On September 19, 1985, and on September 19, 2017, strong earthquakes struck central Mexico. The 2017 one had a magnitude of 7.1, and even so its effects depended heavily on where each building stood and how it was built. A geological process becomes a risk only when it intersects with human activity. That is why managing it is not only Earth science: it is also land-use planning.
What a natural hazard risk is
In 2010, a magnitude 7.0 earthquake caused more than $200{,}000$ deaths in Haiti. Weeks later, another one of magnitude 8.8 in Chile left about 520 victims. Why did a far more violent event cause a catastrophe hundreds of times smaller?
For an earthquake, the hazard is how likely a strong tremor is in that region. The exposure is how many people and buildings are above it. The vulnerability is how well or poorly those buildings withstand it.
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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…
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.
Mexico has both. It sits on the Pacific Ring of Fire, where the Cocos plate sinks beneath the North American plate. This produces earthquakes and volcanoes such as Popocatépetl. In addition, hurricanes in the Pacific and the Gulf of Mexico and intense rains cause floods and landslides.
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Classify these natural hazards as internal or external in origin and justify the classification: earthquake, flood, drought, landslide, and volcanic eruption.
Classify these natural hazards as internal or external in origin and justify the classification: earthquake, flood, drought, landslide, and volcanic eruption.
The National Seismological Service (SSN-UNAM) records thousands of earthquakes a year, but no one can name the day of a strong one. By contrast, Popocatépetl has an alert traffic light that CENAPRED updates according to its activity. Why can science anticipate some geological catastrophes and not others?
The exercises in this section may present other cases of eruptions. The concepts are the same as for Popocatépetl: prior monitoring is prediction, preventive evacuations and codes are prevention, and adjusting measures during the eruption and the care afterward are correction.
Volcanic precursors
A volcano that wakes up gives off signals that can be measured. The earthquakes (swarms) become shallower: the hypocenters, which at first are at 10-12 km, rise to a few kilometers or less than 1 km deep. The ground deforms (swells), and GNSS stations record it in centimeters. Gases (SO₂, CO₂) that escape diffusely also increase. If they all grow at once, the magma is rising and an eruption becomes more and more likely.
In Mexico, CENAPRED communicates Popocatépetl's alert level with a volcanic alert traffic light of three colors: green (low activity), yellow (activity that requires preparation and attention from the population), and red (eruption imminent or underway, with high risk). Each color is divided into phases, and each phase indicates what actions the population and authorities should take. The current phases are always found on CENAPRED's official page.
Earthquakes: preventing without being able to predict
Structural prevention (in buildings): building code, shear walls, bracing, energy isolators and dissipators, retrofitting of old buildings.
Non-structural prevention (population and planning): drills (such as the national one on September 19), the Seismic Alert (SASMEX), evacuation routes and meeting points, zoning maps, civil protection education.
Two hazards, two origins
The Trans-Mexican Volcanic Belt (Popocatépetl, Colima, Pico de Orizaba) and the earthquakes of the Pacific coast have the same origin: the subduction of the Cocos plate beneath the North American plate. But they are managed differently. Magma rises gradually and leaves precursors, so volcanic risk can be monitored and is partially predictable. Subduction earthquakes release stress all at once, so they are managed with earthquake-resistant construction and early warning of a few seconds.
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State two structural prevention measures (in buildings) and two non-structural prevention measures (concerning the population and planning) against seismic risk.
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…
Distinguish between prediction, prevention, and mitigation in volcanic risk management, illustrating each with a real action carried out during the 2021 La Palma eruption.
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…
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.
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.
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.
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.
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…
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.
In the Sierra Madre, in Oaxaca, or in Guerrero, hurricane rains often bring landslides that block roads and bury houses. What makes a slope move, and how can we know beforehand?
Types and signs
Creep: very slow movement of the soil. It is revealed by trees with curved trunks ("saber-shaped"), tilted poles, and small cracks.
Translational slide: a block or layer moves along a plane of weakness (for example, a stratum) in an almost straight line.
Rockfall: blocks that detach from a vertical slope face.
Mudflow or debris flow: the water-saturated material flows like a fluid.
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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…
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").
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.
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.
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.
The return period is a long-term average, not a calendar. Floods do not "take turns": each year is a new chance, so one can happen again the following year.
On risk maps, the zones that would flood with floods of different return periods (for example 10, 50, and 100 years) are drawn. This is how decisions are made about where not to build and which defense works are needed.
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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…
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…
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.
After a major disaster, drills and defense works multiply, but over the years people build again in hazardous areas. Why does society forget the danger precisely as time passes without a disaster?
Return to the beginning: the same earthquake can be minor news or a tragedy depending on exposure and vulnerability. Those two we can change through planning, codes, and risk culture.
Summary
Let's practice! 3 medium exercises in this section