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 at least six major lithospheric plates and indicate on which one the Iberian Peninsula lies and which plate it contacts to the south.

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.

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

Describe the relative movement of the plates at each type of boundary (divergent, convergent and transform) and indicate which process dominates at each (creation, destruction or conservation of lithosphere).

What is a transform fault? Give the best-known example (the San Andreas Fault) and explain which plates it separates and in which direction they move.

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

Two earthquakes of the same magnitude occur beneath the same region, one at $30\ \text{km}$ depth and the other at $300\ \text{km}$. Discuss which one will cause more damage at the surface and why, explaining how seismic waves are attenuated with the distance they travel.

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 centre 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 centre 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.

When two continents collide, neither of them can subduct easily. Explain what happens to the crust in this collision (shortening, thickening, thrust faults), what relief results, and give the example of the Himalayas.

The Mariana Trench reaches almost $11\ \text{km}$ in depth. Explain which tectonic process forms oceanic trenches, what happens there to the sinking lithosphere, and why volcanic island arcs are usually found next to them.

Although the main orogenic collision between Iberia and Eurasia ended more than $20\ \text{Ma}$ ago and current GNSS/GPS measurements record an almost negligible horizontal tectonic shortening rate ($< 0{,}3\ \text{mm/year}$), geodetic levelling profiles reveal that the axial zone of the Pyrenees continues to rise vertically at a net rate of $0{,}5$ to $1{,}2\ \text{mm/year}$.

1. a) Resolve this apparent paradox: if active tectonic convergence is not the primary driving cause of the current uplift, justify what endogenous physical mechanism—based on Airy's isostasy model and the presence of a deep crustal root—drives the mountain range's uplift as external agents erode it.
2. b) In a Pyrenean valley, a stepped sequence of four river terraces incised into bedrock is identified. The upper terrace (T1) is located at $+120\ \text{m}$ above the current riverbed, and its alluvial sediment has been dated to $150\,000$ years ago. Calculate the average rate of river incision (in $\text{mm/year}$) and interpret what this data reveals about the equilibrium between river incision and the uplift of the mountain range during the Quaternary.
3. c) Refute the following statement: "Since erosion removes thousands of tonnes of rock per year, the Pyrenees will be reduced to a peneplain at sea level in a few million years." Explain the role of delayed isostatic compensation in the longevity of mountainous relief.