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Why do earthquake swarms occur in rift zones such as Iceland?
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tectonic basins and rift valleys
Escarpments of the Great Rift Valley rising above the plain north of Samburu Game Preserve, central Kenya. Beisa oryxes graze in the foreground.
In rift zones like Iceland, earthquake swarms occur because the Earth’s crust is subjected to continuous tectonic extension as lithospheric plates diverge.1
This process is analogous to a rubber band being slowly stretched until it snaps. While the separation of plates is a continuous process, the fracturing of the brittle crust is intermittent. When the accumulated stress from this stretching exceeds the strength of the near-surface rocks, they fracture along steeply dipping cracks parallel to the rift. These fractures allow magma to rise, which often triggers earthquake swarms and volcanic activity. Consequently, these swarms are a direct result of the crustal rocks failing under the intense strain of ongoing plate divergence.12