# Stress & Strain

The text assumes that you know the basics of elasticity. Unfortunately, the softening of geoscience degree programs insures that many do not have a firm foundation in continuum mechanics. I can’t spend weeks covering the material, but I started with a review of stress and strain. The plan to insure that you understand derivation of the fundamental equations and boundary conditions of seismic wave propagation and excitation at least in outline form. That is, I want you to be a blue to follow the progression from Newton’s Law and Hooke’s Law to the fundamental equations. The better students will struggle through the details to understand deeply, but hopefully all will appreciate the path through the material.

When we move from elementary physics and the study of the effect of forces on “point” objects, we must deal with how the forces are transmitted through a material, it is convenient to consider forces normalized by the area they operate across. This leads to the idea of stress, a force per unit area. Pressure is a simple example, but more generally, we are also interested on orientation of the surface (the area) on which the force is operating, so the concept of stress is abstracted somewhat. Strain is a deformation, and we are interested in deformations because seismic waves are deformations (disturbances) from equilibrium that transmit energy from the source (earthquake) region (where it was stored as strains) outward, in all directions surrounding the source. Seismic waves are important because they can cause substantial damage to human-made constructions, and they are valuable because the transmit information on the earthquake process and Earth’s interior. We use a rather simple (appropriate for small deformations) definition of strain that is nonetheless precise. Precision is necessary to reason quantitatively about the processes that are involved in seismic excitation and propagation.

Read your old structural geology or continuum mechanics notes. Try to connect them to what we discuss. The Schaum’s Outline by Mase is a good short review of continuum mechanics, and there are many good books on the subject (check out the library). You won’t have time to read a whole book and keep up with the class, but self study at a slower pace is a good way to understand the details. You might have to start with a review of some mathematics (vector calculus and tensors).

You should be reading Chapter 3 of Udias et al. (2014) to see where we are headed.

# Searching For Earthquake Information

In the last 10 minutes of class, to break from the theoretical work, I reviewed three common places for searching for earthquake information: the USGS, the Global CMT Catalog, and the ISC. Examples are shown at http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/AdvSeismo/. You should walk through some examples that interest you to get a feel for how to search these data bases interactively. Each also has more efficient way to search using computer programs or scripts, but I often find myself browsing or checking ideas by searching these sites.