アラスカ中部イーグルサミットにおける地温と凍上および斜面物質移動の観測

抄録

 Ground temperature, frost heave, and slow mass movement were measured on a debris-mantled slope at Eagle Summit, central Alaska from 2004 to 2008. The maximum and minimum temperatures at the ground surface during the measurement period were 25.7°C on 4 July 2007 and −32.7°C on 28 February. The mean annual ground surface temperature was −3.1°C.
 Subsurface isotherms indicate that seasonal freezing starts from late September to early November, and that seasonal frost begins to melt in mid-May. The thawing front reached a depth of 80 cm in 8 days. The maximum height of the seasonal frost heave at the ground surface reached about 2.0 cm during the early period of seasonal freezing.
 Twenty diurnal freeze-thaw cycles occurred in spring, but less than 10 in autumn. Short-term frost heave corresponded to diurnal freeze-thaw cycles at the ground surface. The cumulative amount of short-term and seasonal frost heave between 2004 and 2008 was 22.1 cm. Using this value, the potential frost creep is calculated to be 2.5 cm/yr on the 20° slope.
 On the other hand, the annual average movements measured with two painted stone-lines were only 0.3 mm and 0.4 mm, which is considerably smaller than potential frost creep. The small surface movement probably reflects the presence of vegetation patches and uneven subsurface materials. Therefore, the value of potential frost creep (2.5 cm/yr) probably represents the average surface movement in this area. In terms of the frequency of freeze thaw-cycles and the value of potential frost creep, mountains in central Alaska have intermediate characteristics between the mid-latitude alpine area and Arctic.