Suess, who has written upon the earthquakes of lower Austria, shows how the majority of the disturbances have had their origin along certain lines which form a break in the continuity of the Alps. One line runs north-east from Bruck towards Vienna. Near Wiener Neustadt, where the greatest number and heaviest shocks have occurred, this line is met by a north-north-west line crossing the Danube and following the valley of the river Kamp.[90] Hoeffer has drawn similar lines from the head of the Adriatic, one set running north-north-east to intersect near Litschau, and the other north-north-west to intersect near Frankfort in the valley of the Rhine.[91] _Examples of distribution._—A curious example of the distribution of seismic movement is that of the earthquake of March 12, 1873, worked out by Professor P. A. Serpieri. This earthquake appears to have been simultaneously felt on the Dalmatian coast and in central Italy, in a region lying north-east from Rome and south-east from Florence. In both of these areas the motion was from south-east to north-west. The shock then radiated from the central Italian regions, so that at places on the western shore of the Adriatic it was felt after it had been felt on the Dalmatian coast. Many explanations might be offered for this peculiar distribution of seismic activity. Possibly the shock originated at a great depth beneath the bed of the southern part of the Adriatic, and by following existing lines of weakness simultaneously reached the surface of the earth in central Italy and Dalmatia. In Tokio, which is built partly on a flat plain, partly in valleys denuded from a low tableland, and partly on the spurs of the tableland itself, the distribution of earthquakes is a subject yet requiring attention. Sometimes it has happened that persons in one house have been sufficiently alarmed to escape into the open air, whilst others, not more than a mile distant, have not been aware that the city had been shaken. [Illustration: FIG. 34. Areas almost simultaneously struck from S.E. to N.W. [graphic] Subsequent radial disturbance [graphic]] _Extension of earthquake boundaries._—Natural obstructions which may be sufficient to retard small earthquakes may in certain instances not be found sufficient to retard the larger disturbances. Thus the shocks of Calabria are usually only felt on the western side of the Apennines, but instances have occurred when they have crossed this barrier. In 1801 the earthquake of Cumana crossed a branch of the coast range. Sometimes earthquake boundaries give way, and countries which they sheltered subsequently become exposed to all disturbances. The true explanation of this is probably in a shifting of the centre of seismic activity. Thus up to December 14, 1797, although Cumana was often devastated, the peninsula of Araya was not hurt. On this date Araya commenced to suffer, and has continued to suffer ever since.