Environmental and Exploration Geophysics II tom.h.wilson Department of Geology and Geography West Virginia University Morgantown, WV Brief Overview of Earthquake Seismology

Focal Point, hypocenter and epicenter - Distance to focal point

Three observations are needed to define the location of the focal point Focus is located at the common intersection of spheres centered at three observation points. Radius is determined from the P-S arrival time difference.

Velocity variations within the earth’s interior lead to curved ray-paths and a variety of refraction paths.

Gradual increase of velocity with depth causes seismic waves to refract along curved ray paths that eventually return to the surface.

The appearance of a direct P-wave arrival in a time distance plot also forms a curve whose slope continually decreases. Seismic waves traveling greater distances reach greater depths and travel with higher average velocity

In this time distance plot you will see a variety of curved raypaths. What do you think the linear blue colored event is associated with?

6 hours of recording time are shown in the panels below. Most obvious are the surface waves and P-waves. Note that the surface waves keep circling the earth and the P-waves bounce back and forth from one side of the earth to the other.

In the near-surface applications of refraction seismology we have been able to determine velocity distributions and layer thickness. In earthquake seismology it is possible to analyze time distance relationships to uncover the deep structure of the earth working from similar principles.

As increasing amounts of earthquake observations become available it has been possible to map out fairly detailed variations in velocity down to considerable depth beneath the earth’s surface.

… revealing major and minor structural features within the earth,

The tectonic plates which are carried about on large mantle convection cells.

Seismic travel times reveal velocity layering and also more subtle, non-layered regions of velocity variation.

A puzzle to solve - Given the sums through the various cells noted along the sides of the square, determine the values A through D.

Earthquake tomographic images much like CAT scans reveal subtle details in the velocity structure of the earth’s interior

Earthquake tomography provides evidence for the existence of lithosphere graveyards and super plumes.

An increasingly extensive network of seismographs provides more and more detail views of the earth’s interior with each passing year.

Relatively hot and cold regions are discerned through relative differences in velocity. The shapes of these regions reveals information about plume distribution and irregularities in the cooler materials which have sunk down to their resting place on the outer core.

For the remainder of the day I want to review material we’ve covered up to this point that you may be uncertain about and address any questions you may have about the problems that have been assigned.

1. A reversed seismic refraction survey indicates that a layer with velocity V 1 lies above another layer with velocity V 2 and that V 2 >V 1. We examine the travel times at a point located midway (at C) between the shotpoints (at A and B). The travel time of the refracted ray from end A to midpoint C is less than the travel time of the refracted wave from end B to midpoint C. Show that the apparent velocity determined from the slope of the travel time curve for refracted waves produced from the source at A is less that the apparent velocity for refracted waves produced from the source at B. Toward which end of the layout does the boundary between the V 1 and V 2 layers dip? i.e. where is down-dip? Explain! (Robinson and Coruh, 1988). Homework Problems

2. Suppose that a reversed refraction survey indicated velocities V 1 =1500 m/s and V 2 =2500 m/s from one end, and V 1 =1500 m/s and V 2 =3250 m/s from the other. Find the dip of the refractor. What would be the changes in velocities if the refractor had a slope 10 degrees larger than the one you computed? (Robinson and Coruh, 1988)

In addition - begin working problems 3.2, 3.3 and 3.4 from Chapter 3. First geophone is at 3 meters from the source, the remainder are spaced at 10m intervals. What can you see? Pb 3.2

First geophone is at 3 meters from the source, the remainder are spaced at 10m intervals. What can you see? Pb 3.3

The two problems assigned in class and problems 3-2, 3-3, and 3-4 will be due next Tuesday. There will be no formal recitation section tomorrow, however, if you have questions I will be in my office tomorrow from 1-2 and we will also consider additional questions you may have during Thursday’s class.