Rice University scientists studied how the density of microseismicity, or small tremors, related to the seismic structure of the Pacific Northwest in the United States. Red lines in the graphic at left correspond to cross-sections from northern Washington (top), central Oregon (middle), and northern California (bottom). The researchers determined a strong correlation exists between tremor density and underthrusting sediments (brown material in the graphics on right). Fluids that are released from the downgoing slab are concentrated in these sediments and lead to very slow seismic velocities in the region. (Credit: Jonathan Delph/Rice University)

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