UNDERWATER DISTURBANCES 



same time partook an uneven short pitching motion followed by 

 heavy rolling. The disturbances commenced at 0530 GMT on 4 

 June and the rolling continued for 15 minutes. Seven hours later at 

 1245 GMT in 19°31'N., 105°45'W. the crew aboard the SS TALA- 

 MANCA heard a loud noise like distant gunfire, then experienced 

 .severe vibrations, and at 1337 GMT 2 similar reports were heard 

 about 10 seconds apart but there were no apparent vibrations. How- 

 ever, 20 minutes later the sea surface was littered for 5 or 6 miles 

 with small white oval objects. Several hours later in 19°28'N., 

 106°06'W., the SS HANOVER reported at 1205 ship's time (prob- 

 ably 1905 GMT) 2 violent shocks that rocked the ship like a nearby 

 explosion. Fifteen minutes later 2 more shocks were experienced 

 with only slight vibrations. 



The main quake created considerable damage throughout the 

 country side behind Manzanillo and inundated the immediate coastal 

 area with a minor tsunami. Tide gages in Hawaii recorded a 23/2-foot 

 wave and 12 hours after the earthquake the boxlike harbor of Pago 

 Pago, over 4,400 miles southwestward of the epicenter, experienced a 

 series of sea level fluctuations on the order of 8 feet for over an hour. 



DOMINICAN EARTHQUAKE OF AUGUST 1946 



Another interesting earthquake occurred on 4 August 1946 when 

 the northeast coast of the Dominican Republic was shaken by a de- 

 structive quake at 17h 51m 05s GMT. The epicenter was located 

 in deep water about 9 miles eastward of Cape Samona. Forty miles 

 westward of the epicenter in the bight of Escocesa Bay an 8-foot 

 tsunami surged ashore destroying the town of Matanzas. Four miles 

 northward of Cape Viejo Frances Light, or about 43 miles north- 

 westward of the epicenter, the sea around MS CAMCA became 

 confused at 1310 ship's time and commenced to boil violently, the 

 ensuing vibrations aboard ship became so turbulent the engines were 

 stopped and all machinery secured. The disturbance continued for 

 10 minutes, during which time an enourmous cloud of dust was ob- 

 served rising from the shore behind Cape Viejo Frances. Approxi- 

 mately 30 minutes later the water was observed to be boiling again, 

 but this time to the northward of the ship. Probably, there is no 

 connection but it is interesting to note that approximately 23-2 hours 

 earlier, while steaming northward through the Yucatan Channel, the 

 SS SAMUEL F. DEWING encountered similar areas of turbulent 

 water in an otherwise calm sea at 0915 (local time) near 21° 12 'N., 

 86°16'W. The disturbance appeared close ahead and when the ship 

 entered the area the bow took a 30 degree sheer to starboard despite 

 attempts to check her. Although the fathometer was in constant 

 operation no bottom trace was recorded, but in 1960 a ship did report 

 4J2 fathoms in this general area. 



During a seaquake the water mass in the ocean is seldom affected 

 but areas of boiling water, seiches, and tsunamis have been started 

 by earthquakes. Areas of boiling water that are occasionally sighted 

 by ships during and after seaquakes could be of volcanic origin, but 

 may have mechanics similar to earthquake fountains and sand blows 

 that occur ashore during seismic disturbances. This phenomena can 

 produce at great epicentral distances fountains 6 to 8 feet high that 

 flow up to 3 hours after the earthquake. The alternate tension and 

 compression applied to the ground during the passage of seismic waves 

 may open fissures and suck all the ground water into the opening, then 

 the same force closes the fissures and forces the water out. Under 

 the ocean a related mechanism would create a boiling effect on the 

 surface without an appreciable rise in water temperature, and the 

 unlimited supply of sea water would allow for an earthquake fountain 

 of staggering proportions. 



Another unusual phenomenon is the seiche, a standing wave 

 that has its crest at one end and its trough at the opposite end of an 

 enclosed or partially confined body of water, and may even occur as 

 lateral oscillations in rivers, canals, and ditches. It is only necessary 

 that the limits of the enclosure define a natural period of oscillation. 

 Earthquakes are a comparatively rare cause of seiches which are usu- 

 ally set up by prolonged winds, currents, or tides. Seiches in harbors 

 have been started by the arrival of tsunamis. True earthquake 

 seiches have been set up in graving docks and castle moats by the 

 arrival of seismic waves through the interior of the earth. Regardless 

 of the cause, it may become impossible to hold a ship alongside a 

 dock during such a disturbance. 



TSUNAMI 



Many coasts of the world, notably those bordering the Pacific 



Ocean, have been swept by giant waves surging ashore as high as 60 

 feet above normal tide levels. Although often referred to as tidal 

 waves, they are not associated with the tide in any manner but are 

 somehow related to tectonic earthquakes and are actually seismic 

 sea waves, commonly called tsunamis. The mechanics of a tsunami 

 are not fully understood. These waves usually originate in the vicin- 

 ity of deep ocean trenches after strong earthquakes, and often travel 

 undetected across thousands of miles of open ocean before devastat- 

 ing an exposed coast, island , or harbor. Block tectonics ashore have 

 produced vertical displacements of many feet over hundreds of square 

 miles. Tectonics of this nature under the sea floor would produce 

 an instantaneous movement of a tremendous mass of sea water which 

 would immediately fan out from or pour into the epicentral area with 

 great momentum, depending on whether the crustal block was raised 

 or lowered. Occasionally, tsunamis are created by earthquakes whose 

 epicenters are on shore. Possibly, the vibrations of the .sei.smic wave 

 passing through the sedimentary covering on the sides of trenches 

 and continental slopes are great enough to create great submarine 

 landslides which would again displace a tremendous amount of water 

 in a relatively short time. There are also recorded instances where 

 submarine volcanic explosions and glacier break-ups have created 

 this type of wave in localized areas. There is some thought of the 

 possibility of a tsunami being created by a mechanism related to the 

 seiche being formed in deep trenches or troughs. 



Once a tsunami is spawned it travels across the open seas at 

 speeds approaching 400 knots and because of its low wave height 

 (2 to 3 feet), tremendous distance between crest (100 miles or more), 

 and unusually long period (12 to 40 minutes) is very inconspicuous. 

 When a tsunami enters shoal water it conforms to the same physical 

 laws that govern wind-driven waves, but because of its great forward 

 speed in deep water, the slowing of bottom drag is relatively much 

 greater than for a wind-driven wave. Therefore its forward speed 

 rolls the wavefront over itself to greater heights than is possible with 

 a wind-driven wave. 



Usually, the first indication in a harbor or along an exposed 

 coast of a tsunami is a slight, often unnoticeable, rise in the water 

 level, only to be followed immediately by a noticeable withdrawal of 

 the water, which returns several minutes later to a height well above 

 normal tide level. The initial wave is seldom the highest, the crest 

 is usually reached between the third and eighth wave. Once the crest 

 is reached the wave begins to subside slowly, sometimes requiring a 

 day or more. Because of a tsunami's long period and delayed crest, 

 there is frequently enough time to take eva.sive action if it is initiated 

 immediately upon noticing the first indications. 



There are no known seismographic characteristics by which the 

 seismologist can determine if a particular earthquake will create a 

 tsunami; fortunately few do. However, once spawned the.se waves 

 can be so destructive that a Seismic Sea Wave Warning System was 

 organized and is now in operation in the Pacific Area. The system 

 consists of strategically located seismographic stations, tidal .stations, 

 a communication system, and a Control Center. Each seismographic 

 station is equipped with specially designed instruments and they are 

 located throughout the Pacific in such a manner no really strong 

 earthquake will go undetected. The tide stations are equipped with 

 Seismic Sea Wave Detectors that produce an audible alarm on the 



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