UNDERWATER DISTURBANCES 



F. A belt of deep focus earthquakes originating at depths 



between 200 and 400 miles. 



When the associated hypocenters of a typically active arc system 

 are plotted in profile on a cross sectional chart showing the relief, they 

 seem to dip under the arc, frequently under a continent, at an angle 

 of approximately 45 degrees. The epicenters of the large shallow 

 earthquakes (B) are usually on the landward side of the foredeep, 

 but the epicenters of the deep shocks (F) may fall hundreds of miles 

 away. In South America the epicenters of many of the deep shocks 

 are well to the east of the Andes. 



The chief characteristics of block tectonics are frequent shallow 

 quakes from which arc features are absent. The activity consist of 

 relative displacements of blocks, separated by large faults which are 

 often nearly straight and dip almost vertically into the earth. 



DKKP FOCUS QUAKES 



Most earthquakes occur in the earth's crust, but some originate 

 in the rock mantle at depths as great as .'iW) miles, which .seems to 

 be the floor of earthquake activity. The mechanics of an earthiiuake 

 are the same regardless of the focal depth, however, the surface af- 

 fects in the meizoseismal areas differ greatly. A deep focu.squake does 

 not create a well-defined meizoseismal area, instead there may be 

 several seemingly unrelated geographical areas of equal intensity at 

 varying distances and bearings from the epicenters. Yet in areas 

 closer to the epicenter the earthquake may be imperceptible. The 

 instrumental magnitude of two earthquakes, one normal and one 

 deep focus, may be the same, but the intensity in the meizoseismal 

 area will decrease as the depth of the hypocenter increases. 



VOLCANIC EARTHQUAKES 



There is still another class of earthquakes whose mechanics are 

 somewhat different. They are caused by the pressure of the confined 

 gases or forces brought into play during the swelling and contraction 

 of molten volcanic lava which may result in an explosion, tension 

 fracture, or fault within the structure of the volcano itself. Therefore, 

 volcanic earthquakes are dependent on volcanism and may be defined 

 as the transient elastic vibrations caused by forces originating in the 

 magma chamber and conduits of a volcano. The focal depth of this 

 type earthquake is very shallow. Frequently, the hypocenter is within 

 the volcanic cone structure itself. Volcanic earthquakes may be of 

 considerable local intensity, often resulting in extensive damage to the 

 flank of a volcano, but their total energy is slight and they are usually 

 imperceptible a few miles away. A tectonic earthquake of equal epi- 

 eentral intensity would be perceptible at great distances. 



EARTHQUAKE GEOGRAPHY 



The world chart showing the geographical distribution of seismic 

 activity reveals that the earthquake epicenters fall in narrow belts 

 and zones easily correlated with certain geological features as high 

 mountain chains, oceanic trenches, ridges, and rises. Normally, 

 where there is high-relief there is increased seismic activity, indicating 

 that high seismic activity is related to crustal disturbances of the 

 recent geological past. The highest mountains and deepest trenches 

 are probably products of fairly recent geological time. Crustal 

 irregularities as great as these would be equalized by geological pro- 

 cesses within a single epoch if stress were not continually at work to 

 maintain them. Earthquakes are indications of these stresses. 



Possibly, the present world chart of the seismic zones, even 

 though unchanged since Milne's time, merely represents a fleeting 

 geological snapshot of temporary or unusual seismic conditions. 

 Man's instrumental records span only a few short years of the hun- 

 dreds of millions of years of geological time, and the evidence found 

 in rocks indicate the tectonic geography of the earth has changed 

 considerably during the past eons. 



At present over 1 million perceptible earthquakes occur each 

 year, of which at least 700 are strong enough to cause damage in 

 their meizoseimal area. Fortunately, as the magnitude of earthquakes 

 increase the relative frequency with which they occur decreases. 

 Earthquakes as well as volcanoes are usually located close to large 

 bodies of water. Therefore, it is not surprising that three-quarters 

 of the world's earthquakes occur under the ocean floors. The North- 

 ern Hemisphere is more susceptible to seismic activity than the 

 Southern Hemisphere. The area below 30° South Latitude represents 

 one-fourth of the earth's surface, yet less than 10 percent of our 



earthquakes occur in this area. Earthquakes are unknown on the 

 continent of Antarctica. 



The Pacific Ocean is ringed by a system of active volcanic and 

 seismic belts and has a branch that extends into the East Indies. 

 Another branch extends eastward across Central America into the 

 West Indies. This system accounts for four-fifths of the world's 

 earthquakes. Another belt extends from the Mediterranean region 

 of Southern Europe across Southern Asia and down the Himalayan 

 Mountains into Southeast Asia. Active belts are also found in the 

 oceans along the Tonga Trench, down the Mid-Atlantic Ridge, and 

 southward from the Central Indian Ocean with a branch swinging 

 westward across the South Atlantic towards the Sandwich Trench 

 and another branch swinging eastward south of Australia. 



PROPAGATION' OF SEISMIC WAVES 



When a rock mass fractures, the earth reacts to the resulting 

 seismic waves as an elastic soli<l, propagating them to all parts of 

 the earth, both through the interior and over the surface. Four 

 elementary types of .seismic waves are generated, each having its 

 own characteristic type of motion and velocity that is easily disting- 

 uishable on seismograms. 



The primary wave or P wave, sometimes called the longitudinal 

 wave, is the faster of the two body waves that travel through the 

 interior of the earth. Its velocity varies from about 4 to 7 knots per 

 second, depending on the nature ami density of the material trav- 

 ersed. Because of the variations in density, the path of the P wave 

 through the earth is not a straight line cord but a concave curve bent 

 toward the center of the earth, and it requires approximately '20 

 minutes for it to travel to the opposite side of the earth. The P wave 

 has a period of about 1 .second and travels through the earth in a 

 manner similar to that of a sound wave. Thus as it radiates out from 

 the focal point, the particles of earth immediately ahead of the wave 

 front are compressed forward away from the hypocenter along the 

 path of travel. Then as the wave front passes there is a slight dila- 

 tion of the ground and the earth's particles move back along the path 

 of travel toward the hypocenter. 



The secondary body wave or S wave, commonly referred to as 

 a sheer or distortional wave, travels through the earth at velocities 

 about 0.6 that of the P wave and follows the same general path. 

 However, there must be rigidity in the material through which the S 

 wave travels, and there is no evidence that it penetrates the core. 

 The propagation of the S wave is similar to that of a light wave or 

 to the transverse vibrations of a taut string. Therefore the earth's 

 particles are displaced in a direction that is perpendicular to the 

 direction of travel from the focal point. Whenever a body wave 

 traveling through the earth strikes a region of abrupt density changes, 

 as at the earth's surface, the bottom of the crust, or even the surface 

 of the core, part of the wave energy tends to be reflected in new and 

 complex wave forms, or even transposed into waves of the opposite 

 type. Thus a sheer wave may be changed into a longitudinal wave 

 or vice versa. 



The other 2 basic types of seismic waves are the Love and the 

 Rayleigh surface waves which are created when the P wave reaches 

 the earth's surface; they account for most of the visible ground 

 movement in the immediate epicentral area. The speedier Love 

 wave with a velocity of approximately 2' o knots per second is a sheer 

 wave propagated in a horizontal plane in a manner similar to the way 

 in which a wave travels down a garden hose when it is shaken from side 

 to side. The velocity of the Rayleigh wave is only slightly lower than 

 that of the Love wave and it creates a vertical wave motion on the 

 earth's surface like the wave that travels down a carpet when one 

 end is shaken up and down. 



ENGINEERING SEISMOLOGY 



Man's increasing knowledge of earthquakes and the specialized 

 field of engineering seismology enables architects and engineers to 

 design and build structures with inherently greater earthquake resis- 

 tent qualities. Structures built to withstand great downward or 

 gravitational forces with little thought of a vertical thrusting from 

 beneath or the sheering effect of horizontal twisting may crumble 

 during an earthquake, while properly designed structures closer to 

 the epicenter escape relatively unscathed. Within a structure, the 

 tempo of earthquake shaking in relation to the natural period of 

 vibration of the structure is very important; if the frequency of the 

 induced vibration happens to be the same as the natural frequency 

 of the structure the resulting movement will be the sum of the two 



56 



