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On the distribution of earthquakes by latitude. (Ch. Maurain, C. R. 184 p. 612, 1927). 1. Maurain’s statistics (Ch. Maurain) refer only to strong earthquakes recorded at a considerable distance by a large number of stations, and the geographical coordinates of whose epicenters can therefore be determined with sufficient accuracy. These conditions for proper statistics had previously been indicated by Milne.
The new statistics cover the following earthquakes: 1° 542 earthquakes that occurred from 1914 to 1920, whose epicenters are indicated in the Bulletin of the British Association Seismology Committee or in the International Seismological Summary and which were recorded at least at a distance of 1500 km or 135° of the arc of a great circle; 2° 1009 earthquakes from 1903 to 1910, found in lists published by Milne, and of which he says that they were detected over the whole earth, or in any case over an area exceeding the area of Europe and Asia together.
All these 1551 earthquakes are distributed as follows among zones bounded by the parallels 0°–10°, 10°–20°, etc.
| 0°–10° | 10°–20° | 20°–30° | 30°–40° | 40°–50° | 50°–60° | 60°–70° | 70°–80° | 80°–90° |
|---|---|---|---|---|---|---|---|---|
| 450 | 329 | 244 | 258 | 152 | 87 | 21 | 9 | 1 |
Taking into account the size of the area of these different zones, they find that the number of earthquakes falling on a surface of 10 million square kilometers will be as follows:
\[ 59{,}8 \qquad 38{,}3 \qquad 30{,}4 \qquad 35{,}4 \qquad 24{,}1 \qquad 17{,}1 \qquad 5{,}6 \qquad 3{,}9 \qquad 1{,}3 \]
Thus the density of earthquakes, in general, is the greater the smaller the latitude.
This result, of course, depends to some extent on the distribution of seismic stations by latitude. Since in the northern hemisphere a larger number of stations are situated in the middle latitudes, strong earthquakes should be registered equally well whether they occur in the polar regions or whether their epicenters are located at the equator; moreover, 30 observatories are situated above \(50^\circ\) latitude, and some even above \(60^\circ\). In the southern hemisphere most seismic stations are located between latitudes \(30^\circ\) and \(40^\circ\), and some are above \(40^\circ\); despite unfavorable conditions, seismographs capable of recording earthquakes at a distance of 15,000 km will in general register even those earthquakes that occur in the Antarctic; earthquakes occurring at the south pole will be detected at all observatories situated between the south pole and latitude \(45^\circ\).
542 earthquakes included in the first part of the statistics are divided almost equally between the two hemispheres (273 in the northern hemisphere, 264 in the southern, and approximately 5 on the equator). The distribution is sufficiently developed for the part of the statistics borrowed from Milne; out of all 1551 earthquakes, 927 fall in the northern hemisphere and 624 in the southern, with earthquakes occurring near the equator divided almost in half.
The distribution of 542 earthquakes by zones of \(10^\circ\) is as follows:
| \(0^\circ—10^\circ\) | \(10^\circ—20^\circ\) | \(20^\circ—30^\circ\) | \(30^\circ—40^\circ\) | \(40^\circ—50^\circ\) | \(50^\circ—60^\circ\) | \(60^\circ—70^\circ\) | \(70^\circ—80^\circ\) | \(80^\circ—90^\circ\) |
|---|---|---|---|---|---|---|---|---|
| 164 | 142 | 84 | 77 | 56 | 15 | 2 | 2 | 0 |
For each zone, per area of 10 million square kilometers, the following number of earthquakes occurs:
\[ 18{,}5 \qquad 16{,}5 \qquad 10{,}4 \qquad 10{,}6 \qquad 8{,}9 \qquad 3 \qquad 0{,}6 \qquad 0{,}9 \qquad 0 \]
i.e., the change occurs in the same way as was found for all 1551 earthquakes.
Thus it turns out that there is indeed a decrease in the density of earthquakes with increasing latitude.
- Naturally, the existence of this variation may be explained by the rotational motion of the Earth and the centrifugal force caused by it. One may suppose that the distribution of masses in the layers lying beneath the outer crust does not everywhere satisfy the equilibrium conditions corresponding to the field of forces composed of the forces of gravity and centrifugal forces; deviations from the equilibrium conditions, more considerable at the equator, will decrease from the equator toward the pole.
The works of Milne and Montessus de Ballore have led to the well-known results that, in strongly seismic regions, significant geological evolution takes place. It may be considered that the results of the statistics cited above indicate the influence of the Earth’s rotation on the distribution of earthquakes in these regions of active geological evolution. The first and second explanations attribute the role to the existence of deviations from gravitational equilibrium conditions, causing either slow (geological) or rapid (seismic) movements of the layers of the Earth’s crust lying not far beneath the surface.
M. Polikarpov.