THE CASE FOR TEACHING EARTHQUAKE FOCAL MECHANISM SOLUTIONS IN STRUCTURAL GEOLOGY

The canon of topics typically covered in an introductory structural geology course can be enhanced by the inclusion of earthquake focal mechanism solutions (FMS) in the course content. FMS are useful to structural geologists because they provide information about non-recoverable strain, the geometry of active fault patches, and the orientation of net slip for a given earthquake. A primer for geologists on FMS has been developed and is available online at no cost through serc.carleton.edu/NAGTWorkshops/structure04/resources.html. This site also contains a growing number of related complete exercises that are available to the community for use in teaching structural geology.

Several benefits are associated with inclusion of FMS in structural geology courses, including (1) improved motivation, (2) reinforced fundamental concepts and (3) broadened scope. FMS are essential to the study of active tectonics. Related issues of hazard recognition and risk assessment tend to endow structural geology with a sense of urgency and importance for students – a significant positive motivator. FMS integrate and reinforce key concepts related to the mechanics of crustal materials. The elastic component of crustal rheology is well illustrated with reference to earthquake data. The cycle of elastic loading, frictional failure, radiation of seismic energy, detection by seismograph stations, and analysis of first-motion records in an inverse process to determine the characteristics of the seismic moment tensor is an excellent example of an important 4D structural/geophysical problem. Resulting FMS provide constraints on present-day crustal kinematics. Use of FMS by structural geologists broadens the scope of our potential influence as scientists working in the public interest, as we seek an improved understanding of hazardous active structures.

FMS are routinely and rapidly computed for earthquakes of magnitude greater than 5.5 worldwide, and are easily obtained at no cost from online databases (e.g., neic.usgs.gov/neis/sopar and www.seismology.harvard.edu/CMTsearch.html). Hence, students can easily acquire FMS for use with corresponding geologic maps and literature to investigate the active tectonics of a given area.