Exercises: Hazards of internal origin

Distinguish between hazard, exposure, and vulnerability in the context of an earthquake affecting a city.

What does the Richter scale measure and what does the Mercalli scale measure? Point out a fundamental difference between them.

State three seismic safety measures that can be applied to the design and construction of buildings.

Define the hypocenter and epicenter of an earthquake and explain why damage is usually greatest in the vicinity of the epicenter.

Name three secondary effects of an earthquake, such as liquefaction, landslides or tsunamis, and briefly describe what each one consists of.

What is a tsunami and what characteristics must an earthquake have to generate one? Consider its magnitude, the depth of the focus and the deformation of the sea floor.

Distinguish between prediction, prevention and protection against earthquakes, and give an example of an action of each type.

What does volcano monitoring consist of? Name three parameters that are monitored on an active volcano and explain what a change in each one can signal.

The graph shows, on a logarithmic scale, the energy released by earthquakes of magnitude $5$ and $6$ (one of magnitude $6$ releases about $30$ times more energy than one of magnitude $5$). Indicate where the bar for magnitude $7$ (marked with "?") would approximately be, calculate how many times more energy it releases compared to magnitude $5$, and explain the logarithmic nature of the scale.

In the 2021 La Palma volcanic eruption, lava destroyed many homes, but casualties were very few. Relate this outcome to volcanic monitoring and emergency planning.

The same earthquake can cause very different damage in two towns situated at equal distances from the epicenter. Explain the role of the foundation soil type and building quality.

A magnitude $7$ earthquake releases about $30$ times more energy than a magnitude $6$ one. Estimate how many times more energy it releases than a magnitude $4$ one and explain why comparing magnitudes as if they were linear quantities is misleading.

What is soil liquefaction, in what type of ground does it occur, and why does it cause buildings to settle and overturn even when they are correctly built?

Pacific tsunami warning systems combine seismic networks with deep-sea pressure buoys. Explain what each element of the system measures and why deep-sea detection gives warning before the wave reaches the coast.

What is a seismic microzonation map and what use does it have for urban planning and for the building codes of a city?

Compare the risks to the population of an effusive eruption, dominated by lava flows, and of an explosive one, with an eruptive column and pyroclastic flows. Which one requires wider and faster evacuations and why?

On the seismic hazard map of the Iberian Peninsula, basic seismic acceleration values ($a_b$) exceed $0.16\ g$ in areas of Granada, Murcia, and Almería (Betic Cordillera), while in the Pyrenees they range between $0.08\ g$ and $0.12\ g$, despite both mountain ranges being Alpine collision orogens.

1. a) Justify from plate tectonics why oblique convergence and the strike-slip fault system of the southeastern peninsula contribute, together with the higher present-day deformation rate, to shallow seismicity with shorter return periods (for earthquakes of comparable magnitude) and greater hazard than the Pyrenean shortening.
2. b) An expansion of a municipality in the Guadalentín basin is planned: sector A is located on Quaternary alluvial deposits with a water table at a depth of $2.5\ \text{m}$, and sector B, on a hillside of well-consolidated massive limestones. Evaluate the geotechnical risks derived from wave amplification and liquefaction in both sectors, and design a justified land-use planning proposal to locate a regional fire station and an outdoor sports complex.

The propagation speed of a tsunami in deep water follows the equation $v = \sqrt{g \times h}$, where $g = 9.8\ \text{m/s}^2$ and $h$ is the ocean depth. An earthquake of magnitude $M_w = 8.2$ is recorded with an epicenter in an oceanic trench ($h = 4\,000\ \text{m}$) located $360\ \text{km}$ from a coast whose shallow continental shelf has an average depth of $40\ \text{m}$.

1. a) Calculate the estimated time it will take for the wave front to cross the deep ocean basin and predict how velocity, wavelength, and wave height will vary as it penetrates the shallow continental shelf, justifying the physical principle of conservation of energy involved.
2. b) If the focal mechanism of the earthquake revealed an exclusively horizontal strike-slip motion with no vertical component, should the maximum tsunami warning be maintained with the same level of severity? Justify your answer considering the transfer of energy to the water column.

A city of $50\,000$ inhabitants is located at the mouth of a glacial valley draining the slope of an andesitic stratovolcano $4\,200\ \text{m}$ high with an ice cap at its summit. The volcanological observatory detects a shallow seismic swarm with sustained harmonic tremor, a sharp increase in $\text{SO}_2$ emissions, and a surface bulging rate of $3.5\ \text{cm/day}$ oriented toward the river valley.

1. a) If an explosive Plinian eruption begins in early spring (thaw phase), predict the two volcanic processes posing the greatest lethality and immediate destruction to the city, justifying their trajectories and speeds based on topography and available materials.
2. b) Design a dynamic zoning plan for the emergency: determine which areas of the city must undergo mandatory immediate evacuation and which ones preventive shelter-in-place due to tephra dispersal, justifying which meteorological and topographic variables will determine the delimitation of each sector.

A city of $200\,000$ inhabitants sits on soft sedimentary fill $15\ \text{km}$ from an active fault.

1. a) Break down its seismic risk into hazard, exposure, and vulnerability;
2. b) propose two earthquake scenarios and their differing effect on the old downtown and the new neighborhoods;
3. c) prioritize three measures, two structural and one of management, justifying the chosen order.

In the Doñana marshes and in the Tinto River estuary, decimeter-scale layers of marine sands containing marine bioclastic fauna have been identified chaotically intercalated within calm fluvial silty sequences, dated to the 3rd century BC and November 1755.

1. a) Justify the sedimentary origin of these deposits (tsunamites) and interpret the regional geodynamic framework of the Gulf of Cádiz (plate convergence and structures such as the Gorringe Bank or the Marqués de Pombal fault) that explains the recurrence of these low-frequency, very high-energy events.
2. b) Analyze the evolution of risk along the coastlines of Huelva and Cádiz comparing 1755 with the present day: argue which of the factors in the risk equation ($R = P \times E \times V$) has skyrocketed with modern urban and tourism development, and refute using geological and oceanographic arguments the proposal to build a $2.5\ \text{m}$ high seawall as the sole protective measure.

A paleoseismic trench study across an active fault reveals that the last four major earthquakes ($M_w \approx 6.9$) occurred 780, 590, 395, and 190 years ago. GNSS stations record a continuous tectonic strain rate of $3.0\ \text{mm/year}$, and no earthquake with a magnitude greater than $4.0$ has occurred in the last 190 years.

1. a) Interpret these data using the elastic rebound model and calculate the cumulative displacement deficit on the fault. Justify why this segment can be defined as a "seismic gap" and argue why this method enables medium-term probabilistic forecasts but not an exact deterministic prediction (day and hour).
2. b) After a microearthquake swarm is recorded ($M < 2.0$), a viral message claims that "the big earthquake will strike this weekend." Design a technical communication strategy from the emergency management agency (such as FEMA and the state's emergency management office) that refutes the prediction without undermining the culture of prevention, and explain what a real earthquake early warning system based on P and S waves consists of, along with its useful response window.

During a fissure basaltic eruption on a volcanic island with steep relief, such as that of La Palma in 2021, the eruptive vents simultaneously eject fall pyroclasts, ash, and lava flows exceeding $1\,050\ ^\circ\text{C}$ down urbanized and agricultural slopes with gradients of $15\,\%$.

1. a) Explain the relationship between the viscosity of basaltic magma, the topographic gradient, and the formation of lava tubes, deducing how the formation of these tubes affects the heat loss of the flow and the extent of damage to distal infrastructure.
2. b) A neighborhood committee proposes two emergency measures against the lava tongue: cooling the active front using seawater hoses to halt it and digging transverse trenches on the slope to divert the flow. Justify the physical infeasibility of both measures in this scenario based on the heat capacity, flow rate, and mass of the lava flow, and conclude what types of land-use planning measures are the priority and most reliable ones against this hazard.

Two earthquakes of magnitude $M_w = 7.0$ occur on an active continental margin with subduction of oceanic lithosphere beneath continental lithosphere. Earthquake A originates at a depth of $12\ \text{km}$ in the overriding plate close to the oceanic trench, while earthquake B occurs at a depth of $250\ \text{km}$ in the Benioff-Wadati zone.

1. a) Compare the expected isoseismal maps for both events: predict and justify which of the two will record a much more destructive maximum intensity (EMS-98) at the epicenter concentrated in a small area, and which will produce moderate intensities felt over a radius of hundreds of kilometers, applying the concepts of geometric and intrinsic attenuation.
2. b) If the oceanic plate subducts eastward with an average dip of $45^\circ$, predict the geographical position of the epicenter of earthquake B relative to that of earthquake A and the oceanic trench, and design a construction zoning guideline for the territory that relates focal depth to the earthquake resistance requirements needed from west to east.