D.2 Geophysical hazard risks

As slope angle increases, the component of gravity acting parallel to the slope (shear stress — the destabilising force) increases. Simultaneously, the component acting perpendicular to the slope (which generates normal stress and frictional resistance, contributing to shear strength) decreases. So the numerator (shear strength) decreases relative to the denominator (shear stress).

If shear stress rises while shear strength falls, the fraction $FS=\frac{\text{shear strength}}{\text{shear stress}}$ becomes smaller. The ratio moves closer to 1.0 and can eventually drop below it as the slope becomes steeper.

When $FS<1.0$, shear stress (the force trying to move material downslope) exceeds shear strength (the resistance holding material in place). The slope is no longer stable and mass movement occurs. When $FS>1.0$, resistance exceeds driving force and the slope is stable.

The correct statement is that FS decreases as slope angle increases, and FS below 1.0 indicates that shear stress exceeds shear strength, causing failure. This matches the option: 'FS decreases, indicating that shear stress now exceeds shear strength and mass movement will occur'.

The question tests understanding of the factor of safety concept: specifically how slope angle changes the ratio and what crossing the threshold of FS = 1.0 means physically.

'FS increases, indicating that the slope is becoming more stable as shear strength rises' is incorrect. Steeper slopes do not increase shear strength — they increase shear stress, which is the denominator. FS therefore falls, not rises, as angle increases.

'FS stays constant because gravity acts equally on both components' is incorrect. Gravity's two components are trigonometrically related to slope angle. As angle increases, the parallel (shear) component grows and the perpendicular (normal/frictional) component shrinks — they do not remain proportionally equal.

'FS increases above 1.0, but mass movement still occurs because frictional resistance is eliminated' is self-contradictory. If $FS>1.0$, shear strength still exceeds shear stress and the slope is stable. Movement cannot occur at FS > 1.0 under this model.

The only logically and physically consistent statement is that FS decreases as slope angle increases, and FS < 1.0 signals failure because shear stress has exceeded shear strength.

The question asks why mass movements concentrate in steep, high-relief terrain. The core explanation lies in how gravity is resolved into two components on a slope: shear stress (parallel to slope, destabilising) and normal stress (perpendicular to slope, generating friction and stability).

In mountainous regions, slope angles are large — often exceeding 25°–40° or more. As angle increases, the downslope (shear) component of gravity grows while the perpendicular (normal/frictional) component shrinks. This shifts the balance of forces towards instability, making failure more likely even without an external trigger such as rainfall or seismic shaking.

The steep terrain is a landscape predisposition: it explains where mass movements are likely, not when they will occur. Rainfall or earthquakes may trigger the actual event, but they do so on a slope that is already susceptible because of its angle and relief.

The correct explanation is that steeper slope angles increase the downslope gravitational component relative to frictional resistance, concentrating mass movement susceptibility in mountainous terrain. This is the option that accurately describes the physical mechanism.

The question asks for the most accurate explanation of why mass movements concentrate in mountainous terrain. The answer must reference a clear physical mechanism linking terrain to slope instability.

'High altitude creates colder temperatures that freeze soil particles together, preventing movement on flat terrain but not on peaks' is incorrect. Freeze-thaw weathering can loosen material at altitude, but cold temperatures generally increase cohesion by freezing pore water, rather than preventing movement only at low elevations. This is not the primary mechanism explaining the global pattern.

'Mountain regions receive more annual rainfall than lowlands, so the water trigger alone explains why mass movements are absent from plains' is incorrect. Rainfall is a trigger, not a predisposing terrain factor. Furthermore, many lowland areas receive very high rainfall (e.g. tropical rainforests) yet do not experience mass movements at the frequency seen in mountain zones. Slope angle is the underlying cause.

'Tectonic activity injects hot fluids into mountain slopes, liquefying rock and causing it to flow downhill under pressure' is incorrect. While tectonic activity can contribute to slope instability through seismic shaking or hydrothermal alteration, this is not the general explanation for why all mountainous terrain — including non-volcanic ranges — shows elevated mass movement frequency.

Only the option referencing steeper slope angles increasing the downslope gravitational component relative to frictional resistance provides the correct, generalisable physical mechanism for the global distribution pattern.