Question 2 of 21: Question 1, Part 2: True / False

Question 1, Part 2: True / False (14 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

7) TRUE. The classic Wegener/Du Toit lines of evidence for Pangaea – jigsaw continental fit, matching fossil taxa (e.g. Mesosaurus, Glossopteris) and truncated mountain belts across the Atlantic, and a shared Permo-Carboniferous glacial till (tillite) record across the southern continents – are exactly as stated.

8) TRUE. Magnetic-reversal stripes recorded symmetrically in oceanic crust either side of mid-ocean ridges (the Vine–Matthews–Morley evidence for seafloor spreading) prove Earth's field has flipped polarity repeatedly through geologic time.

9) FALSE. Continents do not independently "plow through" oceanic lithosphere; continents and the oceanic lithosphere around them are carried together, passively, on rigid tectonic plates, and oceanic lithosphere is created (spreading ridges) and destroyed (subduction) at plate boundaries, not shoved aside by an advancing continental "icebreaker."

10) TRUE. Continental crust is felsic/granitic (≈2.7 g/cm³), less dense than mafic/basaltic oceanic crust (≈3.0 g/cm³) – the density contrast is why continents ride high (isostasy) while ocean basins stay low.

11) TRUE. Oceanic crust is progressively older with distance from the ridge axis (seafloor spreading), so it has had progressively longer to accumulate pelagic sediment – sediment thickness increases away from the ridge, thinning to near zero at the freshly-created axis.

12) TRUE. By definition, the focus (hypocentre) is the actual subsurface point of rupture, and the epicentre is its vertical projection onto the ground surface.

13) TRUE. This is the elastic-rebound theory (Reid, 1906): strain accumulates elastically across a locked fault until the rock's strength is exceeded, then ruptures and instantaneously releases the stored strain energy as seismic waves.

14) FALSE. Magnitude measures energy release at the source, but felt intensity (and damage) attenuates strongly with distance and depends on local geology/soil amplification – a given earthquake is NOT felt equally at all distances.

15) FALSE. S-waves are shear waves and cannot propagate through a liquid or gas (no shear strength) – they travel only through solids, which is exactly how seismology proved the Earth's outer core is liquid (S-wave shadow zone).

16) TRUE. Tall, flexible structures are most vulnerable to the lateral, back-and-forth shaking produced by horizontally-polarized S-waves, Love waves, and strike-slip motion, which more strongly excites a building's fundamental lateral sway mode than vertical P-wave motion.

17) TRUE. The P–S arrival-time separation grows with epicentral distance (the basis of the standard P-S travel-time distance formula); near-zero separation between P, S, and surface waves means the travel path was very short, i.e. the station sits essentially at the epicentre.

18) FALSE. The great majority (roughly 70–90%) of earthquakes are SHALLOW (above ~70 km); only a small fraction are intermediate or deep-focus, with true deep-focus events (down to ~700 km) restricted to subduction-zone Wadati–Benioff zones and comprising a small minority of all seismicity – the statement inverts the real proportion.

19) FALSE. The (local) Richter magnitude is based on the logarithm of the MAXIMUM AMPLITUDE recorded on a standard seismograph at a given distance, not directly on total radiated energy (energy is only empirically related to magnitude through a separate logarithmic relationship, and modern practice uses moment magnitude for large events).

20) FALSE. Loose, unconsolidated, water-saturated soils typically AMPLIFY ground shaking and are prone to liquefaction (temporary loss of shear strength), making them POOR foundation material – the opposite of "absorbing and dampening" vibrations; firm bedrock is the preferred foundation material for seismic resistance.