Optional theme: Geophysical hazards practice questions.

Along the Reykjanes Peninsula in Iceland, earthquake foci are almost all shallower than 15 km, fissures open repeatedly and fluid basaltic lava erupts from them. Which plate boundary type is described?

• A conservative boundary, where crust is neither created nor destroyed
• A divergent boundary, where new oceanic crust is formed
• A convergent boundary, where oceanic crust is subducted
• A collision boundary, where two continental plates are crumpled upwards

Answer: A divergent boundary, where new oceanic crust is formed

Fluid, low-silica basalt escaping through fissures together with only shallow foci is the signature of crustal extension; a subduction margin would add a deepening band of foci and viscous, silica-rich magma.

Kilauea in Hawai'i has slopes of only a few degrees, whereas Merapi in Java, Indonesia, has steep, layered slopes. Which explanation of the contrast is correct?

• Kilauea erupts fluid basaltic magma, Merapi viscous andesitic magma
• Kilauea erupts viscous magma that cools quickly on steep slopes
• Merapi lies above a mantle hot spot, whereas Kilauea sits over a subduction zone
• Merapi is the older volcano, so erosion has steepened its cone

Answer: Kilauea erupts fluid basaltic magma, Merapi viscous andesitic magma

Silica content controls viscosity: basaltic lava travels far before solidifying and builds a shield, while andesitic magma is sticky, traps gas and erupts explosively, stacking lava and tephra into a steep composite cone.

The 1985 Michoacan earthquake had its epicentre on Mexico's Pacific coast, yet the most severe Modified Mercalli intensities were recorded about 350 km away in Mexico City. Which statement explains this pattern?

• The earthquake had two different moment magnitudes, one at each site
• Intensity always increases with distance from the epicentre
• Magnitude is a single value, but intensity varies with local ground conditions
• Mexico City lay closer to the focus than the coast did, because the focus was more than 300 km deep

Answer: Magnitude is a single value, but intensity varies with local ground conditions

Moment magnitude measures the energy released by the rupture and has one value per event, while intensity measures shaking at a place; the soft lake-bed clays beneath Mexico City amplified the seismic waves that reached them.

After two weeks of monsoon rain a slope of clay-rich regolith fails as a rotational slump. No earthquake was recorded in the region. Which mechanism best explains the failure?

• Freeze-thaw weathering prising blocks apart along joints
• Abrasion of the base of the slope by wind-blown sand
• Chemical solution of the carbonate cement that binds the regolith together
• Rising pore-water pressure reducing shear strength on the slip plane

Answer: Rising pore-water pressure reducing shear strength on the slip plane

Water filling the pore spaces pushes grains apart and lowers the friction holding the mass in place, so failure follows prolonged infiltration rather than any sudden trigger.

A government cannot change the frequency or magnitude of the earthquakes that affect its territory. Using the relationship risk = (hazard x vulnerability) / capacity to cope, which action reduces disaster risk?

• Recalculating the magnitudes of historic earthquakes with an updated method
• Raising the official magnitude threshold at which an event is called a disaster
• Training and equipping local search-and-rescue teams in every district
• Adding the epicentres of past earthquakes to the national geological archive

Answer: Training and equipping local search-and-rescue teams in every district

Only the trained rescue teams change the denominator, capacity to cope; recalculating or archiving past magnitudes merely refines the description of the hazard, and redefining the word disaster changes the statistics rather than the exposure, the vulnerability or the ability to respond.

Which impact of the 2011 Tohoku earthquake in Japan is best classified as a secondary hazard?

• Meltdowns at the Fukushima Daiichi nuclear plant after its generators flooded
• Ground shaking that continued for several minutes across the whole of northern Honshu
• Vertical displacement of the sea floor along the subduction interface
• Permanent horizontal movement of the coastline along the rupture

Answer: Meltdowns at the Fukushima Daiichi nuclear plant after its generators flooded

A secondary hazard is generated by the primary event rather than by the fault movement itself; shaking, sea-floor displacement and rupture are all direct products of that movement.