Name:

Geology Laboratory:

Predicting Eruptions:

Earthquakes as Warning Signs

A still -- Volcano -- Life --

That flickered in the night --

When it was dark enough to do

Without erasing sight --

A quiet -- Earthquake Style --

Too subtle to suspect

By natures this side Naples --

The North cannot detect

The Solemn -- Torrid -- Symbol --

The lips that never lie --

Whose hissing Corals part -- and shut --

And Cities -- ooze away –

Emily Dckinson

Introduction

The first warning that Mount St. Helens might become active was a small earthquake on March 20, 1980. Before then, the mountain had been quiet for fifty-nine years. The summit was covered with snow. No crater was visible. Then, on March 20 scientists recorded a 4.1 earthquake underneath the mountain. This was followed by other earthquakes. By March 23, earthquakes were occurring below the mountain at the average rate of one every half hour. On March 27, the first eruption of steam and ash occurred at the summit, and as many as 130 minor (< 4.0) earthquakes were occurring each day. It was evident that the mountain was going into an active phase.

In Hawaii, the eruption of Mauna Loa or Kilauea can be predicted by the earthquake activity and the swelling of the summit areas. In fact, the movement of the magma from depth to the summit can be traced by the location of the earthquakes. The first earthquakes occur at depth. Later ones become progressively closer to the surface. Also, many of the earthquakes are a special kind called “harmonic tremors” which are believed to be caused by the movement of magma. Scientists were hoping that by studying the earthquakes – the “warning signs” – future eruptions of Mount St. Helens could be predicted.

Questions/procedure

1. Make a hypothesis .Which of the following kinds of data do you think might be best for making a prediction about an eruption? You may combine choices and add your own ideas.

a) The average number of earthquakes per hour. (If all the small ones – some which were detected only by a seismograph – are counted, this number becomes quite large. For example, on March 27, 135 earthquakes with a magnitude of 3.0 or greater were recorded.)

b) The number of earthquakes greater than 4.0. (Maybe the number of stronger earthquakes which occurred during a given period is more important than the swarms of little ones. Although a 4.0 earthquake probably would not be felt unless the person were directly over it, this strength is still large enough to be significant.)

c) The magnitude of the strongest earthquakes. (Maybe the pattern of when the strongest earthquakes occurred is significant.)

d) The occurrence of harmonic tremors. (As these were detected only on certain days, maybe they could be used to tell when the magma was moving.)

Write your hypothesis below.

2. The volcanologists who were studying Mount St. Helens had to hypothesize just as you did. With little information about former eruptions of Cascade volcanoes, they plotted all available data looking for clues or warning signs.

In this activity you will plot the real data given in the table. If you were a scientist studying Mount St. Helens in 1980, could you have predicted the major eruption on May 18^th from this data?

Table of Earthquake Data from Mount St. Helens

Time period
(3 days)

Ave. No, of earthquakes per hour

No. of earth-quakes greater than 4.0

Largest earth-quake for the 3-day period

Other events:

E=Eruption; H=Harmonic tremors

March 20,21,22

23,24,25

26,27,28

29,30,31

3 1/2

4.1

4.4

4.9

4.6

Quiet

E. (first)

E. and H.

April 1,2,3

4,5,6

7,8,9

10,11,12,

13,14,15

16,17,18

19,20,21

22,23,24

25,26,27

28,29,30

2 ½

1 ½

4.9

4.5

4.7

4.9

4.7

4.6

4.8

4.6

4.7

E. (large)

E. and H.

Quiet

Bulge detected

Quiet

May 1,2,3

4,5,6

7,8,9

10,11,12

13,14,15

16,17

4.7

4.8

5.0

4.8

4.4

5.1

Quiet

E. and H.

Quiet

E. (major)

3. Move to a computer workstation and start Microsoft Excel. In the spreadsheet, list the middle-most date (from the string of 3 days) from column 1 in the table above. Enter it by month and day in the following way: April 2 = 4/2, May 14 = 5/14, etc. Excel will automatically change the format (as you will see). In the second Excel column, enter the associated average number of earthquakes per hour (column #2). Continue entering data from columns 3 and 4 (do not enter data from column 5).

4. When all data has been entered, highlight the first two columns in Excel. Click the Chart Wizard icon from the menu bar (it looks like a bar graph). Continue as follows:

Step 1: Select line graph option. Click Next.

Step 2: Click Next.

Step 3. Click Titles tab, and enter titles for the graph and the axes; Click Legends, uncheck show legend box; click Next.

Step 4: Click Finish.

5. A graph will appear on your spreadsheet. Right click on the x-axis and click Format Axis. Select the Numbers tab. The date category should already be selected. From the type box, select a date style that does not include the year (which, as you know, is 1980.) click OK. Left click on the graph and then Print .

6. Repeat the steps above and create 2 additional graphs, one with data from column 3 and column 4. In each case, also highlight column 1 so that the dates appear on the graph. You can select non-adjacent columns by selecting one column, holding the ctrl key, then selecting the other.

7. Study the graphs you have made to see if you can detect a relationship between the earthquakes (the warning signs) and the eruptions of Mount St. Helens. If you had been a scientist monitoring the earthquake activity, what graph would you use to predict the catastrophic eruption on May 18? You will notice column 5 has data on eruptions and harmonic tremors. On the graph you think best predicts the eruption, develop and write in symbols for both. Your eruption symbol should be scalable to represent minor eruptions (E), large eruptions (E large) and major eruptions (E major).

8. Explain why your chose the graph you did as the best predictor. Do the inclusion of eruptions and harmonic tremors reinforce your argument? How? Would the same information be able to predict other volcanic eruptions at other volcanoes? Defend your position.