412 EARTHQUAKE PHENOMENA. 



Mr. Mallet's Fourth Report on Facts and Theory, &c., "Brit. Ass. 

 Trans.," 1857— J 58, but which space will not permit of transcript 

 here. 



An erroneous notion of the dimensions of the great earth-wave 

 must not be formed from its being called an undulation; its velocity 

 of translation upon the earth's surface is great occasionally in hard, 

 elastic, and unshattered formations, probably as much as thirty miles 

 per minute, and the wave or shock moving at this rate has been re- 

 corded to have taken some seconds to pass a given point; if so, its 

 length or amplitude is often several miles. Its altitude, however, is 

 not great, and, as may be seen from Fig. 1, continually diminishes 

 as the wave passes outwards from the origin. 



Before, during, or immediately after the passage of the great 

 earth-wave or main undulation, a continuous violent tremor or short 

 quick undulation (like a short chopping sea) is often felt. This may 

 arise from secondary elastic waves accompanying the great earth- 

 wave, (like the small curling or capillary waves on the surface of the 

 ocean swell,) produced probably by comparatively minute or 

 secondary impulses, due to the discontinuous and heterogeneous 

 nature of the formations through which the normal wave has been 

 propagated. Sometimes, however, a number of shocks occur so 

 rapidly as to convey the impression of a continuous jar or tremor, 

 and may be succeeded by one or more great shocks; this is probably 

 the source of "tremor observed before the shock," as the subse- 

 quent arrival of the transversal waves is of the tremors after it. 

 (For other complications of the phenomena, see Mallet's 1st, 2d, and 

 4th Reports, Brit. xVss.) It is very desirable that the interval in 

 time between these minor oscillations .should be observed by a 

 seconds watch, and also their total duration at each epoch of motion. 

 Former narrators often confound the whole of each epoch of such 

 rapidly recurrent shocks with one shock supposed to last a con- 

 siderable time. 



2d. When the superficial undulation of the earth-wave, coming 

 from inland, reaches the shores of the sea, (unless these be pre- 

 cipitous, with deep water,) it may lift the water of the sea up and 

 carry it along on its back, as it were, as it goes out into deep water; 

 for the rate of transit is so great that the elongated heap of water 

 lifted up has not time to subside laterally. This may be called the 

 forced sea-wave; its elevation will be comparatively small, and a little 

 less than the altitude of the earth-wave, when close to the shore on 

 a sloping beach; and where the water is still, any observations that 

 can be made as to the height of this fluid ridge will afford rude indi- 

 cations of the altitude of the earth-wave or shock. 



Earthquakes, whether at sea or on land, seem to be only accom- 

 panied with subterranean noises when strata are fractured or masses 

 of matter rent or blown away at volcanic origins. Where such is not 

 the case, the two preceding are the only waves to be expected from 

 an earthquake of inland origin; but when fracture occurs, then at 

 the moment of the shock, or very slightly before or after it, we shall 



