D.3 Hazard risk and vulnerability

Both regions face the same hazard magnitude (M9.0), so hazard magnitude is held constant. The variable that differs is vulnerability, shaped by development level, building codes, and response capacity.

Risk = Hazard × Vulnerability. With magnitude fixed, the region with higher vulnerability — the LEDC with no enforced building codes — will experience greater casualties. Poorly constructed, unreinforced buildings collapse under seismic shaking, as seen in the Haiti 2010 comparison.

The MEDC region has strict, enforced seismic codes that allow buildings to flex and absorb energy rather than collapse. The LEDC city lacks this protection, so the primary cause of death is structural collapse rather than ground shaking itself.

The correct answer is that the LEDC city suffers far greater casualties due to structural collapse and weaker response capacity. This mirrors the Haiti (M7.0, ~230,000 deaths) vs Tōhoku (M9.0, ~16,000 deaths) inversion, where the weaker quake in an LEDC was far more lethal.

The question asks which statement best describes the expected difference in outcomes when hazard magnitude is identical but development level differs.

'Both regions would experience similar death tolls because the hazard magnitude is identical' is incorrect — this conflates hazard magnitude with disaster impact and ignores vulnerability entirely.

'The MEDC region would suffer greater casualties because higher population density increases exposure' is incorrect — the question states the MEDC region is sparsely populated, so density is not an amplifying factor there.

'LEDC informal buildings are more flexible and less likely to collapse' is a misconception — informal, unreinforced concrete construction (typical in LEDCs) is among the most susceptible to seismic collapse, not less so.

By elimination, the correct answer is that the LEDC city suffers far greater casualties due to structural collapse and weaker response capacity, reflecting how vulnerability, not magnitude, drives disaster impact.

The question presents a high-magnitude hazard (VEI 5) with a surprisingly low death toll (57). The task is to explain why hazard power did not translate into high mortality.

Vulnerability at Mount St. Helens was low because: (1) the terrain around the volcano was sparsely populated forested land rather than dense settlement; (2) USGS scientists detected weeks of increasing seismic activity, allowing advance warnings; (3) authorities established and enforced an exclusion zone before the eruption.

Crucially, the evacuation was before the eruption, not reactive. Most deaths occurred among people who ignored evacuation orders or were caught at unexpectedly distant distances from the blast — not from a failure of the overall system.

The correct combination is sparse population, pre-eruption seismic monitoring, and enforced exclusion zones — these three factors together produced low vulnerability despite high hazard magnitude.

The question asks for the most direct explanation of why a VEI 5 eruption caused only 57 deaths.

'The eruption was less physically powerful than reported' is factually incorrect — Mount St. Helens produced the largest landslide in recorded history and an ash column reaching the stratosphere, confirming extreme physical magnitude.

'Dense forest cover absorbed most of the blast energy' is incorrect — the lateral blast destroyed hundreds of kilometres of forest, meaning trees were destroyed by the blast, not a barrier to it.

'Emergency responders evacuated thousands in the hours after the blast' contradicts the actual sequence — evacuations happened in the weeks before the eruption, not as a reactive response after it began.

The correct answer is sparse population combined with USGS monitoring and pre-eruption exclusion zones, which directly reduced vulnerability before the hazard struck.