ES-117-2A-77 Natural Disasters (Online Undergraduate)

Earthquakes result from a sudden release of energy from either a new or existing fault in the Earth's crust. This energy arrives to the affected region in the form of seismic waves - waves of energy that elastically distort the material that they travel through. The origin point of seismic energy from within the Earth during an earthquake is the focus or hypocenter. This is the initial region along with a new fault has formed or an old fault has ruptured. The position on the Earth's surface directly above the hypocenter is known as the epicenter. As the rupture along the fault begins, the resulting seismic waves travel outward in all directions from the hypocenter in a spherical fashion. The waves of energy are strongest nearest the hypocenter but gradually grow weaker further away.

There are basically two types of seismic waves: body and surface waves. Body waves emanate spherically from the hypocenter and travel entirely within the interior of the Earth, while surface waves travel along the surface of the Earth emanating from the epicenter. Body waves are compressional or P-waves and shear or S-waves. Surface waves include Love and Rayleigh waves. For the purposes of this lab, we are only going to focus on S and P waves. P-Waves are compressional waves that displace Earth materials in the direction of travel as the move outward in a spherical fashion from the hypocenter. S-Waves are transverse waves the displace Earth materials in a direction perpendicular to the direction of travel as they too move outward in a spherical fashion from the hypocenter. Near the Earth's surface in continental regions, P-waves can travel at speeds around 6 kilometers/second while S-waves travel at speeds around 3.5 kilometers/second. Because P-waves are faster than S-waves, they arrive at seismic stations earlier than S-waves. They are, therefore, referred to as primary waves while the slower traveling S-waves, arriving a little later than the P-waves at the seismic station, are sometimes called secondary waves.

The amplitude of the squiggly curve of the seismogram provides information about the energy released during an earthquake. The Richter magnitude scale is determined by the amplitude P- and S-waves as measured at a distance of 100 kilometers (62 miles) from the epicenter of an earthquake. For example, a magnitude 2 earthquake produces a signal with an amplitude that is ten times larger than a magnitude 1 signal. A magnitude 3 earthquake would produce a signal with an amplitude one hundred times larger than a magnitude 1 signal. The energy in a wave is actually the function of both amplitude and frequency (number of waves per second). The most energetic earthquakes have a higher proportion of low frequency waves than do the the least energetic. Taking these factors into consideration, seismologists have estimated that there is a 32-fold increase in energy going from one Richter scale unit to the next. In other words, a magnitude 3 earthquake releases 1024 times ( 32 x 32) the energy of a magnitude 1 earthquake.

1. Calculate the earthquake epicenter distance from the seismographs provided.

2. Determine the amplitude of each earthquake from the seismographs provided.

Materials

ruler

pencil

lab sheets (download and print below)

Procedures

1. Download and print the following PDF which contains images of 3 seismographs recorded during an earthquake event: 3 Seismograph Readings

2. Recreate the following data table in your lab report.

3. Using a ruler, measure the distance from the beginning of the P-wave to the beginning of the S-wave (see diagram below for example). Record this measurement in millimeters in the data table. If your ruler doesn't have millimeters, measure the S-P separation in inches to the nearest decimal and then convert to millimeters by multiplying by 25.4.

4. Convert each of your S-P separations in millimeters to seconds. 1 mm = 1 second. Record this in the data table.

5. Calculate the epicentric distance from each seismic station using the approximation that 1 second of difference between P and S arrival times corresponds to an 8 kilometer distance. For example, a 12 second S-P separation would equate to a 12 sec x 8 kilometers/second = 96 kilometer epicentric distance. Record these values in the data table.

6. Measure the amplitude from each seismograph. To do this, you measure from the center horizontal line to the top of the highest point on the wave (see diagram below for example). Record this measurement in millimeters in the data table.

7. Now, determine the magnitude for the each earthquake using the Richter Scale plot. To do this, you find the S-P separation in seconds on the leftmost scale and draw a line across to the amplitude in millimeters on the rightmost scale. Where this line crosses the scale in the middle indicates the magnitude of the quake (see example below). You'll need to download and print the Richter Scale

The following video explains the procedures for this lab .ES117LAB02

Complete a report for this week's lab. Your lab report should be 2-3 pages in length and MUST contain the following: 1. Lab Title 2. Introductory paragraph(s) stating the purpose of the lab and covering all concepts, ideas, and definitions covered in the lab.

3. Overview of the lab procedures and data collection. 4. Presentation of data collected (tables, images, graphs, etc...) 5. Answer to all questions posed in the lab (if any). 6. Concluding paragraph(s) in which you discuss the results of the lab and what you've learned from the experience. Submit your completed lab report to the dropbox by day 4.