Landslides and debris flows associated with forest harvesting can cause much destruction and the influence of the timing of harvesting on these mass wasting processes therefore needs to be assessed in order to protect aquatic ecosystems and develop improved strategies for disaster prevention. We examined the effects of forest harvesting on the frequency of landslides and debris flows in the Sanko catchment (central Japan) using nine aerial photo periods covering 1964 to 2003. These photographs showed a mosaic of different forest ages attributable to the rotational management in this area since 1912. Geology and slope gradient are rather uniformly distributed in the Sanko catchment, facilitating assessment of forest harvesting effects on mass wasting without complication of other factors. Trends of new landslides and debris flows correspond to changes in slope stability explained by root strength decay and recovery; the direct impact of clearcutting on landslide occurrence was greatest in forest stands that were clearcut 1 to 10 yr earlier with progressively lesser impacts continuing up to 25 yr after harvesting. Sediment supply rate from landslides in forests clearcut 1 to 10 yr earlier was about 10-fold higher than in control sites. Total landslide volume in forest stands clearcut 0 to 25 yr earlier was 5·8 × × × × × 10 3 m 3 km − − − − −2 compared with 1·3 × × × × × 10 3 m 3 km − − − − −2 in clearcuts > > > > >25 yr, indicating a fourfold increase compared with control sites during the period when harvesting affected slope stability. Because landslide scars continue to produce sediment after initial failure, sediment supply from landslides continues for 45 yr in the Sanko catchment. To estimate the effect of forest harvesting and subsequent regeneration on the occurrence of mass wasting in other regions, changes in root strength caused by decay and recovery of roots should be investigated for various species and environmental conditions.Figure 2. (a) Map of Sanko catchment showing the year in which artificial forests were replanted. Trees in the control sites were planted between 1912 to 1916. (b) Distribution of slope gradient in Sanko catchment.Figure 4. (a) Maximum hourly rainfall, (b) maximum daily rainfall and (c) maximum rainfall in a given rainy season (1 June to 31 October) for the various periods of aerial photograph interpretation in Sanko catchment.Figure 6. Comparison of rainfall attributes (i.e. maximum hourly rainfall, maximum daily rainfall and maximum rainfall in a given rainy season) and volume of new and expanded landslides (older landslides which grew in size from the previous photograph period).Figure 7. Changes in sediment supply rate from new or expanded landslides and frequency of occurrence of new landslides with time after clearcutting. Landslide rate and frequency are compared with the dynamic root strength values estimated by Sidle's (1991Sidle's ( , 1992 model. augment pore pressure response that triggers slope failure (Sidle and Ochiai, 2006). Thus, rates of sediment supply are affe...

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S U M M A R YThe Miocene basaltic-andesitic lavas and Cretaceous granites were sampled in the Abukuma Terrane of northeast Honshu for geochronological and palaeomagnetic investigations to determine precise timing of counter-clockwise rotation of NE Japan. Characteristic component with unblocking temperature of 560-590 • C is isolated from 26 sites of the Takadate and Ryozen Formations and 12 sites of the Cretaceous granites. Concordant northwesterly palaeomagnetic directions (D = 265 • -331 • ) are observed in the Cretaceous granites of the Abukuma Terrane. The Cretaceous mean-palaeomagnetic direction from this terrane (D = 313.5 • , I = 54.6 • , α 95 = 6.3 • and N = 20) is almost identical to Oligo-Miocene (33-20 Ma) palaeomagnetic direction reported from the Japan Sea side of NE Japan as well as to the Cretaceous palaeomagnetic direction reported from the Kitakami Terrane. This type of behaviour clearly suggests that the Abukuma Terrane underwent a coherent counter-clockwise rotation as a part of NE Japan after 20 Ma. Conventional potassium (K)-Argon (Ar) dating method performed on the plagioclase grains revealed distinctive ages of 14.44 ± 0.88 and 16.52 ± 0.82 Ma for the Takadate Formation (11 sites) and the Ryozen Formation (eight sites), respectively. This type of behaviour indicates that two independent episodes of volcanic activities were probably responsible for the production of igneous rocks in the Takadate and Ryozen areas. Northerly declination is obtained from the Takadate Formation (D = 355.3 • , I = 39.4 • , α 95 = 11.6 • ), while slightly westerly deflected declination appears in the Ryozen Formation (D = 333.5 • , I = 58.0 • and α 95 = 9.6 • ). The here presented results, therefore, suggest that the province of NE Japan was still experiencing a counter-clockwise rotation at 16.5 Ma, however by 14.4 Ma the rotational motion has completely ceased. Compared with the available palaeomagnetic and geochronological data from SW Japan, rotational motion in both provinces of NE and SE Japan has already been ceased by 14 Ma. Thus any additional evidence regarding the complete cessation of rotational motions in NE Japan will provide a key to understand the timing of opening in the Japan Sea.

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