Video summary

自然災害 13 地すべり

Main summary

Key takeaways

Educational

Main ideas and lessons (Lesson 13: Natural Disasters — Landslides / ジスリ)

1) Where the lesson fits in the course

  • The instructor moves from weather-related disasters (heavy rain, typhoons) to urban-related disasters, especially collapse/landslide phenomena.
  • Next week they will cover debris flows connected to these events.
  • In this lesson’s latter half, the focus is on:
    • The characteristics of landslide types (especially ジスリ / self-sliding landslides)
    • The mechanisms for how sediment/landslide disasters occur

2) Review: basic landslide types and how they differ

Using a diagram, the instructor distinguishes among:

  • Surface landslides (surface collapse)

    • Collapse occurs on/near the surface.
    • Typically small in scale and lower volume.
    • Material (soil/debris) is not as extensive as in deeper collapses.
    • Can happen suddenly (seconds to minutes), with few warning signs.

  • Self-sliding landslides (dis-slips / ジスリ)

    • Collapse involves a larger mass and deeper movement than surface collapse.
    • Scale can be large.
    • Movement can occur over minutes but also can develop over tens of thousands of years (wide range).
    • Retains some resemblance to original form because the failure extends deeper to the substrate.
    • Often shows distinctive terrain shapes and “internal structure” elements.

  • Debris flows (泥流 / water-soil mixed flows)

    • Can occur when large collapses contribute enough material to a drainage system.
    • Important hazard: a large collapse can block a river, forming a natural dam, which increases upstream risk and can trigger further debris-flow-like impacts.

Key distinction summarized (3 characteristics)

  • Scale
    • Surface landslides: small
    • Self-sliding landslides: large
  • Time
    • Surface: sudden (seconds to minutes)
    • Self-sliding: can be minutes, but also long-term (tens of thousands of years)
  • Shape / depth
    • Surface: mostly surface layer fails; original form is lost
    • Self-sliding: failure extends to deeper layers; original form partially remains

3) Hazard “signs” and predictability

Terrain and precursors differ by type.

Self-sliding landslides: observable warning signs

Before occurrence, you may see:

  • Cracks (especially in the upper ground / specific upper zones)
  • Crack development or ground deformation
  • Changes such as:
    • Ground may shift/submerge
    • Air flow may be noticed (interpreted as part of deformation signals)
    • Groundwater level changes (decrease/increase)

These fluctuations increase the chance of identifying where failure may occur.

Surface landslides: limited warning

  • They tend to collapse suddenly
  • Few reliable pre-signs are available
  • This makes advance identification of failure location difficult

4) Morphology (shape) of self-sliding landslides: “scooped out like a spoon”

Self-sliding is part of broader “true layer collapse” terminology, where deeper structure fails rather than only the surface.

Method/metaphor: spoon-gouged “true slip” landform

  • Analogy: scooping food with a spoon leaves a spoon-shaped gouge, while contents remain in place in a way that creates a characteristic pattern.
  • In nature: a similar spoon-shaped excavation pattern appears in the terrain.

Terminology of key moving parts (topography elements)

Within a self-sliding landform, the slide describes named components:

  • Cracks (fissures)

    • Marked as an initial sign (e.g., “No. 1” area)
    • Can lead to collapse from the top, or from multiple cracks lower down

  • Active collapses / separations

    • Starting points for the movement:
      • “Active collapses” for certain patterns
      • “Separations” for others

  • Cliff at the beginning of the slip

    • A distinct boundary where movement initiates

  • Blocks

    • Parts that retain original structure after sliding/rotation
    • Common on flatter slopes more than steep ones

  • Groove-like separated areas / depressions

    • Depressions between separated blocks and the cliff
    • Often lead to distinctive water-related ecosystems in older slides

  • Compression-rich area (pressure zone)

    • Where separated blocks push down onto original terrain
    • Produces rising/jumbled/muddy bumpy topography

  • Debris-flow-like rich area

    • Down-slope jumbled material can behave similarly to surface-collapse debris flows in some sections

  • Debris-flow cone

    • At the terminal area where deposition forms a cone shape

Practical teaching point

The instructor emphasizes that landform classification isn’t purely academic: these elements can help identify whether a site experienced a self-sliding landslide in the past—even if human perception is confused by recovery of vegetation.

5) Two main terrain patterns in self-sliding (A and B)

Both patterns involve a sliding surface (shown in red on diagrams) but differ in slope conditions.

Pattern A: “ecosliding” (steeper, more readily moving)

  • Terrain resembles the spoon-gouge pattern.
  • The sliding surface is activated more readily.
  • Because the slope is relatively steep, movement occurs if factors act on the slip plane.

Pattern B: “flat-body sliding” (gentler slope, tends to slide slowly)

  • Terrain slope is much gentler than Pattern A.
  • Movement may proceed slowly if:
    • The sliding surface is clayey
    • Groundwater remains high
  • Even on gentle slopes, huge ground masses can still move if enough activation occurs.

Shared mechanism concept

  • In both patterns:
    • A sliding surface governs movement
    • The sliding surface acts as the boundary of deformation/motion

6) Mechanism of occurrence: groundwater + progressive failure stages

The instructor states the biggest causal factor, regardless of other geology, is:

  • Groundwater

Conceptual mechanism progression (diagram stages A → B → C)

  • Stage A: initial gravitational instability

    • Active erosion/valley deepening due to:
      • Tectonic setting (mountains with active drift/uplift context)
      • Warm climate with heavy rainfall
    • Over time:
      • Rivers deepen valleys
      • Mountains rise higher relative to valleys
      • Supporting lower parts erode away
      • The mass becomes increasingly unstable in the direction of gravity

  • Stage B: cracks form and deformation begins

    • A first crack forms near the top.
    • Swelling/deformation occurs as mass shifts downward under gravity.
    • A “chair crack” concept is referenced as a previous model/idea.

  • Stage C: groundwater seepage increases and triggers slip

    • Rainwater seeps through cracks and reaches the sliding surface.
    • Water makes clays/slip layers:
      • slipperier
      • and mechanically destabilized
    • As water accumulates between structures:
      • the system may lift slightly or lighten in key zones
      • leading to further loss of stability
    • Collapse may initiate from the lower slope first, then propagate upward.

Role of rainfall

  • Groundwater infiltration is emphasized as the bridge between rainfall and mechanical failure.

7) Case studies: examples of self-sliding leading to hazards and community response

A) 2008 Iwate–Miyagi inland earthquake: large self-sliding example

  • A very large self-sliding landslide occurred after the earthquake.
  • Scale described:
    • Length: about 1.4 km
    • Width: about 800 m
  • Notable feature:
    • The area collapsed all at once but retained parts of original structure (consistent with self-sliding morphology).

B) 2013 Harunomachi / Kadoshima district (Hamamatsu, Shizuoka): tea plantation area

  • Setting:
    • Tea plantations on flat terrain connected to a river valley.
    • Soil collapse partially blocked the river → natural dam formed.
  • Why it matters:
    • Natural dam risk: if left, it could collapse → trigger mudslide/debris flow.
  • Mitigation:
    • Local residents + administration installed a temporary drainage system to prevent water accumulation and reduce risk.

Terrain reconstruction / causation logic

  • The instructor explains slope geometry as “flat → steep → flat → steep” repeating:
    • Interpreted as:
      • older self-sliding topology with repeated movement phases
      • a newer rupture occurring within an earlier self-sliding area
  • Key hazard insight:
    • Even after the land “settles” and becomes flatter, it may remain prone to future collapse if people live there.

Warning and response timeline (why countermeasures worked)

  • Self-sliding wasn’t “without warning”:
    • Residents discovered cracks in the tea fields before major movement.
  • Reporting and institutional response:
    • Residents reported quickly to Hamamatsu City.
    • Civil engineering staff came immediately.
    • This enabled evacuation and an effective response system.
  • Lesson emphasized:
    • Citizen observation + rapid reporting can strongly reduce damage.

Rainfall context

  • A graph was used: daily rainfall vs. the time window before the event.
  • Key points described:
    • Crack discovery around March 21 after roughly ~100 mm/day rainfall.
    • Major movement around March 23.
  • Conclusion:
    • The main driver was groundwater/instability consistent with the self-sliding mechanism, not only extreme rainfall at the moment of movement.

8) Distribution across Japan: where self-sliding is common and why

The instructor shows landslide distribution is uneven, not uniform.

Regional concentration pattern (examples)

  • Hokkaido: concentrated in Hidaka mountain ranges
  • Tohoku: more common on the Sea of Japan side (also noted as similar in Niigata/Toyama)
  • Additional zones: Shizuoka-related areas, Shikoku, and key peninsula regions with east-west undulating slopes

Main geological explanations: “three main types of geology” (as introduced)

  1. Neo-Triassic system (K-character rocks / marine sedimentary origin)

    • Rocks/sediments formed in earlier geological periods (described relative to “current era” and preceding period).
    • Often associated with frequent self-sliding on the Sea of Japan side of Tohoku.

  2. Disaster-causing landslides along tectonic/fault/structural lines

    • Includes references to major fault/structural frameworks like the Central Tectonic Line.
    • Weakening occurs where bedrock rubs along faults.
    • Weaker geology → more landslide occurrences.

  3. Hydrothermal/hot-spring related self-sliding

    • Hot water alters rocks via hydrothermal fluids (heat + water → breakdown and weakening).
    • Can create slower downslope movement.
    • Often tied to hot springs where earthquakes can flatten terrain and enable hot-spring town development.

9) Why Shizuoka has many self-sliding sites: faults + sedimentary history

The instructor links Shizuoka’s concentrated landslides to:

  • Fault rupture zones
  • Structural lines (deep-history lineages with severe weathering)
  • Fault-related weak geological corridors

Fault/structural line mechanism (simplified)

  • Along faults/structural zones:
    • repeated rubbing weakens bedrock into crumbly material
    • rain infiltrates more easily
    • weathering increases collapse likelihood
  • Structural lines (deep and long history) tend to crumble more than typical faults.

Basin sediment supply concept

  • Using the Abe River basin / Setogawa group context:
    • Landslides continuously supply large amounts of sediment to the river.
    • This is described as one reason the river is notable for sediment intake.
  • Example mentioned:
    • Oosuzure as one of Japan’s largest landslides, supplying much rock/sediment.

10) Additional concepts: “compound/organized” geology and tectonic plate history

Geological terms are introduced to explain why certain regions contain landslide-prone compositions:

  • Oceanic deposits accumulate on the subducting plate and later become part of Japan’s geology.
  • Different “bodies” (e.g., Simanto and similar groups) are described as products of oceanic accretion over time.
  • Compression and long-term pressure/mixing change rock structure (the lecture uses analogies such as “substitute seat” / a crystal-structure concept).

Linking geology map and landslide distribution

  • A comparison is made between:
    • a geological map (dots marking central structural areas and fault/structural settings)
    • self-sliding distribution and other landslide categories
  • Also noted:
    • snow accumulation and melting can add groundwater, increasing instability in some regions (especially when combined with geological conditions)

11) Positive/usable outcomes: landslides can create valuable landforms

The instructor concludes with “benefits” and adaptation:

  • Hot springs can develop on relatively flat terrain formed by past slides.
  • Another example from Niigata (Uonuma):
    • flat terrain with ponds formed due to past self-sliding landform depressions
    • ponds used for fish breeding, with cultural/economic use dating back to the Edo period
  • Overall message:
    • Japan’s disaster-prone landscapes can be resiliently utilized rather than only feared.

Speaker / sources featured

  • Primary speaker: an unidentified instructor/lecturer leading “Lesson 13” (auto-subtitle voice; no name provided).
  • Referenced real-world events/places (mentioned in the content):
    • Iwate–Miyagi Inland Earthquake (2008)
    • Kadoshima district, Hamamatsu City, Shizuoka (self-sliding event described as 2013)
    • Hamamatsu City / Shizuoka Prefecture (local government/emergency response role)
    • Ikenohira landslide / Uonuma region, Niigata (pond/fish breeding example)
    • Shizuoka fault/tectonic zones and basins (including Abe River basin / Setogawa context)
    • Hot spring areas such as Hakone (and other onsen examples listed in the lecture)

Original video