The theory of plate tectonics explains that the majority of Earth’s geological features—from mountain ranges to ocean trenches—result from interactions between slabs of rock that make up Earth’s outer shell. The slabs—collectively called tectonic plates—vary in size, composition, and movement, and have been dragged by convection currents in Earth’s mantle for at least 3 billion years.
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Spanning 40,000 kilometers as it circles the Pacific Ocean, the region experienced about 90% of the world’s earthquakes and contains over 70% of its active volcanoes. The horseshoe-shaped belt features convergent, divergent, and transform plate boundaries.
Because primary waves are faster than secondary waves, seismic stations will detect longer gaps between these waves’ arrivals the farther an earthquake is. Calculating the distance to the earthquake from three stations allows its origin to be triangulated.
Matching glacial, fossil, and rock records, mountain ranges, and continental shapes across oceans was insufficient evidence for continental drift. Precise measurements of magnetic minerals on the ocean floor revealed seafloor spreading, which could only be explained by moving landmasses.
After the 1995 Kobe earthquake killed over 6,000 people, Japan created E-Defense to reproduce the force of earthquakes and test them against structures as tall as 10 stories, leading to changes in Japanese building codes to reduce collapse rates.
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