Abstract

An objective of many entry-level earthquake exercises is to use S-P wave travel time differences from multiple seismic stations, and then "triangulate" to determine the location of the epicenter of a seismic event. Our lab activity does this, but our students then go on calculate a body-wave magnitude for the event and also analyze 1st P-wave "ups" and "downs" to narrow down the focal plane solution and the possible types/orientation of faulting that could be responsible. Follow-up topics include: why there is variation of magnitude as measured at different stations; why the epicentral distance circles don't perfectly intersect at a point (like they do in textbooks); whether the faulting matches the plate tectonic setting; whether a nuclear bomb test could have been responsible for the waves rather than fault displacement; and how likely it was that the earthquake would have generated tsunami.

Context

This lab exercise is used in an entry level Natural Hazards course for non-STEM students, most of whom are taking the course to fulfill their lab science requirement. They come to the exercise equipped with background in plate tectonics, the types of faulting expected at different types of plate boundaries, and seismic waves (e.g how body waves travel through Earth, the difference between P and S waves, which travel through solids vs. liquids).

Why It Works

While epicenter location is important and probably the first thing you would want to know about a seismic event, we believe even entry-level students should be exposed to the breadth of information that can be obtained from the analysis of seismic waves. This goes beyond epicentral location and includes other information about the event (magnitude, type of mechanism, tectonic location) as well as information about Earth, the medium the waves travel though (e.g. Earth is heterogeneous).