Geological Hazards
In January 2010, a magnitude 7.0 earthquake shook Haiti, the other half of our island: more than 100,000 people died. Weeks later, in Chile, a magnitude 8.8 earthquake (about 500 times more energetic) caused a few hundred deaths. How is it possible that the less powerful quake was the deadlier one? The answer is that risk does not depend only on the energy of the event.
Geological risk: hazard probability, vulnerability, and exposure
An earthquake in the middle of the ocean, far from any coast, harms no one. The same earthquake beneath a city with poorly built buildings can be a tragedy. To understand this, geologists separate three ideas.
A geological risk map shows the zones of a territory according to their hazard probability and vulnerability (active faults, unstable slopes, flood-prone areas). It is used when planning a city: for example, to decide that a school or a hospital should not be built on a slope at risk of landslide, and that urban growth should be directed toward safer land.
Geological risks are classified according to the origin of the process. Internal risks come from the energy of the Earth's interior (earthquakes, volcanic eruptions, tsunamis of seismic origin). External risks come from processes at the surface, driven by gravity and water (landslides, floods, avalanches, coastal erosion).
Distinguish between internal and external geological hazards, and give two examples of each type.
Distinguish between internal and external geological hazards, and give two examples of each type.
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.
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.
Seismic risk: magnitude and intensity
An earthquake occurs when a fault suddenly breaks and releases the stored energy. The point of rupture, inside the Earth, is the hypocenter; the point on the surface directly above it is the epicenter. Seismic waves radiate from the hypocenter, and a seismograph records them in a seismogram. On the island, the National Seismology Center monitors this activity, which is linked to the Caribbean plate, the Septentrional fault, and the Puerto Rico Trench.
Two different ideas are used to measure an earthquake.
Different effects with the same magnitude. Besides magnitude, the effects depend on the depth of the hypocenter (the shallower, the more damage), the distance to populated areas, the type of ground (soft soils amplify the shaking), and the quality of the buildings. That is why Haiti in 2010 (vulnerable buildings, a dense city, and a nearby epicenter) suffered far more than Chile (strict earthquake-resistant codes) even with less energy.
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…
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.
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…
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.
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…
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."
- 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.
- 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.
- 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.
Volcanic risk
Depending on the viscosity of the magma, eruptions are of two types.
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…
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.
Risk prevention and management
Since the hazard probability cannot be eliminated, we act on vulnerability and exposure.
Earthquake-resistant measures in construction include: foundations suited to the type of ground, reinforced concrete structures with well-reinforced and braced columns and beams, regular and symmetrical shapes, ductile materials that deform without breaking, joints between neighboring buildings, and base isolators to absorb the shaking. In buildings that are not new, the structure can be reinforced.
Describe three seismic risk prevention measures that can be applied in building construction in high-risk areas.
Describe three seismic risk prevention measures that can be applied in building construction in high-risk areas.
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…
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.
- 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.
- b) Explain the conceptual difference between the "magnitude" and the "intensity" of an earthquake using data from both earthquakes to illustrate your answer.
- 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.
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…
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.
- a) Justify why Section 1 suffered a catastrophic landslide while Section 2 remained stable, analyzing the influence of dip, lithology, and plant cover.
- 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.
Back to the beginning: The Haiti quake (7.0) was far deadlier than the Chile quake (8.8) because risk is hazard probability × vulnerability × exposure: in Haiti the buildings were fragile, the population dense, and the epicenter close, while Chile was prepared with earthquake-resistant codes.