UC San Diego Bulletin of the Scripps Institution of Oceanography Title The Tsunami of April 1, 1946 Permalink https://escholarship.org/uc/item/89040915 Authors Shepard, F P Macdonald, G A Cox, D C Publication Date 1949-01-19 Supplemental Material https://escholarship.org/uc/item/89040915#supplemental Peer reviewed eScholarship.org Powered by the California Digital Library University of California THE TSUNAMI OF APRIL 1,1946 BY F. P. SHEPARD, G. A.MACDONALD, AND D. C. COX UNIVERSITY OF CALIFORNIA PRESS BERKELEY AND LOS ANGELES 1950 BULLETINOF THE SCRIPPSINSTITUTION OF OCEANOORAPHY OF THE UNIVERSITPOF CALIFORNIA LA JOLLA,CALIFORNIA EDITOW: C. E. ZOBELL, C~RLECKART, C. L. HUBBS Volume 5, No. 6, pp. 391-528, plates 6-33, 21 figures in text Submitted by editors January 19,1949 Issued March 24,1950 Price, $1.75 [CONTRIBUTIONSFROM THE SCRIPPS INSTITUTION OF OCEANOORAPHY, NEW SERIES, NO. 4411 PRIITED IN THE UNITED STATES OF AMERICA CONTENTS PAQE Abstract .................................................................... 391 Introduction ................................................................392 Acknowledgments ..........................................................393 General Features of Tsunamis .................................................. 393 Name ................................................................ 393 Origin ................................................................ 394 Nature of the waves .................................................... 395 Geographic and Geologic Setting of the Hawaiian Islands ........................... 397 History of Tsunamis in Hawaii ................................................398 TheTsunamiofAprill, 1946 .................................................. 404 General features ........................................................... 404 Source of the waves .....................................................404 Time of arrival in Hawaii ................................................ 404 Speed of the waves ..................................................... 406 Period of the waves ..................................................... 406 Wavelength .......................................................... 407 Relative size of successive waves .......................................... 407 Nature of the waves striking Hawaiian shores .............................. 409 Measurementofwaveheightsonshore ........................................ 410 Description of effects on individual islands ..................................... 411 Island of Kauai ..........................................................411 Island of Niihau ..........................................................418 IslandofOahu ...........................................................418 WaveatKawelaBay, Oahu .............................................424 IslandofMolokai ........................................................ 427 IslandofLanai .......................................................... 431 IslandofMaui ........................................................... 431 IslandofHawaii ......................................................... 436 Coastalgeography .....................................................436 Sea-floortopography ................................................... 437 Elevations attained by the tsunami ....................................... 438 EKectsinHiloBay ..................................................... 441 The tsunami along the eastern shores of the Pacific .............................. 443 High-water marks ....................................................... 443 Tide-gauge records .....................................................445 Comparisons with observations of Japanese tsunamis ............................ 445 Variation in intensity and effects of the waves in Hawaii ..........................447 Effects of coasbline orientation ..........................................449 Effects of shape of the island ............................................. 449 Exposure of the coast to storm waves ............................. z ........ 449 Prcsence or absence of coral reefs ........................................ 450 Shore-line configuration ................................................. 451 Submarine topography ..................................................452 Promontories bordered by deep water .....................................453 Interference of refracted and reflected waves ...............................454 [iii] iv Contents PAGE Effects on wells and springs in Hawaii .........................................455 Damage by the tsunami .....................................................456 Structural damage .....................................................456 Damage to vegetation .................................................. 459 Erosion ................................................................... 460 Subaerial and shore-line changes .......................................... 461 Submarine erosion ..................................................... 462 Casualties in the tsunami .................................................... 463 Conclusions ................................................................. 464 Danger areas .......................................................... 464 Mitigation of future damage ............................................. 466 Future avoidance of loss of lie ........................................... 466 Plans for observing future waves ............................................. 468 Literature Cited ............................................................. 469 Plates .......................................................................... 471 THE TSUNAMI OF APRIL 1,1946 BY F. P. SHEPARD, G. A. MACDONALD, AND D. C. COX ABSTRACT THE TSUNAMI which devastated the shores of the Hawaiian Islands on April 1, 1946, was caused by a movement of the sea bottom on the northern slope of the Aleutian Trough, south of Unimak Island. The waves traveled southward to Hawaii with an average speed of roughly 490 miles an hour, a wave length of nearly 100 miles, and a height in the open sea which is thought to have been 2 feet or less. The height and violence of the wave attack on Hawaiian shores varied greatly: at some points the waves dashed up on the shore with great violence and to heights as great as 55 feet above sea level; elsewhere they rose slowly and without turbulence. A detailed account is given of the heights the waves reached on the shores of the major islands, and damage done is described. Waves were generally highest on those sides of the islands which were toward the wave origin, and at the heads of sub- marine ridges projecting into deep water. They tended to be lower at the heads of submarine valleys, along shores protected by wide coral reefs, and at the ends of peninsulas projecting into deep water without corresponding submarine ridges. Waves were refracted around circular or nearly circular islands much more effectively than around elongate and rectangu- lar islands. Locally, storm waves superimposed on the crest of the broader swells of the tsunami did considerable damage. The areas suffering heavy damage in the 1946 tsunami are in general those most subject to damage by future tsunamis originating in the North Pacific. Eastern and southern coasts are subject to damage by tsunamis from South America. Western coasts are comparatively safe, although they have suffered some damage from Japanese tsunamis. All Hawaiian shores are subject to possible damage from tsunamis of local origin. Damage from future tsunamis can be lessened by avoiding construction in known danger areas; by construction of suitable sea walls, with open strips behind them; by limiting construction in heavily populated areas of danger to reinforced concrete or other wave-resistant structures; and by raising on stilts the frame structures in rural areas. Loss of life can be lessened or elimi- nated by establishing a suitable warning system, the warnings being based on instrumental detection of the small preliminary water wave, or the observation of ocean waves by shore stations around the borders of the Pacific Ocean and on mid-Pacific islands nearer the origin of the waves. INTRODUCTION ON THE morning of April 1, 1946, the Hawaiian Islands experienced the most disastrous tsunami in their history. More than 150 persons were killed, prin- cipally by drowning, and 163 others were badly injured. Hundreds of houses and other small buildings were destroyed or badly damaged. Property damage probably amounted to $25,000,000. Many other tsunamis have struck the Hawaiian Islands. In fact, such waves are recorded on the Honolulu tide gauge on an average of about one a year. Most of them cause no damage, being so small that their arrival is generally unnoticed, but in the past 125 years several have been severe enough to cause loss of life and damage to property. The Hawaiian shores must, therefore, be recognized as subject to occasional attack by tsunamis. The severity of the wave onslaught was not uniform along the entire shore, but varied greatly from place to place, and over short distances. Locally, one house escaped severe damage while another near by, situated as close to the strand line and at about the same altitude above sea level, was destroyed. Damage was severe on the shores of one bay, whereas in adjacent bays it was light. The reasons for some of these variations are obvious, but those for others are not immediately apparent. Thus, the delineation of those areas which