Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

Structure of Landslides
The head scarps of the landslides are easily distinguished
on the mound. Seven landslides developed on the slope
of the tomb mound (Fig.2). Horizontally layered small,
hand‐made soil blocks comprised the fill (embankment) of
the mound. The remains of‘scaly mosaic’ (fish skin)
structures corresponding to each soil block are oriented
almost horizontally in the original non‐disturbed part of
the mound. Thus, deformation of these horizontal units of
the scaly mosaic structure is a valid indicator of the
deformation caused by landslides and other artificial
disturbances of the mound.
Both landslide 1 and 2 are deep‐seated landslides along
with almost horizontal slip surface while landslide #3 is a
shallow slope failure on steep slope. Both types of
landslides involved mass transfer in a highly fluid state
after collapse, resulting in long travel length relative to
the height of the mound. The deep‐seated landslides are
distributed mainly on the northern side of the mound
which coincide with the downward side of the Ai fault
indicating that the landslides were affected by differences
in the geological conditions of the foundation.
Fig.2 Plan view of the Imashiro‐zuka. Shadowed areas show
location of landslides. Line a,b,c are the course of the
surface wave exploration.
Process of landslide
Observations in the trenches of landslide 1 and 2 (Fig.3)
indicate the occurrence of both co‐seismic stages of
landslide movement; an initial stage with a small cyclic
displacement corresponding to the basal shear and an
advanced stage of one‐way sliding corresponding to
development of the slip surface in the clay of the
basement. These two co‐seismic stages of movement
indicate that the changing boundary conditions such as
increasing of pore water pressure are significant factors in
the landslide movement. The shear resistance should
significantly decrease with increasing pore water pressure
along the slip surface as shown by the liquefied clay‐
injection structure (flame structure) in the middle part of
landslide 2. The fully landslide structure including flow
1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology
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slide in the toe part might be completed in the post‐
seismic stage (Fig.4).
Fig.3 Photos and sketches of the inner structure of landslide #2
(trench C)
a) Longitudinal section: Slump blocks were observed on both
sides of the central block of almost horizontal movement.
b) Squeezed (liquefied) clay structure (flame structure). Clay
was injected into the basal part of the mound along the
slip surface.
Fig 4 Schematic model of landsliding in Imashiro‐zuka
LANDSLIDES OF THE NISHIMOTOME‐ZUKA
The Nishimotome‐zuka is located in Kobe City, Hyougo
Prefecture, approximately 60 km southwest of Imashiro‐
zuka. The current coastline located 200 m from
Nishimotome‐zuka is the result of reclamation after the
19 th century. When Nishimotome‐zuka was constructed it
was located at 6 m elevation, and 100 m inshore of the
ancient coast.
The mound is located on the fan of the Tsuga River. The
main foundation bed of the mound is a gravel layer
containing boulders, however, the southern half of the
rear square of the mound is situated on shore sand. The
intensive disaster zone of the 1995 Hyougoken‐nanbu
earthquake included the location of the mound.
Liquefaction of natural ground induced by the 1995
Hyougoken‐nanbu earthquake are scattered around the
ancient shore sand region that surrounds the mound,