Earth Structure and Seismic Phases
The most basic and most important processing task in seismology is to determine the arrival times of seismic phases. When the first seismic stations with reliable timing appeared more than 100 years ago, they provided, through the observation of travel times, the first real evidence of the internal structure of the earth. Initially there were many unknown wiggles (phases) on the seismogram, however with better and better earth models, we now understand the presence of most of the phases but there are still “abnormal” arrivals to be investigated. In order for the analyst to correctly interpret the seismograms it is therefore important to have a basic understanding, not only of how seismograms look, but also of earth structure, seismic rays and phases. Reading phases is as important as ever since they provide the basic material for improved earth structure (1 and 3D) as well as being used for event location (see Chapter 5). Many phase readings are now done automatically, particularly for the first arriving phase.