How do earthquake waves help scientists find the inside structure of Earth?

Seismic waves change speed and direction as they pass through materials of different density and rigidity. S waves cannot travel through liquid, which helped scientists determine that Earth's outer core is liquid. P waves bend and reflect inside Earth, creating patterns that reveal boundaries between the crust, mantle, and core. This field, called seismology, became especially important in the 20th century as global seismic networks expanded. Without earthquakes, scientists would know far less about the planet's interior.