Nakamura Saemon Taro

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Nakamura, Saemon Taro

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On the Seismic Energy and the Age of theEarth.著者 Nakamura  Saemom, Taro雑誌名 Science reports of the Tohoku University. Ser.5, Geophysics巻 2号 3ページ 206-208発行年 1950-11URL http://hdl.handle.net/10097/44450ON THE SEISMIC ENERGY AND   AGE OF THE EARTHTHEInstitute    Saemon Taro NAKAMURA of  Gecphysics, Faculty of Science,  Thhoku        (Received April 3, 1950)  The maximum seismic energy estimated hitherto is of the order of  1025 ergs. It is only 1/5 of total solar energy received by the earth in one minute. If only a small part of this solar energy can be available to the occurrence of earthquake, it is  sufficient to it. But the solar energy cannot penetrate into the deep part of  the earth's crust where earthquake occurs. It is, therefore, not im-portant in the seismology.  One who observe faults or warping of the earth's crust in the cases of large earthquakes tends to consider the yielding of the earth's crust as the cause of the earthquake. But yielding of the earth's crust itself must have some other source of energy. Such a source of energy must be the inequality of  temper-ature. Inequality of temperature must be caused by either inequal cooling or heating. JOLY attributes it to the distribution of the continental mass, which covers the layer of sima, and is main source of radioactive heat. He tried to explain the geological revolutions by it. But there are much larger  sources of energy ever ignored.  In this paper, the author tried to compare the amount of heat evolved by various sources. § 1. Radioactive Heat.  The heat evolved by the radioactive disin-tegration in the earth's crust is some 15.9Universitycalories or  66.8X107 ergs per year per metric tons of rock. If the density.of rock be 2.8, one cubic kilometer of rock has the mass of 2.8  x  1015 grs. or  2.8X  109 metric tons, and it evolves the energy of 1.87 x  10'8 ergs per year. The radioactive heat evolved in one cubic kilometer of rock can cause a single earth-quake every  5.4  X 106 years. As the total area of the earth is about 5.1  X 108km2, if such a radioactive layer has the thickness of 10.6 m, it can cause  one largest earthquake every year. But as the heat evolved in oceanic area is continually radiate out as  joLY stated, thickness of active layer must necessarily be 36  m. This is  sufficiently large to explain the present seismic activity.  But the fact that igneous rocks intruded in sedimentary rocks in different geological eras have different uranium-lead ratios, must be explained by that uranium had not radioactivity before it intruded into surface layers, because uranium is quite dispersed mass in  rock's and cannot be totally separated from lead yielded by it when it intruded. On this point the author will discuss in details in other case. The radioactive heat ever esti-mated cannot, therefore, be acceptable. • § 2. Depresson of Rock Layer.  Geographer ever classified earthquakes into three different kinds. One of them was caused by the depression of rock into under-                           ON THE  SEISMIC  EN] ground cavity. One cubic kilometer of rock with the weight of  2.8X  1015 grs. is assumed  Eleme to  fall down 1 m.  Its energy is  2.74X  1020 ergs.                                        in th(To cause a large earthquake of the energy of there 1025 ergs, rock of  10km3 must  fall down a  knowr distance of  36  5 m. It may be, therefore, very  oxidal rare to cause the largest earthquake in this  impor mechanism. quite                                          rare  i  3. Atomic Explosion.                                               earth.  Atom-bomb yields considerably large  ener-gy.  neous  When a large atom-bomb explodes in  follovvthe interior of the earth, will  it cause  large'  earthquake?  92U235 yields an energy of  2X 108  Elemer electronvolts or 3.2 x  10-4 ergs per atom. As the weight of an atom of  92U235 is about 3.92  Si  X10-22 grs, energy of 8.2  x  10' ergs can be evolved by 1 gr. of 921.235. 12.2 metric tons of Al                                                  Fe92U235 is, therefore, necessary to cause a  Ca largest earthquake. As ordinary uranium  Na contains only  0.71% of  82U235 and only  5  X  10' grs of  uranium in one gram of rock,  0.34X Mg                                             fI  1026 or  12.3 cubic megameters of rock is  need to cause largest earthquake. It is quite impossible to collect such a large mass of Thi  92U235 from such a large volume of rock. It posed is quite impossible to collect uranium at a ergs place to cause spontaneous  explosion. it is                                          of su§ 4. Latent Heat of Fusion of Rock.                                  If th   The latent heat of fusion of rock when it densi crystallizes from amorphous state is about it to 100 cal/gr. or  4.2  X 10° ergs per gr. If a mass Then of 1  km3 of rock whose density is 2.8 crystal- is su lizes, energy of  1.18X  1025 ergs can be freed. If  tl To cause a single largest earthquake, only  whol, 0.85  km3 of rock is necessary to crystallize.  0.2-1 If this is distributed on the whole area of in c the earth, its thickness is only 1.67 microns. 0.3 r  If the oceanic area is neglected as  jOLY to di stated, it is 5.7 microns. of si tC  ERGY 207 § 5.  Heat of Oxidation of Rock.  Elements contained in igneous rocks are all in the state of oxides. What elements are there in the interior of the earth is little  n. But it is certain that in volcanoes  i Lion of gases at the crater plays an  i portant role. From this fact it may be quite natural to suppose that oxygen is very rare  n the interior of the d ep part of the If exclude oxygen from ordinary  ig-neous rock, its constitution is as given in the  ll wing table. Elements Percentage:    of  elementsOxides51.46  15.08  9.48  6.74  5.28  4.81  3.90  0.24Si02Al203  FeOs  CaO Na20  K‘20  MgO  H20Heat of oxidation per grain 29580  X  107  ergs  3U;60  7180  15800   9022   4616  156[0 142380 Heat of oxidation of each element in 1 gr. of mother rock.15200 X  107   ergs   4700  700   1100    500    200   600  34200 f-  urn  57200  .  The heat of oxidation of 1 gr. of the sup-posed mother material of rock is 57200  x  107 ergs per gram. Even if hydrogen is excluded, it is 23000  X  i07 ergs per gram. On the density  ch a mother material is nothing known. If the elements are purely mixed, its mean density is 2.7. Therefore tentatively assume it to be the same of ordinary rock or 2.8. Then 0.1-0.05  km3 of such a mother material is sufficient to evolve an energy of 10" ergs. If this mother material is distributed on whole area of the earth, its thickness is only  0.2-0.1 microns. And if it be distributed only in continental area the thickness is  0.6— 0.3 microns. It is of cource not necessary to distribute in a uniform layer. 0.1-0.05  km3 of such a mother material of rock at the208 S. T.  NA origin of earthquake oxidizes, then one lar-gest earthqauke can be caused. § 6. Conclusion.  The largest possible energy source of seis-mic activity is the oxidation of the mother material of rock. Only 0.05-0.1 micron of layer or 0.05-0.1 km3 of it is  sufficient for it. The heat of crystallization of amorphous rock is next large one 5.7 micron or 1 km3 of rock is  sufficient to crystallize. Radioactive energy is also possible. But further large mass of rock must be necessary to attribute to the action.  Depression of rock is not  important. On the evolution of heat from radioactive substances, the author cannot accept it without further discussion.  Considering intermediate layer of the earth constructed by  metallic alloy and oxidation of it at its very thin layer at the uppermost part or in a very small volume at the origin of earthquake, seismic activity can be easily explained. Volcanic and other tectonic activ-ities are also explained by this assumption.   If 20 largest earthquakes to be assumed to occur every year, 10 microns of the mother material under continental area is need toAKAMURAoxidize. If in this same rate oxidation has undergone, formation of continents of 60 km thick takes  6X  10'° years or thousand milion years. It is about 200 times of the age of Archaean rock, and nearly the  same of that of the sun.  In this estimation the heat radiated out  from the earth's  surface is neglected. But the work done as tectonic, seismic, and vol-canic actions must finally generate heat. These actions are to be regarded as non-efficient heat engine. The thickness of the radioactive layer is several km to maintain the observed geothermal gradient. The thickness to main-tain present seismic activity is only 36 m, or about only 1/1000 parts are used for seismic action.   If other heat sources are taken in consid-eration the effeciency may be much lower.  References  : (1)  H. JEFFREYS,  Geophysical  svpy. Roy.  Astr. Soc.  (2) J. JOLY,  Ralioactivity and the Surface History     of the  Earth, Oxford Universty Press 1924.  (3) S. T.  NAKAMURA, Some Criticism on the Age of     the  Ear  th and Temperature Distribution in It,    (Japanese)  Report  Jap. Ass. Adv.  Sci., Vol. 8. 190     (1933).

On the Seismic Energy and the Age of the Earth.

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On the Seismic Energy and the Age of the Earth.

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