Exercises: Natural hazards

The diagram shows the three components that, when combined, result in a geological risk. Name them (1, 2, and 3), define each one, and apply them to the example of an earthquake.

Classify these natural hazards as internal or external in origin and justify the classification: earthquake, flood, drought, landslide, and volcanic eruption.

State two structural prevention measures (in buildings) and two non-structural prevention measures (concerning the population and planning) against seismic risk.

Distinguish between prediction, prevention, and mitigation in volcanic risk management, illustrating each with a real action carried out during the 2021 La Palma eruption.

Explain what the return period of a flood is and how it is used in risk maps, and reason what it means for a flood with a "100-year return period" to potentially occur again the following year.

In northern Mexico, there are housing developments built in dry riverbeds (arroyos) and ravines that have been dry for decades. Explain why building there increases risk even if it does not rain for years and which spatial planning principle is violated.

During the week prior to the Tajogaite eruption (La Palma, 2021), the scientific committee recorded the following monitoring data:

• September 11 to 13: seismic swarm with hypocentres at depths of 10-12 km and a maximum magnitude of 2,8.

• September 15: migration of hypocentres to depths of 2 to 4 km and ground uplift of +10 cm detected by GNSS stations.

• September 18: earthquakes with magnitudes greater than 3.2 at depths of less than 1 km, cumulative deformation of nearly 20 cm, and a sharp increase in diffuse SO2 and CO2 emissions.

1. a) Interpret the evolution of these precursors: deduce what was happening with the magmatic intrusion at each stage and justify why evacuation was not ordered on September 11, but was essential to initiate on the 18th.
2. b) Although the lava destroyed more than 1300 hectares and around 3000 buildings, there were no direct fatalities. Justify which risk factor (R = P · E · V) was successfully reduced to zero for people thanks to this instrumental network, and which one proved impossible to mitigate in the short term.
3. c) If similar seismicity were recorded on another island of volcanic origin but without ground deformation or anomalous gas emissions, could an imminent eruption be ruled out? Justify your answer considering magma viscosity and the presence of pre-existing crustal fractures.

A developer plans a housing development of 30 villas on a slope with an average gradient of 32°. The geological profile reveals permeable sandstone strata resting on impermeable shales and marls, with a dip of 28° in the same direction as the slope of the hillside (concordant strata). At the foot of the slope, a vertical cut excavated for a previous road is observed, and in the middle section several pine trees have curved trunks ("sabre-shaped").

1. a) Identify and classify the type of mass wasting event most likely to be triggered. Justify your prediction by analysing the relationship between the slope angle, the dip direction, and the contrast in lithological permeability.
2. b) Explain the physical mechanism by which an episode of intense and prolonged rainfall could cause the sudden collapse of the slope, making explicit reference to pore pressure and the coefficient of internal friction at the contact surface between strata.
3. c) The developer proposes to resolve the instability by covering the cut with continuous shotcrete without drainage holes. Rebut their proposal technically, showing why this measure would worsen the risk instead of preventing it, and design two viable alternative corrective measures.

Compare the volcanic risk in the Trans-Mexican Volcanic Belt with the seismic risk in southern Mexico: the geological origin of each, the feasibility of short-term prediction, and the most suitable prevention strategies in each case.