Sound waves,eimic waves travel through different kinds of materials at different rates. For example, a type of seismic wave called a P wave travels through crust material at an average speed of 6 km/sec.But through the uppermost mantle material,P waves travel at an average speed of 8 km/sec. geologists use their knowledge of this difference in speed to stations to record the arrival of seismic waves. computers then help in analyzing the data and creating a picture of earth's interior. The figure below shows two P waves from an earthquake whose travel times are recorded by a receiving station. use the figure to answer the questions that follow.