Exercises: Geological hazards

Distinguish between internal and external geological hazards, and give two examples of each type.

Look at the diagram of an earthquake, which shows the hypocentre, epicentre, seismic waves and a seismograph recording. Using the diagram, explain the difference between the magnitude and intensity of an earthquake, and state which scale or instrument is used to measure each one.

What is a geological hazard map? Explain its purpose and give an example of how it can be used when planning the growth of a city.

Two earthquakes of the same magnitude hit two different regions: in one there is barely any damage, while in the other thousands of casualties are reported. Explain what factors, besides magnitude, determine the effects of an earthquake: hazard, vulnerability and exposure.

Describe three seismic risk prevention measures that can be applied in building construction in high-risk areas.

During a volcanic eruption, lava flows, ash, gases and pyroclastic flows are emitted, and lahars can form. Choose three of these products and explain the type of damage each one can cause.

Compare the data from these two real seismic events:

• Earthquake A (Lorca, Spain, 2011): Magnitude $M_w = 5,1$; hypocenter depth = $1\ \text{km}$; epicentral distance to urban center = $2,5\ \text{km}$; maximum intensity = VII-VIII (EMS); 9 deaths and hundreds of damaged buildings.

• Earthquake B (Sea of Okhotsk, 2013): Magnitude $M_w = 8,3$; hypocenter depth = $609\ \text{km}$; distance to nearest inhabited area = $350\ \text{km}$; maximum surface intensity = V (MMI); 0 deaths and no property damage.

1. a) Based on the seismic risk equation ($\text{Risk} = \text{Hazard} \times \text{Exposure} \times \text{Vulnerability}$), analyze what specific factors caused the Lorca earthquake to be devastating despite its moderate magnitude.
2. b) Explain the conceptual difference between the "magnitude" and the "intensity" of an earthquake using data from both earthquakes to illustrate your answer.
3. c) Design two preventive measures (one structural and one related to land-use planning or education) to mitigate seismic risk in municipalities with characteristics similar to Lorca.

Following a series of tremors in a region crossed by an active geological fault, a local official declares: "During this month we have experienced $100$ small magnitude $3$ earthquakes. This is good news, because the fault has gradually released the energy equivalent to a major magnitude $6$ earthquake, thus avoiding a greater catastrophe."

1. a) On the earthquake magnitude scale, each increase of $1$ unit of magnitude multiplies the energy released by approximately $31,6$ ($10^{1,5}$). Calculate how many magnitude $3$ earthquakes would actually be needed to equal the energy released by a single magnitude $6$ earthquake, and how many for a magnitude $7$ one.
2. b) Using the numerical results obtained, refute the official's statements and justify why the presence of small, shallow earthquakes by no means guarantees that the risk of a major earthquake has dissipated.
3. c) Explain what relationship exists between the accumulation of elastic strain energy along the boundaries of a locked fault (elastic rebound theory) and the destructive potential of a high-magnitude earthquake.

On a slope above a road, two sections with the same gradient ($35^\circ$) are analyzed after a torrential rain event of $120\ \text{l/m}^2$:

• Section 1: alternating limestone and marl strata dipping at $30^\circ$ parallel to the terrain slope, recently deforested.

• Section 2: horizontal strata of compact sandstone covered by native Mediterranean forest.

1. a) Justify why Section 1 suffered a catastrophic landslide while Section 2 remained stable, analyzing the influence of dip, lithology, and plant cover.
2. b) To stabilize Section 1, a company proposes spraying a continuous layer of shotcrete over the surface without drilling drainage weep holes into the interior. Predict what will happen during the next storm and technically justify why this measure may increase the risk.