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.

Distinguish between a natural risk and a natural disaster, and discuss why the same magnitude $6$ earthquake would be minor news in a desert but a catastrophe beneath a city, introducing the role of exposure.

Explain what the vulnerability of a population to a hazard is and give two examples of how it can be reduced without the hazard changing (reinforced buildings, education and drills, insurance, spatial planning).

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

Name three natural hazards of external origin frequent in Spain (floods, droughts, landslides, heat waves, coastal storms) and associate each one with an especially exposed area.

What is a risk map? Explain which three types of information it combines (hazard, exposure and vulnerability) and what spatial planning and insurance companies use it for.

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.

The official seismic hazard map of Spain sets a basic seismic acceleration of a_b = 0,19 g (for a return period of 475 years) for Lorca (Murcia), whereas for Valladolid it sets a_b < 0,04 g.

1. a) Interpret these data: explain what basic acceleration measures exactly with respect to the acceleration of gravity (g = 9,8 m/s²) and justify why this mapping constitutes a probabilistic forecast useful for engineering rather than a deterministic prediction of date and time.
2. b) During the 2011 Lorca earthquake (Mw = 5,1), two buildings with the same structure located at a similar distance from the hypocenter suffered very disparate damage: one built on sound limestone rock suffered minor cracks, while another on soft floodplain sediments suffered severe collapses. Explain this contrast based on the phenomenon of soil seismic amplification and how the seismic-resistant building code (NCSE) accounts for it.
3. c) A developer argues that it is preferable to invest public funds in researching earthquake early-warning systems that provide minutes of warning rather than making construction more expensive by applying the NCSE code. Justify why this stance is unfeasible for shallow intraplate earthquakes, considering the velocity of P and S waves and the distance to the epicenter.

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 eastern Spain, there are housing developments built in dry riverbeds (ramblas) 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 October 2024 DANA catastrophe in Valencia, the headwaters of the Poyo ravine basin in Chiva recorded precipitation of nearly 500 mm in just a few hours. In municipalities located 30 km downstream, heavy rainfall barely occurred in the early afternoon, but two hours later they experienced a flash flood with a peak discharge exceeding 2000 m³/s that inundated urban areas.

1. a) Explain hydrologically why the catastrophic downstream flood occurred without it raining heavily there at that moment, defining the concept of "concentration time" and analysing the typical shape of the flood hydrograph of a Mediterranean rambla.
2. b) In recent decades, the floodplain of this basin has experienced intensive occupation by industrial estates, housing, and transversal barriers (motorways and railway lines). Analyse how these anthropogenic actions alter: 1) the surface runoff coefficient, 2) the natural attenuation of the flood, and 3) the rise in water levels due to the damming effect at embankments and bridges.
3. c) Evaluate why warning the population when water was already entering urban areas proved ineffective, and propose how to redesign the early warning chain by integrating real-time stream gauging stations with binding protocols for work shutdown and vertical evacuation.

Explain what risk culture is (the social perception of hazards) and discuss why in areas where disasters are very frequent, or very rare over time, the population tends to undervalue them.

In a citizen survey conducted in a municipality in south-eastern mainland Spain and in another in the Canary Islands, more than 80% of respondents stated that "living in their town is completely safe from earthquakes or eruptions because nothing serious has happened in the last 70 years". However, geological records indicate return periods (T) of about 100 years for earthquakes of magnitude M > 6 on that south-eastern fault and about 60 years for eruptions on that Canarian volcanic ridge.

1. a) Rebut the respondents' logic using the concept of return period and the stress-accumulation recurrence model: why does the fact that no destructive event has occurred in the last 70 years not reduce the geological risk and, according to that model, indicate a higher probability of occurrence in the short to medium term?
2. b) Analyse the geological mechanisms underpinning this hazard accumulation: contrast the progressive accumulation of elastic strain along the plane of a locked fault with the progressive pressurisation of a magma chamber due to deep replenishments.
3. c) Design three non-structural land-use management measures that increase resilience and risk perception among the population of south-eastern mainland Spain without inducing unfounded alarm or paralysing economic activity.

A tourist campsite located on the floodplain of a rambla (preferential flow zone with a return period T = 10 years) has been flooded and destroyed by flash floods on three occasions during the last 25 years. After each disaster, the Insurance Compensation Consortium (Consorcio de Compensación de Seguros) has fully compensated for the material losses, allowing the owner to rebuild the permanent facilities at the same elevation in the channel.

1. a) Analyze this case using the risk formula (R = P · E · V): deduce which factors are kept artificially high and explain what the "moral hazard" resulting from economically compensating disasters in recurrent risk zones without requiring mitigation measures consists of.
2. b) According to fluvial dynamics, preferential flow zones fulfill the geomorphological function of evacuating flood discharges and transporting bedload. Predict the hydraulic and geomorphological consequences on downstream land of erecting a perimeter concrete wall to shield said campsite against T = 10-year floods.
3. c) As a geological hazard specialist in public administration, draft three binding technical criteria based on the National Flood Zone Mapping System (SNCZI) to condition the Consortium's compensation and mandate the setback or rezoning of activities to areas outside the T = 100-year flood zone.