パプアニューギニア北東部のサンゴ礁段丘の総合研究　パプアニューギニア，ヒュオン半島の古崩壊地の年代・分布・形成過程に関する研究

Abstract

Landsides occur widely on the coral terraces of the tectonically active Huon Peninsula, Papua New Guinea. Large historic earthquakes in Papua New Guinea have frequently triggered large landslides. Landslide that disrupt the coral terraces at Huon Peninsula have a variety of forms including ; 1) large translational block slide features that carry essentially intact coral terrace remnants along, without large debris flows ; 2) complex slides that may begin as rockslide or deep-seated rotational slide features that continue downslope as debris flow features. In addition, small rockfall and rockslide features that commonly occur on steep valley walls and headwall (type III). Large scale landslides of types I and II occur more frequently in the southeast of the study area where uplift rate is high, up to 3.5 m/ka.Approximate ages of 26 large, late Quaternary landslides of types I and II have been obtained by radiocarbon dating of corals from immediately above or below the debris flow units that have extended and across the Holocene reef sequence, or by U-series ages of older coral terraces that have been disrupted by landslides or where terrace have developed on older landslide debris. The youngest debris flow (type I) directly overlies a coseismically uplifted terrace dated at ca. 1, 250 yr BP. At three sites, at least, an older debris flow is intercalated with reef material that was forming during a late stage of the post-glacial transgression about 7-8 ka. More than 10 large blockslides without debris flow (type I) disrupt ca. 33 ka terrace, but these are trimmed at outer edge by the ca. 6.5 ka Holocene terrace, thus providing bracketing ages.Historic precedent of large landslides generated by major earthquakes, and the close association of debris flow deposits with coseismically uplifted terraces of Holocene age lead us to propose that many, and perhaps most, o f the large scale landslides on Huon Peninsula are earthquakegenerated.