UNDERWATER DISTURBANCES 



Today, scientists generally agree on the concept of the structure 

 and nature of the earth as an elastic sphere slightly flattened at the 

 poles with an equatorial radius of 3,444 miles. The mean rigidity of 

 the earth is of about the same order as that of steel, and it consists 

 of 3 principal parts — the crust, mantle, and core. The crust is an 

 encircling outer shell of heterogenous rock varying in thickness from 

 about 6 miles under the oceans to 15 or 20 miles beneath the conti- 

 nents, but increasing to as much as 30 miles under some mountain 

 chains. The irregularity of the base of the crust is, possibly, the 

 principal departure from the spherical symmetry in the physical pro- 

 perties of the earth. Below the crust is the mantle, believed to be a 

 thick massive shell of dense fine-grained rock that has solidified from 

 a hot molten condition. The thickness of the mantle is about 0.45 

 of the earth's radius and its volume accounts for appro.ximately 85 

 percent of the earth's mass; for practical purposes, the mantle may 

 be considered concentrically homogeneous. Inside of the mantle is 

 the core, a hot dense molten sphere with a radius about 0.55 that of 

 the earth's. Apparently, the core has no rigidity and its exact content 

 is unknown, although it reacts to many investigations in the same 

 manner as would a highly compressed ball of molten nickel-iron. 



Over long periods of geological time, stresses build up within the 

 rock structure of the crust and upper mantle. If the pressure in- 

 crea.ses slow enough, the confined rocks will bend and flow like plastic, 

 and .the rock structure will adjust itself to the changing stress. How- 

 ever, when pressure is applied too fast or the structure is rigid enough 

 to resist slow deformation, the stress will accumulate until the elastic 

 limit of the rock mass is reached. When this happens, somewhere 

 the rock will break. The accumulated pressure will be released in- 

 stantly and the energy transformed into shTjck (seismic) waves. This 

 is the mechanics of an earthquake. 



How the pressure originates is unknown. Many theories have 

 been advanced but none satisfies the scientist completely. The pre.s- 

 sure seems to be connected, in some manner, to the expansion and 

 contraction of the rock mass due to differential heating. The weight 

 of the material above induces another pressure, but this can be 

 calculated. In the crust this pre.ssure increases one atmosphere 

 (14 psi) for approximately each 13 feet of depth. However, the rate 

 of increase is more rapid at greater depths and the pressure at the 

 center of the earth has been e.stimated to be as high as 57 million 

 pounds per square inch. Under great pressure within the earth's 

 mantle, the rocks .-seemingly become stronger and far more plastic, 

 tending to distort rather than break. Although most of the earth- 

 quakes occur within 25 miles of the earth's surface, .some have oc- 

 curred as deep as 390 miles, which seems to be the floor of earthquake 

 action. The pressure as this depth is estimated to be about 3 ' •_. 

 million pounds per square inch, and possibly beyond this point the 

 rock mantle becomes so plyable it will not fracture. 



TECTONIC EARTHQUAKES 



The majority of earthquakes involve a relative movement of 

 rock masses along either side of a fracture or fault plane in the earth's 

 crust. When this relative movement is in a vertical direction, there 

 will be either an elevation or depression of one of the faces. Should 

 an entire crustal block be defined by faults, the entire block may ri.se 

 or fall as a unit. More often however, only one side of the crustal 

 block will move, resulting in a tilting or folding of the crust. Fre- 

 quently, the relative movement along the fault is in a horizontal 

 direction resulting in a twisting effect or horizontal displacement. 

 Earthquakes involving this sudden crustal movement are calleil tec- 

 tonic because they are structural in nature. Normally, in a tectonic 

 earthquake the area of greatest shock intensity lies along or parallel 

 to a fault. Often tectonic earthquakes occur in groups with the epi- 

 center of each subsequent quake migrating along parallel to the fault. 

 The perceptible effect tends to travel great distances down the fault, 

 but decreases rapidly as the perpendicular distance from the fault 

 increa.ses. Although tectonic earthquakes may occur in regions of 

 volcanic activity there is no correlation between the two. The con- 

 trolling factor determining the magnitude of a tectonic earthquake is 

 the relation between the size of the rock mass and the relative dis- 

 tance of movement. In a tremor the displacement may be slight, but 

 visible evidence on the surface have indicated abrupt displacements 

 on the order of 50 feet after some major earthquakes. 



During most earthquakes, the maximum intensity of observed 

 vibrations occur in an elliptical area enclosing the epicenter, which 

 normally lies along the major axis of the ellipse near one end. Usually, 



the orientation of this axis is parallel to major structural trends or 

 faults in the area. The intensity of an earthquake is represented by a 

 roman numeral rating (I to XII) on a descriptive scale that indicates 

 the physically perceptible earthquake motion and is based solely on 

 the effects observed on people and inanimate objects such as buildings 

 and their contents. The area representing the highest intensity is 

 called the meizoseimal area and outside this area, usually elliptical, the 

 intensity falls off rapidly in a uniform manner. Each earthquake 

 will have several intensity ratings from the highest in the meizoseimal 

 area to the lowest rating where the motion is barely perceptible on 

 the outer fringes. Scientifically, an earthquake is described by its 

 magnitude which is derived instrumentally, from the seisomogram, 

 and is a logarithmic m.easure of the total earthquake energy released 

 at the focal point. There is only one magnitude for an earthquake. 



Although the mechanics are similar, there are two distinct clas- 

 ses of tectonic earthquakes — the arc and block types. Arc tectonics 

 constitute the majority group, but most of the knowledge of this type 

 is derived from the analysis of instrumental findings recorded at great 

 epicentral distances. However, on-the-spot observations of earth- 

 quake mechanics including studies of the visible results in the meiz- 

 oseimal area are plentiful from regions of block tectonics. Therefore 

 present earthquake theories are based primarily on information ob- 

 tained from this type. Block tectonics are dominant in California 

 and New Zealand and probably in most regions where only shallow 

 earthquakes occur, as along the Mid-Atlantic Ridge. In sections of 

 Japan, Peru, the Philippines, and North Island (New Zealand) both 

 types occur. The remainder of the seismic activity throughout the 

 world is predominantly arcuate. Arc tectonic are three-dimensional 

 in scope and require a profile view as well as a plan for their study. 

 Normally, they are associated with certain geological features usually 

 found in the following order, beginning on the outside of an arc and 

 traveling toward the center: 



A. A deep oceanic trench or trough. 



B. The principal tectonic line, with a narrow belt of shal- 

 low earthquake epicenters along a non-volcanic up-folding or 

 rise of the earth's crust which may form a submarine ridge or 

 possibly emerge as a chain of small islands. 



C. A belt of earthquake originating at a depth of approxi- 

 mately 40 miles. 



D. The principal structural arc, often consisting of large 

 islands or even a coastal mountain range containing active vol- 

 canoes, and a belt of earthquakes originating at depths of 60 

 miles or more. 



E. An inner structural arc of older mountains, extinct vol- 

 canoes, and earthquakes originating at depths between 120 and 

 180 miles. 



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