Seismic Beachballs - Focal Mechanisms Explained
Channel: Tracy CA Seismograph
Date: 2023-08-23

When an earthquake occurs, seismologists create graphics of focal mechanisms, informally referred to as beachballs, to show the faulting motions that produce the earthquake. They use the patterns of compressions and dilatations received by seismometers. Simply put, the focal mechanisms are based on the direction of the first arriving P wave. This is a difficult concept for many, but this animation walks viewer through steps to understand how scientists know what kind of fault motion occurred deep underground. The different faulting mechanisms for each focal-mechanism end member includes: strike-slip fault, normal fault, and thrust fault.

Creative Commons Attribution (CC BY 4.0) license
SAGE - Seismological Facility for the Advancement of Geoscience
National Science Foundation