 Preliminary reference Earth model

The Preliminary Reference Earth Model (PREM) is a spherically symmetric model of the Earth's internal structure and composition. Developed by Dziewonski and Anderson in 1981, PREM provides a standard reference for understanding the Earth's physical properties, including density, elastic moduli, and seismic wave velocities as a function of depth. Key features of PREM include: 1. **Radial Variation**: PREM describes how various physical properties change with depth from the Earth's surface to its center.