30 REPORT OF THE CALIFORNIA EARTHQUAKE COMMISSION. 
We should expect that the slow accumulation of strain would, in general, reach a 
maximum value and bring about a rupture in a single, comparatively narrow fault-zone; 
and this is probably what occurs for the majority of tectonic earthquakes, but it is quite 
conceivable that the strains should become so great along two or more separated zones,. 
that the vibrations, set up by the rupture of one, might be sufficient to begin the rupture 
of the second; or indeed, that the relief of strain at one might cause additional strain at 
the other and thus start the rupture there, tho this seems improbable if they are as much 
as 20 or 80 km. apart. But it does not seem possible that large blocks of the earth’s 
crust could be suddenly moved as a whole; if the material under the block slowly sank, 
the elasticity of the rock would allow the block to follow, still resting upon the substratum, 
and only a zone between the sinking area and the surrounding regions would be elastically 
strained and experience a sudden elastic rebound when the rupture occurred; and if the 
sinking area were large, the irregularity of the movement would probably bring about rup- 
tures on different sides at widely different times. If a limited region should be elevated, 
exactly the opposite movements would take place. It must not be inferred, from what 
has been said, that small narrow blocks, from a few meters to a few kilometers in width, 
may not be raised or dropt as a whole, but they should be lookt upon as small blocks, 
forming a part of a single fault-zone and playing a very minor part in the general dis- 
turbance of the earthquake. 
The Mino-Owari earthquake of 1891, the Formosan earthquake of 1906, and the Cali- 
fornia earthquake of 1906 are good cases of earthquakes practically with a single fault- 
zone; whereas, the great earthquake in the central part of Japan in 1896 resulted from 
fractures along two roughly parallel fault-planes 15 to 18 km. apart, and the intervening 
region was elevated 1 to 3 meters; one of the fractures was considerably longer than the 
other; and there is no evidence of any connecting fractures, which would separate the 
elevated region into a block; the faults apparently die out, as faults usually do, and 
the elevation diminishes towards their ends and finally disappears completely. The two 
fractures occurred at about the same time, but no determinations were made exact enough 
to show that they occurred simultaneously. The sharply defined areas in Iceland over 
which the earthquakes of 1896 were severally felt suggest that they were due to the set- 
tling of successive blocks, and this ideais strengthened by the fact that the region is deprest 
and separated from the higher adjacent region by a fault. But the description given by 
Dr. Thoroddsen * does not indicate that the individual areas mentioned are bounded by 
faults, nor does he adduce any evidence that they sank at the time of the shocks, tho 
he does describe some large fissures which ran across several of them. Iceland is actively 
volcanic, and the descriptions of it suggest a very mobile condition not far below the sur- 
face. If this condition really exists, it would be much easier for cracks to form at approxi- 
mately the same time and break up the crust into blocks there than in regions where the 
crust rests on a firmer foundation. 
The elevations and depressions about Yakutat Bay, Alaska, which Messrs. Tarr and 
Martin have described as due to the earthquake of 1899, strongly suggest the movement of 
blocks ;? but they did not find evidences of faultings on more than three sides of a block, 
and that in only one instance; tho it must be noted that they were unable to examine 
more than a very limited area and could not determine where the lines of fracture ended. 
It seems possible that the displacements they describe might be accounted for by an up- 
ward pressure, with or without a compression in a direction running north-northwest 
and south-southeast. Such a pressure and compression would bend the rocks into an 
arch, with the surface under tension, and the rupture would occur when this tension 
reached the limiting strength of the rock; the rupture would begin at the surface and 


? Das Erdbeben in Island im Jahre, 1896. Petermann’s Mitt. 1901, vol. xiv, pp. 53-56. 
* Recent Changes of Level in the Yakutat Bay Region, Alaska. Bull. Geol. Soc. Amer. 1906, vol. 
XVII, pp. 29-64. 
