In 2001, seismometers in the Spokane, Washington, area recorded more than 100 earthquakes, some as large as magnitude 4.0. One of the characteristics of the 2001 activity was the reports of loud booms, which are often associated with small quakes. Examining data recorded by the European Space Agency’s GOCE satellite during the 2011 Tohoku earthquake, an international team of researchers has determined that large earthquakes also produce sound waves, but at infrasonic wavelengths. The satellite’s accelerometer detected the passing sound wave at an altitude of 270 km. It was also able to detect changes in atmospheric density due to the sound wave’s compression of the air. The compression increased the drag on the satellite, causing it to activate control jets to maintain a precise orbit.
Researchers from the US Geological Survey and the Pacific Northwest Seismic Network have begun using satellite data to understand the nature of seismic events in the area. Interferometric synthetic aperture radar measurements from European Space Agency and Canadian Space Agency satellites showed surface movement of 15 mm along a thrust fault that passes through the city’s center. Computer models suggest that the quake activity was centered at depths of 0.3 to 2 km. – Ars Technica