Zhu and his colleagues are studying three major physical seismic properties of the Earth, particularly in North America: seismic wave speed, seismic anisotropy and seismic attenuation. “As seismologists we can measure quantities such as seismic wave speed, seismic anisotropy and seismic attenuation,” he said. Seismic attenuation is used by seismologists to measure the decay of seismic energy — which is a mechanical energy — over time and distance. “Seismic attenuation is important, but it is also a poorly understood property of the Earth,” Zhu said. “It allows the mechanical energy of a seismic wave to be converted into heat via the friction of moving rocks.