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Earthquakes in the sky

Four balloons located at an altitude of 20 kilometers (km) in the atmosphere simultaneously and accurately recorded a high-magnitude earthquake that occurred 3,000 km away. Vibrations on the planet’s surface emit low-frequency sound waves—in the infrasound region, inaudible to people—that propagate into the atmosphere. Pressure sensors on the balloons launched by Strateole 2, an international project led by the National Center for Space Studies (CNES) in France, detected these vibrations and inferred the location and magnitude of the quake. Based on data collected by the balloons in late 2021, the group, led by geophysicist Raphael Garcia of the University of Toulouse, France, correctly identified a magnitude 7.3 earthquake that occurred in the Flores Sea in Indonesia (Geophysical Research Letters, July 13). Atmospheric balloons had previously detected lower magnitude tremors closer to their location. The approach could be useful for identifying tremors in remote regions of the Earth and even on other planets, potentially providing a greater understanding of their internal structure.