UNDERWATER DISTURBANCES 



arrival of waves with periods between 10 and 40 minutes. A high 

 priority radio system permits rapid collection and dissemination of 

 tsunami information by the Control Center at the Honolulu Mag- 

 netic Observatory. SEISMO is the characteristic identifying sym- 

 bol for this type radio traffic. 



UNCHARTED OBSTRUCTIONS 



Since the advent of the fathometer the mushrooming supply of 

 new sounding data adds daily to man's increasing knowledge of the 

 topography of the ocean floors. No longer is the bottom of the ocean 

 believed to be as monotonously void of irregularities as is its surface. 

 Man realizes the magnitude and grandeur of many bottom irregular- 

 ities are unmatched anywhere on earth. However, even today vast 

 reaches of the ocean floors remain inadequately surveyed, averaging 

 in some areas only one sounding to many square miles of ocean floor. 

 Consequently, within these areas, particularly where irregularities 

 exist, must lie countless uncharted shoals and banks. Today the 

 increasing drafts of ships and operational depths of submarines 

 greatly magnify the importance of these uncharted shoals as dangers 

 to navigation. 



Modern charts carry many doubtful shoals and reefs that mar- 

 iners have encountered in the past. Many of these obstructions have 

 been confirmed but others defy confirmation. Frequently, there is 

 an astounding similarity between the reports of vessels encountering 

 uncharted obstructions and the reports of vessels undergoing a seismic 

 disturbance. A similarity also exists between ships reports describing 

 distant reefs and rocks awash and vessels reporting early stages of 

 submarine volcanics. A comparison of the chartlet showing the earth- 

 quake belts of the world and the chartlet showing concentrated areas 

 of unconfirmed obstructions reveals a remarkable analogy. For this 

 reason observers should always include the precise GMT of an obser- 

 vation when reporting a suspected obstruction. 



SUMMARY 



Psychologically, the mariner's sensation of grounding is common 

 to most seaquake reports. 



A seaquake will affect each ship differently. The type, construc- 

 tion, and amount of cargo are internal factors affecting the intensity 

 of the vibrations. 



Seaquake vibrations may become severe enough to warrant 

 stopping the engine and securing machinery. 



Frequently, there are two distinct series of closely related vi- 

 brations, representing the arrival under the ship of the P and S waves, 

 respectively. 



Usually shipboard vibrations cease with the terminaton of the 

 seaquake, but, under unusual conditions, vessels have been known to 

 undergo unexplained heavy rolling or pitching motions for several 

 minutes after the original disturbance ceased. 



Visibly, the state of the sea is unchanged during a seaquake. 

 However, areas of turbulent water and locally confused seas are 

 occasionally encountered during seismic disturbances, and this con- 

 dition may exist for some time afterward. 



Marine life can be destroyed by a seaquake. 



There is no correlation between weather and earthquakes, but 

 the condition of the sea at the time of a seaquake may have a bearing 

 on the shipboard intensity of the vibrations. 



Sound may precede the perceptible affects of a seaquake. 



Although the epicenter of an earthquake is ashore the effects 

 may be perceptible aboard ship. 



Almost all seaquakes are the result of tectonic earthquakes. 



The mechanics of tectonic earthquakes are the same whether on 

 land or under the sea. When the rock structure deep within the earth 

 is rigid enough to resist slow deformation by changing stress patterns, 

 the stress will accumulate until the elastic limit of the rock mass is 

 reached, then somewhere along a fault plane the structure will snap, 

 instantly readjusting itself to a condition of no stress. 



Geologically, earthquakes are associated with high relief as 

 mountain chains and deep oceanic trenches. 



Exactly what triggers an earthquake is unknown and they can- 

 not be predicted. 



Tectonic earthquakes tend to occur in swarms, with the epicenter 

 of each subsequent quake migrating parallel to a fault or major struc- 

 tural trend. 



Earthquake energy at sea or ashore tends to travel considerable 

 distances down or parallel to the fault with little loss of energy, but 



the intensity tends to fall off uniformly, rather rapidly as the perpen- 

 dicular distance to the fault increases. 



A study of the chart showing, approximately, the fault zones in 

 the earth's crust and earthquake epicenters for the world will give the 

 mariner an indication of the areas in which he may expect to encounter 

 seaquakes. 



The tsunami chart depicting the coasts that have been inundated 

 by these waves will also furnish the mariner an indication of the gen- 

 eral areas in which tsunamis may be encountered. 



Tsunamis tend to originate near deep ocean trenches and are 

 somehow connected to tectonic earthquakes that usually have their 

 epicenter on the landward side of deep oceanic trenches. 



The most probable explanation of the creation of a tsunami calls 

 for the vertical displacement of a block of the earth's crust under the 

 sea. However, the epicenter of an earthquake does not have to be 

 under the ocean to create a tsunami. 



Fortunately, earthquakes rarely produce tsunamis, but there is 

 no known method to foretell which earthquake will produce a tsunami. 



Usually, the first noticeable indication of an approaching tsunami 

 is a lowering of the water level along an exposed beach or harbor. 



At sea a tsunami is almost imperceptible, although thousands of 

 miles from its origin a tsunami may surge ashore as high as 60 feet 

 above normal tide level. 



The period of a tsunami in deep water is between 12 and 40 

 minutes and its height is under 3 feet. The wave length or distance 

 between successive tsunami wave crests may be 100 miles or more 

 and it may reach forward speeds in excess of 400 knots. 



The first wave of a tsunami is seldom the greatest. Usually, the 

 crest is reached between the 3rd and 8th wave. Therefore, there is 

 frequently enough time to take precautionary measures, if the action 

 is initiated immediately. 



Obey local tsunami warnings issued by competent authorities 

 and work your vessel into deep water when possible. 



, CRLISTAL FEATURES 



:-v;:wU.^^;^;t;;^Vli;:.i 



Seaman's Description 

 Weak seaquake 



SEAQUAKES 



Shipboard Effect 



. Slight vibrations, normally felt in ex- 

 ceptionally fine weather only, as the 

 vibrations would be lost in an easy roll. 



59 



