| Locality | Moment observed of shock | Distance from presumed origin | Velocity in feet per second | ||
|---|---|---|---|---|---|
h. | m. | ° | ′ | ||
| Presumed focus, lat. 43°, long. 11° W. | 11 | 51 | — | — | |
| Ship at sea in lat. 43°, many leagues from coast of Portugal | 11 | 52 | 0 | 30 | 3,091 |
| Ship in lat. 44° 8′ and about 80 leagues from coast | 11 | 54 | 1 | 45 | 3,607 |
| Corunna | 11 | 51 | 2 | 30 | 2,576 |
| Ship lat. 44° 8′ and 80 leagues NNW. of Cape Finisterre | 11 | 58 | 3 | 30 | 3,091 |
| Lisbon | noon | 4 | 30 | 3,091 | |
| Madeira | 12 | 6 | 10 | 0 | 4,122 |
| Cork | 12 | 11 | 9 | 30 | 2,937 |
These tables, owing to the nature of the materials which Mallet had at his disposal, are but rude approximations to the truth. Two interesting facts are, however, observable: the first being that the velocities for the earthquake of 1761 are much higher than those obtained for the earthquake of 1755; and, secondly, that in both cases the velocities as determined from the observations of ships at sea closely approximate to each other, in all cases being nearly the same as that with which a sound wave would travel through water.
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 with direct radiation | Velocity in feet per second by 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 |
| |||
| 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 |
| „ „ |
The following are examples of approximate earthquake velocities which have been determined in Japan.