INTERACTIVE PRESENTATION: SEISMOGRAM INTERPRETATION OF LOCAL AND DISTANT EARTHQUAKES

The interpretation of seismograms in texts and laboratory exercises is usually presented as identification of Primary (P), Secondary (S) and surface waves of distant earthquakes. Usually the S-P time is determined and the travel-time curves are used to estimate the distance to the epicenter and if data from 3 stations is available the epicenter will be located by triangulation. Occasionally the magnitude of the teleseism may be calculated by measuring the amplitude and period of the surface waves. Usually seismograms of local earthquakes are not presented or interpreted.

The purpose of this session is to study both local and distant earthquakes (teleseisms). Comparisons of the characteristics of seismograms of both local and distant quakes will be studied. A comparison of the frequencies of waveforms will be made and methods of calculating distance will be discussed. Formulas will be derived that may be used to determine distance to local earthquakes and the magnitude of an earthquake. The location of the hypocenter by graphical methods of a local earthquake will be demonstrated.

Teleseisms will also be studied and compared with local earthquakes. The distance, depth, and magnitude of a teleseism will be determined. The various phases PP, PPP, PcP, S and the paths they travel will be studied. Seismograms, travel-time curves of distant and local earthquakes will be distributed for classroom use.