UNDERWATER DISTURBANCES 



vibrations and create a greater amount of movement. This condition 

 is called resonance, and the probability of damage from shaking is 

 greater when this occurs than if the two components are completely 

 out of phase. Another closely related phenomenon is when two 

 adjacent structures having different natural periods of vibrations get 

 out of step and tend to batter each other down when shaken by an 

 earthquake. 



SEAQUAKES 



The mechanics of an earthquake are the same, regardless of 

 whether occurring under a land mass or the ocean floor. Earth- 

 quakes ashore are .sometimes called land quakes and the seismic 

 vibrations frequently felt aboard ships are commonly referred to as 

 seaquakes. Although the majority of the world's earthquakes occur 

 under the ocean floors, the mariner is in a position far superior to his 

 land bound brother. A .ship built to withstand the rigors of the sea 

 and remain a cohesive unit regardle.ss of the direction from which an 

 external force is applied becomes "earthquakewise" an engineering 

 marvel. The intensity of shipboard seismic effects varies greatly, 

 but an analysis of ship reports describing .seaquakes in view of local 

 geological conditions and available instrumental data reveals several 

 interesting generalities that are in line with the facts a.ssociated with 

 land quakes. 



The principal .seismic effect aboard ship is the jackhammering 

 vibrations induced on the hull by the arrival of the P wave. I'pon 

 entering the le.ss den.se water from the .sedimentary covering on the 

 ocean floor this compre.ssional-type wave is bent by refraction anil 

 deflected almost vertically to the .sea .surface at 0.8 knot per second. 

 Frequently, the first P waves to arrive on the surface of the sea are 

 not strong enough to be felt aboard .ship, and will pass into the 

 atmosphere above the .sea to create a .sound wave. When the fre- 

 quency of the .sound wave is high enough to be audible a loud bomb- 

 bing noise will immediately preceed the actual vibrations. Sound is 

 not heard in all earthquakes, and it is possible under certain condi- 

 tions to have the .sound without the vibrations. 



The duration of the.se vibrations may vary from a fraction of a 

 second to .several minutes. Ship reports indicate a usual duration 

 of between 15 and 60 seconds. When the P wave arrives at the .sea 

 surface its period is short and amplitude very low. Normally, the 

 amplitude is so small there is no indication of a disturbance by the 

 appearance of the sea surface. Yet this wave front simultaneously 

 striking the complete under water portion of the hull produces enough 

 energy to cause severe vibrations. Rarely have these vibrations 

 damaged ships, but a report from a ship off Point San Telmo, 

 Mexico, stated that on 15 April 1947 earthquake vibrations caused a 

 well-secured deck cargo of heavy steel prefabricated construction 

 sections to walk 6 inches. 



The intensity of shipboard vibrations are determined by internal 

 as well as external factors. Internal variables that affect this vibra- 

 tion include the type and construction of the ship, the nature and 

 amount of cargo on board as well as the manner in which it is stowed, 

 and under certain conditions the position in which the cargo-handling 

 gear or other heavy equipment is seciu-ed. Among the external fac- 

 tors the magnitude of the earthquake, determining the amount of 

 energy released, and the depth of the original fracture (hypocenter) 

 are very important. Frequently, the perpendicular distance to the 

 fault along which the original fracture occurred may be more impor- 

 tant than the epicentral distance. Again at sea, as in a land quake, 

 the earthquake energy tends to travel great distances down or paral- 

 lel to the fault with little loss of intensity, but the perceptibility tends 

 to fall off rather rapidly as the perpendicular distance from the fault 

 increases. Although weather has no bearing on the origin of an earth- 

 quake, the effects (vibrations) on a ship may be magnified by un- 

 favorable weather conditions. 



Occasionally, there is a weaker but definitely distinguishable 

 second set of vibrations closely following the original jolt. This is 

 not a twin quake but the arrival of part of the energy from the slower 

 S wave. When this sheer wave strikes the density discontinuity at 

 the ocean floor part of the energy is reflected back into the earth and 

 part of the energy is transformed into a compressional-type wave and 

 deflected almost vertically into the less dense water. There is an 

 energy loss in this transformation, but the arrival of the S wave under 

 the hull is quite perceptible under proper conditions. If a vessel is 

 in the immediate epicentral area of a shallow quake, the arrival of the 

 second group will only tend to intensify the original vibrations and 



may not be detected. When the epicentral distance is too great the 

 arrival of a weak second group may be imperceptible. However, 

 there is a definite area between the two extremes in which the arrival 

 of the S wave group is easily distinguishable. 



MEXICAN EARTHQUAKE OF JUNE 1932 



An excellent example of the varied manner in which individual 

 ships, located at random, about the epicenter of an earthquake are 

 affected is available from the review of the ship reports describing 

 their experience during the large shallow Mexican earthquake of 3 

 June 1932. Although slight motions were felt throughout the early 

 morning hours in the mountainous area behind Manzanillo, it was 

 not until lOh 36m 50s GMT (03h 6m 50s Local Time) that the prin- 

 cipal shock occurred. The epicenter, located about 30 miles inland 

 near 19.5''N., 104.3°W. fell in the chain of volcanic mountains that 

 traverse Mexico in an east - west trend and are, probably, a conti- 

 nental continuation of the long straight Clarion Fracture Zone that 

 originates in the Central Pacific and passes through the volcanic 

 Revilla Gigedo Islands before emerging on the Mexican Coast. 



During the early morning hours of 3 June 1932, the 55 SOLAN A 

 was steaming through a smooth sea with light variable winds in 

 18°30'N., 104°08'W., at 1037 GMT she was violently shaken for 

 about 7 seconds. The .ship was then about 60 miles 170° from the 

 epicenter in approximately 800 fathoms of water and did not detect 

 any change in the state of the sea. The perpendicular distance to 

 the fault zone was also about 60 miles. 



A few miles to the southwestward in 18°20'N., 104°32'W., the 

 MV SKVKXOR experienced at the same time less severe vibrations 

 but of a longer duration (1 minute). The SEVENOR was approxi- 

 mately 70 miles 191° from the epicenter and the perpendicular dis- 

 tance to the fault zone was between 65 and 70 miles. The ship 

 reported a calm sea and slight westerly swells and detected no no- 

 ticeable change in the surface of the sea. 



Conditions aboard the MV NORTHERN SUN in 19°56' N., 

 106° 14' W. were entirely different. Although the vessel was 115 

 miles 285° from the epicenter the perpendicular distance to the prob- 

 able fault zone was probably not more than 10 miles. Vibrations, 

 commencing at 1039, continued for 3 minutes and became so violent 

 that the engines were stopped. Before the earthquake, the sea had 

 been smooth with a slight westerly swell, but by 1046 GMT the swell 

 pattern had changed and the sea was confused. 



Farther to the northward in 20°28'N., 106°20' W., the 55 ARI- 

 ZONA N commenced to vibrate at 1039 GMT and continued to do 

 .so for about 75 seconds. The ship was about 130 miles 297° from 

 the epicenter with a slight southwesterly sea and did not notice any 

 change in the state of the sea. The perpendicular distance to the 

 fault was probably somewhat over 40 miles. 





fSacture zone 



(^J^^f^^MV SVENOR * 



Manzanillo 



S. S. SOLANA 

 + \Pta. San Telmo 



110° 



MEXICAN EARTHQUAKE 

 JUNE 1932 I 



105° 



The after shocks continued for many days. Ship reports indicate 

 that during the next 36 hours several strong underwater disturbances 

 were experienced in the area. TheMV5/LV£'J?lV/LL0W in 18°45'N., 

 104°34'W. began to vibrate dangerously in every part and at the 



57 



