D.2 Geophysical hazard risks

Earthquakes are the release of built-up stress in the Earth's crust, and this stress accumulates most intensely where tectonic plates interact. As a result, the global distribution of earthquakes is highly clustered rather than random: the vast majority of seismic energy is released along a relatively narrow set of linear zones that correspond almost exactly to the edges of tectonic plates.

At plate boundaries, rocks on either side of the boundary are being pushed, pulled, or sheared past one another. Friction locks the rocks together temporarily, allowing elastic strain to accumulate. When the accumulated stress exceeds the strength of the rock, it fractures or slips suddenly along a fault, releasing the stored energy as seismic waves — an earthquake. Away from plate boundaries, in the stable interiors of plates, this stress accumulation is far weaker and slower, which is why intraplate earthquakes are comparatively rare and usually lower in magnitude.

The clearest real-world illustration of this pattern is the Pacific Ring of Fire, a roughly horseshoe-shaped belt encircling the Pacific Ocean that follows a near-continuous chain of convergent and transform plate boundaries. It stretches from the western coasts of the Americas, through the Aleutian Islands, down through Japan, the Philippines, and Indonesia, to New Zealand. This single belt accounts for the large majority of the world's earthquakes and a comparable share of active volcanoes, because it traces the margins of the Pacific Plate as it interacts with numerous surrounding plates through subduction.

Mass movement is the downslope transfer of rock, soil, and debris under gravity, and its global distribution is closely tied to mountainous and steeply dissected terrain, as noted in the general pattern of geophysical hazard distribution. Within these mountainous zones, slope angle (the steepness of the surface, measured in degrees from horizontal) and gradient (the ratio of vertical rise to horizontal distance) are the most direct controls on whether, and how, material fails.

As slope angle increases, the component of gravitational force acting parallel to the slope (the shear stress) increases, while the component acting perpendicular to the slope (which generates frictional resistance) decreases. Beyond a critical angle -- which varies with material type, moisture content, and vegetation cover -- shear stress exceeds the shear strength of the material and failure occurs. This is why very steep slopes (often greater than 40°) are disproportionately associated with rapid, dry mass movements, while moderate slopes (5°-25°) are more associated with slower, saturation-driven movements.

$FS=\frac{\text{shear strength of slope material}}{\text{shear stress (gravitational force component parallel to slope)}}$

Volcanoes are the surface expression of magma reaching the Earth's crust, and their global distribution is not random -- it is tightly controlled by plate tectonic processes. Roughly three-quarters of the world's active volcanoes occur along the Pacific Ring of Fire, a horseshoe-shaped belt encircling the Pacific Ocean where subduction dominates. The remaining volcanoes are concentrated at two other distinct tectonic settings: divergent plate boundaries (rift valleys and mid-ocean ridges) and intraplate hotspots. Understanding these three settings explains not only where volcanoes occur, but also why they differ so much in eruption style and hazard potential.

Subduction zones form at convergent plate boundaries where a denser oceanic plate is forced beneath a less dense oceanic or continental plate. As the subducting slab descends into the mantle, it heats up and releases water, which lowers the melting point of the overlying mantle wedge and generates magma. This magma rises to form chains of steep-sided composite (stratovolcanoes), typically arranged in curved volcanic arcs parallel to deep-ocean trenches -- for example, the Andes, the Cascades, and the island arcs of Indonesia and Japan. Because subduction zone magma is viscous and gas-rich, eruptions here tend to be explosive and highly hazardous.

Rift valleys occur at divergent plate boundaries, where two plates move apart and the crust thins and fractures. This decompression allows the underlying mantle to partially melt, producing basaltic magma that rises through fissures. On continents, this creates rift valleys such as the East African Rift, dotted with volcanoes like Mount Nyiragongo and Erta Ale. On the ocean floor, the same process forms mid-ocean ridges, the longest chain of volcanoes on Earth, though largely hidden beneath the sea (Iceland is a rare example where the ridge emerges above sea level). Rift and ridge eruptions are generally effusive (lava flows) rather than explosive, because the basaltic magma is less viscous and gas-rich.

Hotspots are the exception to the plate-boundary rule: they are fixed plumes of unusually hot mantle material rising independently of plate boundaries, burning through the overlying plate wherever it happens to sit. As the tectonic plate slowly moves over the stationary plume, a linear chain of progressively older, extinct volcanoes is left behind, with only the volcano directly above the plume remaining active. The Hawaiian-Emperor seamount chain is the classic example, formed as the Pacific Plate has drifted north-westward over a fixed hotspot for millions of years. Yellowstone in the United States is a continental hotspot example.