The great differences in transit velocity obtained for different earthquakes is a point worthy of attention.
Seebach’s velocity is a _true_ transit velocity, and its determination is dependent on the assumption that the shock radiated from the _centrum_ and not from the _epicentrum_, Seebach’s method is explained when speaking about the determination of origins.
Some interesting observations on the velocity with which the earthquake of October 7, 1874, was propagated, are given by M. S. di Rossi.[18]
One assumption is that the disturbance radiated from an origin to surrounding points of observation, whilst another is that the disturbance followed natural fractures, the direction of which is derived from the crest of certain mountain ranges. These velocities are as follows, Maradi being at or near the origin of the disturbance:—
+-----------------------------+-----------------------------------+ | Velocity in feet per second | Velocity in feet per second by | | with direct radiation | propagation along mountain chains | +-----------------------------+-----------------------------------+ | Modigliana 820 | By the Valley of Marenzo 1,080 | | Bologna 656 | „ „ Saveno 1,080 | | Forli 874 | „ „ Montone 1,080 | | Modena 518 | „ „ Panaro { 1,080 | | | { 984 | | Firenze 273 | „ „ Sieve 540 | | Compiobbi 328 | „ „ „ 540 | +-----------------------------+-----------------------------------+
Another set of interesting results are those of P. Serpieri on the earthquake of March 12, 1873. The curious manner in which this shock radiated is described in the chapter on the Geographical Distribution of Earthquakes (see p. 231). Two large areas appear to have been almost simultaneously struck, so that, there being no time for elastic yielding, the velocities calculated between places situated on either of the areas are exceedingly great.[19]
From Ragusa to Venice the velocity was 2,734 feet per second „ Spoleto „ „ 4,101 „ „ „ Perugia to Orvieto „ 601 „ „ „ „ „ Ancona „ 1,640 „ „ „ „ „ Rome „ { 1,640 „ „ { or, 2,186
The following are examples of approximate earthquake velocities which have been determined in Japan.
_The Tokio Earthquake of October 25, 1881._—From records respecting this earthquake it appears to have been felt over the whole of Yezo and the northern and eastern coast of Nipon, a little farther south than Tokio. It was severest at Nemuro and Hakodate, and at the former place a little damage was done. From these facts, together with the indications of instruments recording direction of movement and a general inspection of the time records, it seems that the disturbance must have originated beneath the sea on the east coast of Yezo at a very long distance to the north-east of Tokio, from which place it passed in a practically direct line on to Yokohama.