428 



NA TURE 



[February i i, 1909 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opiniom 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications.] 



Seismograms of the Earthquake of January 23. 



With reference to the article on " Recent Earthi.|uakcs " 

 iii Nature of January 2S, the accompanying records of 

 the earthquake of January 23 may perhaps be of interest. 

 These are from negatives printed from the original curves 

 taken with the Milne twin-boom seismograph at the new 

 magnetic observatory at Kskdalcmuir, Dumfries-shire, of 

 which Mr. G. W. Walker is superintendent. 



The interval between the breaks in the curve (not shown 

 in the accompanying reproduction) is one hour, and the 

 hour mark near the commencement of the earthquake 



Of these two types we may regard the San Francisco 

 earthquake of 1906 as an instance of a slip-fault move- 

 ment. The later \'alparaiso earthquake, on the other 

 hand, would appear to have been due to movement on 

 planes of overthrust faulting in an anticline, and this alone 

 would account for the greater and more widespread 

 devastation witnessed in that case. 



Applying this reasoning to the earthquake of December 

 28 last, we should expect to find that the movement was 

 of the Valparaiso type, and this receives confirmation from 

 the external Assuring of the ground at the surface. A 

 reference to Prcstwich's geological map of Europe throws 

 light at once upon the problem. The Messina Strait is 

 seen to be on the axis of an anticlinal flexure, the sea 

 being there less than 100 fathoms deep. The loo-fathom 

 contour is seen to approach the strait at both ends, and 

 then to double rather sharply back, especially on the 

 Ionian side, while the looo-fathom contour runs in 

 approximate parallelism to it. and much nearer to it on 



CAST - WE ST TRACE 



NORTH- SOUTH TRACE 



corresponds, as nearly as could be ascertained, to 

 3h. 5m. a.m., G.M.T., January 23. The natural period 

 of both booms is jS-bs., and the sensitivity is such that 

 I mm. = o"-44. 



The east-west is clearer than the north-south trace, and 

 a short piece of the latter is omitted, as it is not possible 

 10 reproduce it with certainty from the negative. 



R. T. Glazebrook. 



The National Physical Laboratory, Teddington, 

 Middlesex, February i. 



The Italian Earthquake. 



The able article in Nature of January 7 (p. 277) by 

 " R. D. O.," and the useful notes appended thereto, have 

 no doubt been read with interest by students of physical 

 geology all the world over. I should like to add a few 

 remarks, which may perhaps be also found useful. 



Thanks to the masterly teaching of the veteran geologist 

 Prof. Suess, of Vienna, as first outlined in his smaller 

 work, " Die Entstehung der Alpen," and to the teaching 

 of the Swiss school, we have learned in the last two 

 decades to trace a clearer causal connection between dis- 

 turbances of this sort and the local architectonic structure 

 of the lithosphere. The essential factor of such phenomena 

 would seem to be the local weakness of the crust, result- 

 ing in its yielding, in this area or in that, to variations 

 of stress in those potentially molten portions of the litho- 

 sphere, which, while practically rigid under the rapid 

 rotatory motion of the earth (see my letter to Nature, May 

 4, 1905, on "The Rigidity of the Earth's Interior"), 

 exist under planetary pressure at temperatures above the 

 solid-liquid critical temperature of the mineral masses of 

 which they are composed. 



The variations referred to (whether from cosmic or 

 terrestrial causes) compel portions of the overlying crust, 

 of course, to adjust themselves under the influence of 

 gravitation to the altered mechanical conditions. Such 

 adjustments may, and generally do, occur on lines of 

 ancient " faulting," and may be classified as positive and 

 negative. The former we should expect to occur as down- 

 ward movements under the direct action of gravitation 

 where faulting occurs in a synclinal flexure, the tendency 

 of the bed-rock being to sag down, and in such cases we 

 get a slip-fault movement. On the other hand, where any 

 part of the force of gravity is resolved into tangential 

 thrusts on or near axes of anticlinal flexures, the fault- 

 movements are almost bound to be of an overthrust nature. 



NO. 2050, VOL. 79] 



the Ionian side than on the Tyrrhenian side. The fault- 

 ing, which Prof. Suess is reported to have sketched in the 

 Vienna papers, seems to cut through the Archaean crystal- 

 line mass in the north of Calabria, and then to follow its 

 western boundary for some distance further south, coin- 

 ciding in part with the shore-line. Under the strait itself 

 it seems to bifurcate " in the direction of Etna," accord- 

 ing to Suess, but I would suggest along the southern 

 limit of the exposed crystalline mass, which forms the 

 high promontory of the Peloritan mountains, since 

 Taormina appears to have escaped the effects of the earth- 

 quake. The point of bifurcation would be the weakest 

 place, and therefore the locality in which the upthrust 

 would be most perceptible. If this is admitted, we may 

 discern here Ihe true cause of the dual wave which 

 swamped the low-lving portions of both Messina and 

 Reggio. 



Further, the steepness of the submarine gradients on 

 the south or Ionian side of the area, as compared With 

 those on the Tyrrhenian side, seems to indicate the exist- 

 ence, on a much smaller scale, of conditions which hold 

 good in Japan, where the bed of the ocean rapidly descends 

 to the greatest oceanic depth known on the Pacific side, 

 the " concave " side of the " mountain-wave " (Suess), as 

 compared with the gradients of the " convex side," the 

 shallow Sea of Japan. Prof. Suess (one of our greatest 

 masters) will therefore perhaps allow me to suggest that 

 the seismic movement in the present instance occurred 

 rather on or just outside the rim of the disc-like area of 

 subsidence which is occupied by the Tyrrhenian Sea, an 

 area of which the Lipari Islands with their volcanoes 

 mark an incidental fracture-feature (as worked out years 

 ago by Judd) rather than the centre. 



In the view here put forward the minor earthquake 

 shocks felt a fortnight or so later in the Tuscan region, at 

 Ravenna, and other places, would follow as incidents in 

 the more complete adjustment of the geologically young 

 range of the Apennines to the disturbances of previous 

 mechanical equilibrium, caused by the greater disturbance 

 on the other side of the Tyrrhenian Sea, which has startled 

 the world by its results. The differential results in the 

 Messina-Reggio region would seem, further, to be accounted 

 for by sidelong movements of the ground due to over- 

 thrust faulting, so terrible always in its effect upon build- 

 ings badly constructed and erected upon such loose and 

 incoherent rock-materials as those which constitute the 

 Quaternary and later Tertiary strata, upon which the low- 



