Slowness (seismology)

In seismology, slowness refers to the reciprocal of seismic velocity. It quantifies the inverse of how fast a seismic wave travels through a particular medium. Mathematically, slowness (often denoted as p or u) is expressed as:

p = 1 / v

where v is the seismic velocity.

Slowness is a useful parameter for several reasons:

• Ray Tracing: Slowness is a fundamental quantity used in ray tracing algorithms, which are used to model the propagation of seismic waves through the Earth. By understanding the slowness variations within the Earth, seismologists can better predict the arrival times and paths of seismic waves.
• Velocity Modeling: Slowness variations reflect changes in the physical properties of the Earth, such as density, composition, and temperature. Analyzing slowness patterns helps in developing detailed velocity models of the Earth's interior.
• Slowness Stacks: A slowness stack (also known as a tau-p stack) is a technique used to analyze seismic data to identify coherent signals and estimate their apparent slowness. This technique is particularly useful in identifying and characterizing seismic phases.
• Compact Representation: In certain mathematical formulations and computational applications, working with slowness can simplify equations and data handling compared to directly using velocity.

Slowness is often expressed in units of seconds per kilometer (s/km) or seconds per meter (s/m). The concept of slowness is applicable to both body waves (P-waves and S-waves) and surface waves. Changes in slowness with depth and location provide vital information about the Earth's structure and composition.