Exercises: Plate tectonics and relief

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.

Name the three types of boundaries between lithospheric plates and give a real example of each (a well-known mountain range, ridge, or fault).

Distinguish between the hypocentre and epicentre of an earthquake, and explain what the moment magnitude scale measures compared to the macroseismic intensity scale.

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 Iberian Peninsula experiences earthquakes despite not lying on a simple plate boundary. Explain the role of the contact between the Eurasian and African plates (Azores-Gibraltar) in the seismicity of southern and eastern Iberia.

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.

Consider the following data from two historic earthquakes in the Iberian Peninsula:

• Earthquake A (Lisbon, 1755): Estimated magnitude $M_w \approx 8.5$; submarine focus southwest of Cape St. Vincent at a depth of about $40\ \text{km}$; generated a transoceanic tsunami that devastated the coasts of the Gulf of Cádiz and caused building collapses in Madrid and Salamanca.

• Earthquake B (Lorca, 2011): Moderate magnitude $M_w = 5.1$; onshore focus at a depth of only $1.5\ \text{km}$ on the Alhama de Murcia fault; record ground acceleration of $0.37\ g$; caused $9$ fatalities and severe collapses in the town, but at $30\ \text{km}$ away it was barely felt.

1. a) Justify the specific geodynamic context of each seismic event based on the relative motion between the Eurasian and African plates (diffuse transpressive boundary in the southwest versus strike-slip fault system of the Betic Cordillera in the southeast).
2. b) A news reporter states: "The destructive impact of an earthquake depends solely on its magnitude on the Richter scale." Scientifically refute this statement by analyzing how focal depth, wave attenuation with distance, peak ground acceleration, and structural exposure/vulnerability influenced each case.

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 $5\ \text{mm/year}$.

1. 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.
2. 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.

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.

1. 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.
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.