- The Association of Japanese Geographers
- 地理学評論 (ISSN:13479555)
- vol.80, no.5, pp.259-271, 2007-04-01 (Released:2010-03-12)
Present-day distribution and ongoing degradation of permafrost were evaluated by geophysical means in the source area of the Yellow River, located at the northeastern margin of the Tibetan Plateau. Seismic, electrical and/or thermal soundings were undertaken at 15 sites between 3260m and 4790m ASL in 2003-2005. High P-wave velocities (>2kms-1) and relatively high DC resistivities (650-1100Ωm) below a thin uppermost layer show that permafrost 10-30m in thickness occurs above 4300m ASL. In contrast, low P-wave velocities (<1kms-1) throughout the uppermost ten to fifteen meters of sediments indicate that permafrost is absent below 4000m ASL. On widespread alluvial plains between 4200m and 4300m ASL, some sites show subsurface intermediate P-wave velocities (1.5-1.7kms-1) and low resistivities (30-140Ωm) indicating the presence of unfrozen-saturated sediments, while others show high DC resistivities possibly indicating the presence of permafrost. Negative values of the mean annual ground surface temperature (MAST) also indicate widespread permafrost only above 4300m ASL under the present climatic condition. Assuming that the inter-annual variation in MAST follows that in the mean annual air temperature, permafrost is estimated to have significantly thawed on the alluvial plains at 4200-4300m ASL during the last half-century.