TIDAL DATUM PLANES 41 
60 miles south of Unimak Island, Alaska Peninsula. The curve of Figure 20 reproduces 
the tide curve at Valparaiso, Chile, for the first 10 hours of April 2, 1946. For the 
first six hours the curve is the normal tide curve for that place, but a little after 6, 
large seiches are recorded, these seiches resulting from the seaquake which occurred 
8,000 miles to the northwest 18 hours previously. 
At some places, seiche is an almost constant accompaniment of the tide. Figure 
21 reproduces on a reduced scale the tide curve at Mormon Island inside of Los Angeles 
Fic. 20.—Tide curve, Valparaiso, Chile, showing seiches due to distant seaquake. 
Harbor for the forenoon of January 10, 1951. The seiche here is practically a constant 
feature, although varying in amplitude throughout the year. For the forenoon shown, 
the range of tide was 4.5 feet and the seiche had a range of about half a foot, but at 
times the seiche here may have a range of 1} feet. 
In the tabulation of the tidal record ‘‘saw teeth”’ and seiches introduce difficulties. 
For use in the determination of tidal datum planes it is preferable to consider a smooth 
curve drawn through such irregularities and tabulate the hourly heights directly from 
this smooth curve. The times and heights of the high and low waters should also be 
949995—51——-4 
