GEOS2111 Chap.9 Tsunamis and mass movement
Tsunamis and mass movement
The unit groups tsunamis and mass movement because each sits in two classes at once, primary hazard and secondary hazard. A tsunami is described here as a major secondary hazard associated with earthquakes, volcanoes and landslides, and yet once generated it behaves as a hazard in its own right with its own warning architecture.
Mass movement is the same: a landslide can be triggered by rain, by earthquake shaking or by excavation, and can then dam a river or generate a wave. That double classification means each sits in two places in every framework from Week 2, appearing as an outcome in one causal chain and a trigger in another, and can therefore be reduced from two directions.
Tsunami generation requires the sudden vertical displacement of a large volume of water, which is why fault geometry matters more than magnitude. The unit supplies the comparison ready-made: an offshore subduction thrust event of magnitude 9.0 produced run-up reaching about 40.5 metres and over 19,000 deaths, while a shallow strike-slip event of magnitude 7.0 produced liquefaction and landslides and no tsunami at all.
Thrust faulting lifts the seafloor and displaces the whole water column; strike-slip motion moves ground horizontally past itself. The other distinguishing feature of a tsunami is that where it is felt is not where it was triggered, and that separation is the basis of warning: seismic waves cross an ocean in minutes while the tsunami takes hours.
Australia's alert ladder runs from no threat through watch, marine warning and land warning to cancellation, with a separate notice for a felt earthquake carrying no tsunami threat, and it deliberately separates marine from land risk.
Mass movement covers rock-falls, landslides and lahars, and the unit's five controlling factors include two that are properties of the slope rather than of the trigger, which is why one susceptibility map serves both seismic and non-seismic failures and why drainage, grading and vegetation are the available levers.
What this chapter covers
- 01
Hazards that are also consequences, and why that puts them in two places in every framework
- 02
What generates a tsunami: vertical displacement of a water column, not energy alone
- 03
Thrust against strike-slip, and the comparison the unit supplies ready-made
- 04
The three generating processes: earthquake, eruption and landslide entering water
- 05
Where it is felt is not where it was triggered, and why that separation enables warning
- 06
The alert ladder, and why marine and land warnings are kept apart
- 07
Who does what: the geoscience agency, the meteorological bureau and the state service
- 08
Rock-falls, landslides and lahars, and the five factors that must line up
- 09
Why the same susceptibility map serves seismic and non-seismic failures
- 10
Susceptibility against probability, and what a hazard map cannot decide on its own
Deciding, in minutes, whether to issue a land warning
- +1Separate what you know from what you do not. You know magnitude, depth and that the setting is a subduction margin, which makes thrust faulting and vertical seafloor displacement plausible. You do not yet know the focal mechanism, so you cannot rule out a strike-slip rupture on a nearby transform, which would displace very little water. That uncertainty is the whole decision.
- +1Use the travel time as an asset rather than skipping it. At 900 kilometres a tsunami, if there is one, arrives in the order of an hour or more while the seismic information reached you in minutes. That gap is what the alert ladder exists to spend.
- +1Issue a watch immediately. It costs almost nothing, puts emergency services and coastal communities into a prepared state, and preserves your ability to escalate or cancel once the mechanism and gauge data arrive.
- +1Keep marine and land separate. If evidence supports a modest wave, a marine warning is proportionate: get people out of the water and away from moorings without evacuating a coastline. Escalating straight to a land warning on incomplete information damages the next warning, because compliance is a finite resource that a false alarm spends.
- +1Name your escalation triggers in advance, and close the loop. A focal mechanism showing thrust faulting with a large rupture area justifies escalation before any gauge reports; a confirmed gauge observation of a significant wave near the source justifies a land warning immediately. Whatever is issued must eventually be cancelled explicitly, because an alert allowed to lapse silently leaves some people displaced and others returning during the later waves of a sequence.
Key terms
- Secondary hazard
- A hazard produced by another hazard, such as a tsunami generated by an earthquake or a landslide triggered by shaking. In this week both hazards are also primary, which is why they can be reduced from two directions.
- Run-up
- The vertical height above sea level that a wave reaches on land. It is the quantity that describes how high the water got, and it is distinct from how far inland the water travelled.
- Wave setup
- The raised mean water level near a shoreline produced by breaking waves, which adds to the still water level and therefore to inundation.
- Meteotsunami
- A tsunami-like wave generated by an atmospheric pressure disturbance rather than by ground movement. It is flagged in the unit as a variant worth knowing.
- Marine warning
- An alert level indicating dangerous currents and waves near the shoreline and in the water, without expected land inundation. Keeping it distinct from a land warning protects compliance with the more serious message.
- Land warning
- An alert level indicating that inundation of land is expected, requiring movement inland or to high ground.
- Rock-fall
- The detachment and free descent of rock from a steep face. It is the fastest form of mass movement and the one with the least warning.
- Lahar
- A volcanic mass movement in which loose material is mobilised by water and travels far beyond the eruption's own reach. It is named in the unit's mass movement scope alongside rock-falls and landslides.
- Slope saturation
- The condition in which pore water fills the spaces in slope material, reducing the strength that holds it in place. It is one of the two pre-existing slope conditions in the unit's five factors and is seasonal, which is why season appears on the list.
- Susceptibility map
- A classification of terrain from highest to lowest likelihood of failure, based on slope, material and history and independent of what will eventually trigger it. It supports zoning and does not, on its own, support a statement of probability.
- Hazard cascade
- A chain in which one hazard triggers another, such as shaking triggering a landslide that dams a river whose failure then floods downstream. The deaths usually occur at a step downstream from the hazard people were watching.
- Alert cancellation
- The formal closing of a warning. Including it in the ladder matters because an alert that is never closed leaves people either displaced or quietly returning without permission.
Tsunamis and mass movement FAQ
Why is a big earthquake not enough to produce a tsunami?
Because a tsunami requires the sudden vertical displacement of a large volume of water, and not all large earthquakes displace water vertically. Thrust faulting on a subduction interface lifts the seafloor and moves the entire water column above it, which is why the unit's magnitude 9.0 offshore example produced run-up reaching about 40.5 metres.
Strike-slip faulting moves the ground horizontally past itself, so even a shallow magnitude 7.0 close to a coast can produce liquefaction, landslides and no tsunami at all, which is exactly what the unit's second example did.
Three questions therefore replace magnitude when you are asked about tsunami risk: was the rupture beneath water, was the faulting vertical in character, and how much seafloor area was displaced, which is where magnitude finally enters as a proxy for rupture area.
Why does the alert ladder separate marine and land warnings?
Because they describe different hazards to different people, and conflating them destroys the credibility of both. A wave that will capsize boats, tear out moorings and generate dangerous currents in a harbour may never cross a dune, and telling an entire coastal town to evacuate for that spends public compliance that will be needed for the warning that matters.
Separating the two lets the agency issue a proportionate instruction: leave the water, or leave the low ground. The ladder also includes an explicit cancellation state, which is a design feature rather than an afterthought, since an alert that is never formally closed leaves some people displaced longer than necessary and others quietly returning to the shore during the later waves of a sequence.
And a separate earthquake notice exists so that a felt event with no tsunami threat is answered rather than met with silence.
Why do tsunamis have a warning system when most hazards in this unit do not?
Because of arithmetic rather than technology. A tsunami is the only hazard in this unit where the process that generates it and the place it does damage are separated by hours. Seismic waves cross an ocean basin in minutes and are detected almost instantaneously by the global seismic network; the tsunami itself travels far more slowly.
That gap is where a warning centre operates: it decides whether a tsunami was generated, projects arrival times along coastlines, and gets people moving.
The consequence is that where modern systems exist the technical chain generally works, and what fails afterwards is human: people who do not receive the message, do not believe it, cannot move because of who they are caring for, or return too early because a sequence is not over. An hour of warning is worth nothing to someone with nowhere higher to go.
What has to line up before a slope fails?
The unit lists five factors for earthquake-triggered failures, and two of them are about the slope rather than the earthquake, which is the useful observation. The trigger side contributes magnitude above about 4.0 and ground shaking. The slope side contributes gravity acting on an over-steepened slope and water-saturated slope material, and season enters because saturation is seasonal.
Because two of the five are pre-existing conditions in exactly the sense the unit defines, the management lever is real: the trigger cannot be controlled, and drainage, grading and vegetation can.
The other important structural point is that once movement begins, the morphology and processes are the same whether the trigger was seismic or not, which is why a single susceptibility map serves both cases and why the expensive part, characterising the terrain, only has to be done once.
What is the difference between susceptibility and probability?
Susceptibility says which slopes could fail; probability says how likely a failure is in a stated period. A susceptibility map classifies terrain from highest to lowest on the basis of slope, material and history, independent of what will eventually set it off, and that is genuinely useful for zoning, foundation requirements and route selection. It is not a statement about when.
Converting susceptibility into risk requires a trigger frequency, which means a rainfall or seismicity model, and an exposure layer, which means people and assets, and both are frequently missing. This is the same gap that runs through the whole GIS chapter: a hazard map on its own supports zoning, and only a hazard map crossed with exposure and vulnerability supports a decision about people.
Exam move
Hold the fault geometry picture as your central object for tsunamis, because almost every question in this area reduces to it. Draw two panels, a thrust fault lifting the seafloor and a strike-slip fault moving sideways, and write the two real examples underneath with their magnitudes and their outcomes. That single sheet answers the generation question, the magnitude question and the comparison question.
Second, memorise the alert ladder in order with the action attached to each rung, and be able to say why marine and land are kept apart and why cancellation is on the list; the design reasoning is worth more than the labels. Third, learn the agency split, because questions about institutional failure in a tsunami are almost always questions about a handover between two of the three bodies involved.
Fourth, treat mass movement as a conditions problem rather than a trigger problem: write the five factors, mark the two that are properties of the slope, and note that those two are the ones that can be managed.
Finally, collect two or three hazard cascades in one place, since cascade questions appear across several weeks and the characteristic feature is always the same: the deaths occur one step downstream of the hazard everybody was watching.
Working through Tsunamis and mass movement in GEOS2111? Sia is AskSia’s AI Environmental Science tutor — ask any GEOS2111 Tsunamis and mass movement question and get a clear, step-by-step explanation grounded in how GEOS2111 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
