Earthquake swarms can be driven by both fluids and slow slip together.

Earthquake swarms are long clusters of small quakes. Scientists often debate what drives them: moving underground fluids, or slow fault slipping that releases no seismic waves. Studying them inside quiet tectonic plates helps answer this, because faults there are especially sensitive to temporary forces. Researchers analyzed about ~50,000 earthquakes from the 2019-2020 Palghar swarm in western India. They mapped the quakes with high precision to see where and when rocks broke underground. They discovered the shaking began on a shallow western fault, jumped to an eastern fault, and then spread into the damaged rock between them. While the overall swarm spread out like diffusing fluid, smaller bursts moved too fast for fluid alone. The speeds and durations matched both fluid flow and slow slipping events. In the end, neither single model explains the swarm. Instead, moving fluids and transferring stresses worked together across interacting faults to trigger the shaking.

Why it matters

For people living in stable continental regions far from plate boundaries, earthquake swarms are poorly understood. This study shows that high-resolution earthquake catalogs can reveal the hidden processes driving these swarms, which could improve hazard assessment in areas not typically considered seismically active.

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