Geophysical hazard risks practice questions.

In November 1985 a relatively small eruption of Nevado del Ruiz, Colombia, melted part of its summit ice cap and destroyed the town of Armero, more than 40 km away. Which hazard travelled that distance?

• A pyroclastic flow channelled down the valleys
• An ash fall carried by strong upper-level winds
• A lava flow that followed the river valleys
• A lahar generated by meltwater and loose ash

Answer: A lahar generated by meltwater and loose ash

Lahars are why a small eruption killed about 23 000 people: meltwater mobilises loose volcanic debris into a dense mudflow that follows valleys tens of kilometres from the vent, far beyond the reach of a pyroclastic flow.

Across a long earthquake record for one region, events of Mw 5 occur roughly ten times as often as events of Mw 6, and events of Mw 6 roughly ten times as often as events of Mw 7. Which statement about geophysical hazards does this pattern illustrate?

• Large events are impossible until the small ones have released the stored strain
• Hazard magnitude and hazard frequency are inversely related
• Every magnitude class contributes an equal share of total energy release
• The recurrence interval of an event is fixed by the depth of its focus

Answer: Hazard magnitude and hazard frequency are inversely related

Frequency falls by about an order of magnitude for each step up the scale, so the rare large event dominates losses while the common small one dominates experience. Planning built only on what residents remember therefore underestimates the hazard.

A coastal city is shaken by an offshore earthquake, and forty minutes later a series of waves inundates the harbour. Which term describes the second event in a hazard event profile?

• A secondary hazard triggered by the primary event
• An unrelated coincidence of two separate hazards
• The primary hazard, since it caused more damage
• A slow-onset hazard, because it arrived after a delay

Answer: A secondary hazard triggered by the primary event

Secondary hazards are caused by the primary event and often account for most of the losses - tsunami after an earthquake, lahar after an eruption, fire after ground shaking. Ranking by damage rather than by causation is the common error.

During shaking, water-saturated sandy ground beneath a district of Christchurch, New Zealand behaved temporarily like a liquid, and buildings tilted and sank. Which process is this?

• Liquefaction
• Subduction
• Solifluction
• Isostatic rebound

Answer: Liquefaction

Shaking raises pore water pressure until grains lose contact and the deposit loses bearing strength. It explains why identical buildings on different ground behave completely differently in the same earthquake, and why microzonation maps matter for planning.

Which characteristic of a hazard event profile most strongly affects how much warning a population can be given?

• Speed of onset
• Areal extent
• Magnitude
• Frequency

Answer: Speed of onset

A slow-onset hazard such as drought allows months of preparation, while an earthquake gives seconds. Magnitude determines potential losses and areal extent determines how many are exposed, but neither creates the opportunity to act.

A student writes: 'A tsunami crosses the deep ocean at the speed of a jet aircraft, so no coastline can ever be warned in time.' Identify the single error in this reasoning.

• Tsunami waves in fact travel more slowly in deep water than in shallow water.
• Tsunami waves lose almost all their energy before they leave the deep ocean.
• The speed of a tsunami cannot be estimated from the depth of the water.
• Coasts thousands of kilometres away still have hours before the waves arrive.

Answer: Coasts thousands of kilometres away still have hours before the waves arrive.

Even at jet speed, roughly 700–800 km/h in deep water, a tsunami needs several hours to cross thousands of kilometres of ocean, so distant coasts can be warned; the real limit is the near field, where the first waves arrive within minutes of the shaking. Tsunamis slow down, not speed up, as the water shallows.