UNDERWATER DISTURBANCES 



O. L. Martin Jr. 



Maritime Safety Division 



U. S. Naval Oceanographic Office 



This article is being published to create a measure of seaquake con- 

 sciousness in the minds of watch standers so that they will be in a posi- 

 tion to recognize and therefore be more able to cope with the forces acting 

 upon a ship during an underwater disturbance. The article is not 

 intended to alarm the mariner about a natural phenomena which, al- 

 though of considerable interest, may never be experienced to a degree which 

 could cause distress. 



INTRODUCTION 



Slowly pacing the wing of the bridge, the mate watched the sun 

 rise above the horizon as the ship steamed eastward in a slight south- 

 westerly sea at a speed of 17 knots. Morning stars placed the ship 

 approximately 20 miles off the southwest coast of Central America 

 in 2,000 fathoms of water. Suddenly, there was a thunderous boom, 

 the ship commenced to shake violently and seem to rise up in the 

 water. Although the fathometer registered no bottom, the ensuing 

 vibrations became so severe that the engines were stopped; the mate 

 was certain the ship had grounded. An inspection of the hull, bilges, 

 and machinery revealed no apparent damage. However, the master 

 believed that the ship had at least struck a submerged object with 

 such force that there must be underwater damage to the hull. Subse- 

 quently, the ship was drydocked, at considerable expense, only to 

 find no evidence of underwater damage. 



Later, an investigation of seismographic data provided by the 

 U.S. Coast and Geodetic Survey disclosed that a few seconds before 

 the vibrations commenced aboard the ship a strong earthquake oc- 

 curred under the ocean floor about 35 miles southeast of the ship. 



EARTHQUAKE HISTORY 



Psychologically, an earthquake affects man in a rather strange and 

 awesome manner. When the earth begins to lurch beneath his feet 

 and he realizes that "terra firma" is a rather ambiguous term, his 

 first impulse is usually to run. Often, this sudden undulation of the 

 earth's surface subjects man to nausea and headache. Even the 

 equilibrium mechanisms in his ears and eyes can be affected in such 

 a manner that small vibrations in solid structures are magnified to 

 the extent that if real the structures would be torn asunder. Broken 

 columns and sheared arches of ancient ruins are majestic witnesses 

 to catastrophic earthquakes of the past. Throughout recorded his- 

 tory, man has wondered about the cause and effect of the earth- 

 quakes that have continually plagued him. Two thousand years ago, 

 the Chinese devised primitive instruments for detecting distant earth- 

 quakes. Other civilizations, particularly the Japanese, meticulously 

 recorded the observed effects in the shaken area of each earthquake. 

 These studies, although useful, revealed little of the geography, 

 geology, and mechanics of earthquakes. Man did not begin to compre- 

 hend the nature of earthquakes until an English geologist and mining 

 engineer named John Milne founded modern seismology. 



Milne, already an engineer of considerable renown, accepted in 

 1875 a position as professor of geology and mining at the Imperial 

 College of Engineering in Tokyo. Immediately upon arrival in 

 Japan he was greeted by an earthquake and at once became involved 

 in earthquake research. 



Literally, an earthquake is a shaking of the earth in which the 

 ground surface moves back and forth, side to side, and up and down. 

 If man could detach himself from the earth and remain suspended 

 above the affected area, it would be simple to determine the relative 

 magnitude and direction of ground motion during an earthquake. 

 This is impossible. However, Newton's "First Law of Motion" 

 states that a body at rest will remain at rest until acted upon by an 

 external force. Thus, as with a pendulum, if the connection between 

 a body and the earth is as loose as possible, the movement of the 

 earth will exert a minimum external force on the body. Then, when 

 an earthquake does occur, the body will tend to remain stationary 

 while the earth moves beneath it. Milne incorporated this principle 

 in the instrument he developed to scientifically detect and record 

 local ground movements caused by the Japanese earthquakes. Basi- 

 cally, the instrument, firmly secured to the ground, consisted of the 

 loosely connected body (pendulum) to which a marking pen was at- 

 tached in such a manner that it lightly scribed a line on a contin- 



uously moving roll of paper powered by a clock mechanism. Thus, 

 when earthquake vibrations moved the paper in relation to the mark- 

 ing pen, Milne was able not only to obtain a written record of these 

 vibrations in the form of jagged lines, but he was also able to deter- 

 mine the time of the movements. 



Normally, seismographic stations are equipped with 3 instru- 

 ments — 2 orientated at right angles, to record movements from east- 

 west and north-south directions, and 1 to record the vertical move- 

 ments. Thus, a record can be obtained of all 3 basic earthquake 

 movements. Modern engineering and electronics have greatly im- 

 proved todays' seismographs permitting even more detailed study of 

 the phenomena. Each variation in the jagged lines as recorded on 

 seismograms have meaning to the seismologist who can, by comparing 

 the seismograms of a single distant earthquake from several stations, 

 determine the location of its epicenter and the depth beneath the 

 earth's surface (hypocenter) at which the original fracture occurred 



Milne did not actually invent the seismograph, the principle and 

 idea had been advanced about 1830, but he did develop and field 

 test a very practical instrument by setting up a network of seismo- 

 graphic stations throughout Japan. Data obtained from the network 

 enabled him to determine the geographic positions of the epicenters 

 for 8,331 earthquakes in the next 8 years. When the earthquake 

 epicenters were plotted, he discovered they fell in a relatively narrow 

 belt along the East Coast of Japan, and that the interior of the Islands 

 were relatively free of earthquakes. Upon returning to England, in 

 1885, Milne presented his findings to the Seismological Society, where 

 they created such a great interest that a new science was launched — 

 the study of earthquakes by an interpretation of the shock (seismic) 

 waves they generate in the earth. 



Milne proposed, and by the turn of the century had in operation, 

 a world wide system of 34 seismographic stations. The information 

 obtained from these stations enabled him to chart areas of earthquake 

 activity throughout the world; basically the seismic zones he charted 

 are unchanged today. Milne furnished the tools and established the 

 methodology of the new science, but discoveries and advancements 

 came fast as man began to seismically probe the interior of the earth. 

 By 1907 the velocities of the seismic waves in the earth's mantle 

 were accurately known and the existence of a core had been discov- 

 ered. Soon Andrya Mohoroviac discovered that a discontinuity 

 formed the lower boundary of the crust, and by 1913 Beno Gatenberg 

 had determined the radius and nature of the core. Only in this cen- 

 tury, with the aid of seismology, has man been able to investigate 

 earth beneath its surface to study scientifically the nature of its in- 

 terior. 



OUR QUAKING EARTH 



53 



