNAMI MECHANISM 
Most tsunamis result from earthquakes hav- 
ing focal depths of less than 60 km. Iida 
(1958) has derived an empirical relation giv- 
ing the maximum focal depth H (in km) for 
an earthquake of magnitude M which has re- 
sulted in a detectable tsunami: 
M i§ 6.42 + 0.01 H (1) 
where M is the Richter magnitude given by 
log E(ergs) = 11.8 -f- 1.5 M (2) 
The focal depth of the Alaska earthquake was 
about 20 km. This was shallow enough to create 
tsunami waves even though the epicenter of the 
main shock was as much as 100 km inland from 
the coast. A number of shallower aftershocks 
over a large area ranging from Hinchinbrook 
Island to southeast Kodiak Island indicate that 
crustal movements over a wide area were in- 
volved. Undoubtedly these shallow aftershocks 
created smaller waves that could not be sepa- 
rated, in the tide gauge records, from reflections 
of the initial tsunami. 
If the tsunami waves that hit the island of 
Kodiak were the result of crustal movements 
only, then the first wave could be expected to 
