The detection of secondary tsunamis

The long wave data recorded at Wake Island during the time period of 9–12 March 1957 is analyzed through the application of a variable period band pass filter with a specified width. The filter band is successively centered at the periods of 6, 20, 55 and 100 min. The primary tsunami of 9 March 1957 generated in the Aleutian Islands is observed in all the filtered records as well as in the original record. Several hours after the primary tsunami, additional bursts of wave energy begin to appear in the filtered wave records. The later wave arrivals are well correlated with the expected arrival times of long surface gravity waves generated by the aftershocks of the initial tsunamigenic earthquake. These secondary tsunamis are most distinctive in the 55 and 100 min period filtered records. Since permanent ground displacements are not always expected with aftershock activity, these secondary tsunamis are theorized to have been generated by nonpermanent or transient ground displacements.