396 L. MARTIN ALASKAN EARTHQUAKES OF 1899 



ever, 60 miles farther from Yakutat, along the same line, jarring appara- 

 tus and causing lamps to swing in the U. S. Weather Bureau office at 

 Eagle and being observed by prospectors panning gold in the Forty-Mile 

 district. 



Local topographic or geological conditions also explain observation or 

 failure at great distances. In the Koyukuk and lower Yukon observa- 

 tions (shown on figure 3 as detached areas) weak tremors seem to have 

 been naturally amplified in the unconsolidated Pleistocene silts. 



SQUARE MILES SHAKEN 



The shocks of the great earthquake on September 10 (figure 4) were 

 felt at all places within 250 miles of Yakutat and at other points up to 

 480 miles from the origin. This minimum area of sensible shocks in- 

 cludes 216,300 square miles on the land alone, and if an equal area in the 

 Pacific Ocean was disturbed (figure 1) the minimum area includes 

 432,500 square miles. This takes no account of the detached observation 

 on Lake Chelan, Washington. 



The observations by Mr. Schrader on the Koyukuk and by Father 

 Amcan on the lower Yukon are detached areas beyond the compact mini- 

 mum area of the shock of September 3 (figure 3). If these points, 670 

 and 730 miles respectively from Yakutat, were included, together with 

 other points equally distant, a circular area with a radius of 700 miles 

 was shaken, and this area included about 1,539,000 square miles. This 

 is considered quite probable, first, because this part of Alaska was, in 

 1899, an almost vacant wilderness, and, secondly, because people in 

 Alaska become so accustomed to earthquakes that they do not notice or 

 record weak tremors at great distances like these. In Alaska, too, there 

 were no high buildings to naturally amplify weak tremors not sensible to 

 persons on the ground, as was the case in La Crosse, Wisconsin; Boston, 

 Massachusetts, and Few York City^^^ during the Charleston earthquake 

 of 1886. 



The inclusion of the disturbed areas within circles follows the assump- 

 tion that Yakutat Bay was the center of disturbances, outside of which 

 there was little or no movement. This hypothesis seems warranted, in 

 the absence of other evidence, and the plotting of the minimum areas 

 (figures 3 and 4) bears this out somewhat. An alternate hypothesis 

 would consider the tectonic disturbances to extend in the direction of the 

 axis of the Saint Elias-Chugach range. Facts are not available for set- 

 tling absolutely between these hypotheses. 



"0 C, E. Dutton : Ninth Annual Report of the U. S. Geological Survey, 1889, 



