On September 19, 1985, and again on September 19, 2017, Mexico City shook violently, even though the epicenter was far from the capital. Earthquakes in Mexico do not happen at random: how is the Earth's outer layer divided, and why does it move?
Lithospheric plates
If you mark the epicenters of earthquakes and active volcanoes on a world map, they do not appear scattered at random: they form narrow bands, like the Pacific Ring of Fire, and leave huge stable areas in between.
Mexico lies in the contact zone of several plates: the North American, Pacific, Cocos, Caribbean, and Rivera plates.
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What is a lithospheric plate and over which layer does it move? State which force is considered today to be the main driving mechanism of its movement.
What is a lithospheric plate and over which layer does it move? State which force is considered today to be the main driving mechanism of its movement.
GPS stations show that North America and Europe are moving apart about 2.5 cm per year in the Atlantic, while the Cocos plate sinks beneath the North American plate off the coasts of Guerrero, Oaxaca, and Chiapas at about 5 to 7 cm per year. What types of contact exist, depending on relative motion?
Examples: divergent, the Mid-Atlantic Ridge or the Gulf of California (the Baja California peninsula is separating from the mainland); convergent, the Middle America Trench (Cocos beneath North America) and the Himalaya mountain range; transform, the San Andreas Fault in California.
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Name the three types of boundaries between lithospheric plates and give a real example of each (a well-known mountain range, ridge, or fault).
Name the three types of boundaries between lithospheric plates and give a real example of each (a well-known mountain range, ridge, or fault).
Explain what occurs at a mid-ocean ridge and at a subduction trench, stating in which of the two locations new oceanic lithosphere is created and in which it is…
Explain what occurs at a mid-ocean ridge and at a subduction trench, stating in which of the two locations new oceanic lithosphere is created and in which it is destroyed.
The diagram shows the three types of boundaries between lithospheric plates. a) State which type of boundary is hidden beneath the "?" symbol and describe the relative…
The diagram shows the three types of boundaries between lithospheric plates. a) State which type of boundary is hidden beneath the "?" symbol and describe the relative motion of its plates. b) Give a real example of each of the three boundaries. c) Indicate in which new oceanic crust is created and in which it is destroyed.
Geologists express energy with a logarithmic relationship: $\log E = 4.8 + 1.5\,M_w$, with $E$ in joules. Each unit of magnitude adds 1.5 to the exponent, and that is why the energy is multiplied by $10^{1.5} \approx 32$.
Tsunamis: earthquakes under the sea
When the hypocenter lies beneath the ocean floor, the earthquake can cause another effect besides the shaking: a tsunami (seismic sea wave). In a subduction zone, the overriding plate gets "stuck" and builds up stress; when it breaks, the seafloor suddenly rises or drops several meters and pushes the whole column of water. That disturbance travels across the ocean as a very long wave, at hundreds of km/h in open water, and grows taller when it reaches the coast, where the water is shallow.
For this reason, a submarine subduction earthquake can damage distant coasts through the tsunami and, at the same time, buildings through the shaking, though at different distances from the epicenter. In Mexico, the Pacific coast (Guerrero, Oaxaca, Chiapas, Jalisco, and Colima) is exposed because the Cocos plate subducts off its shore. The earthquake of September 7, 2017 (magnitude 8.2, off the coast of Chiapas) produced a tsunami of about one meter on the coast, and the 1932 Jalisco earthquakes also generated significant waves.
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Distinguish between the hypocentre and epicentre of an earthquake, and explain what the moment magnitude scale measures compared to the macroseismic intensity scale.
Distinguish between the hypocentre and epicentre of an earthquake, and explain what the moment magnitude scale measures compared to the macroseismic intensity scale.
A seismological observatory records two seismic events in the same province: Earthquake 1: Magnitude $M_w = 7.0$; hypocentre at a depth of $60\ \text{km}$ in a…
A seismological observatory records two seismic events in the same province:
Earthquake 1: Magnitude $M_w = 7.0$; hypocentre at a depth of $60\ \text{km}$ in a mountainous rural area. The nearest city is $90\ \text{km}$ away, founded on competent granite massifs and with buildings constructed under earthquake-resistant regulations.
Earthquake 2: Magnitude $M_w = 5.0$; hypocentre at a depth of only $2\ \text{km}$, located beneath a historic city center built on an unconsolidated alluvial plain.
a) The energy released by an earthquake is governed by the relationship $\log E = 4.8 + 1.5 M_w$. Calculate with justification how many times more mechanical energy Earthquake 1 released than Earthquake 2.
b) In the city near Earthquake 1, the recorded intensity was grade IV (widely felt, no damage), whereas in the city center of Earthquake 2 the intensity reached grade VIII (severe structural damage and partial collapses). Interpret this apparent discrepancy by explaining the role of hypocentral distance (geometric attenuation), the "site effect" due to wave amplification in soft sediments, and building vulnerability.
In the Atlantic, an underwater mountain range rises with a cleft down its center; off the coast of Chile, a trench 8,000 m deep sinks next to active volcanoes; and in central Asia the Himalaya rises without volcanism. In Mexico, Popocatépetl and Pico de Orizaba are part of a volcanic arc linked to the subduction of Cocos. Why does each boundary build a different landscape?
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Currently, the Atlantic oceanic lithosphere is expanding at about $2.5\ \text{cm/year}$ from its mid-ocean ridge, while its continental margins are almost entirely…
Currently, the Atlantic oceanic lithosphere is expanding at about $2.5\ \text{cm/year}$ from its mid-ocean ridge, while its continental margins are almost entirely passive. In contrast, the Mediterranean represents a relict basin where remnants of the Tethys oceanic crust actively subduct beneath southern Europe at a convergence rate of about $5\ \text{mm/year}$.
a) Reasonably predict the geodynamic evolution of the Atlantic-Iberian and North American margins: explain what critical physical change the oceanic lithosphere will undergo as it moves away from the ridge and cools, and deduce what new plate boundary structure will form spontaneously so that the Atlantic Ocean initiates its closure within the Wilson cycle.
b) Design a tectonic model for the Mediterranean Sea $40-50$ million years from now: deduce what resulting landform will replace the current marine basin after the definitive continental collision between Africa and Eurasia, what deformation geological structures will predominate in the suture zone, and how this closure will affect thermohaline exchange between the Atlantic and Indian Oceans.
Go back to the beginning: the 1985 earthquake was due to the subduction of the Cocos plate beneath North America, and the 2017 one to the deformation of the Cocos plate itself as it sank. In both cases the waves were amplified in the soft sediments of the capital's ancient lake.
Summary
Let's practice! 3 medium exercises in this section