Homework 3: Earthquakes & Volcanoes

Earthquakes: Southern Peru Earthquake, June 23, 2001

On June 23, 2001, a massive earthquake of moment magnitude 8.4, the largest earthquake recorded worldwide in 35 years, struck off the coast of southern Peru. Widespread structural damage was reported in Peru's second largest city, Arequipa, known for historical colonial buildings. Landslides were common in the mountainous regions, blocking roads, disrupting communication lines, and contributing to the loss of property and life. After repairing roads and clearing landslides, relief workers headed to Moquegua where they found widespread destruction. Preliminary estimates indicate at least 80% of the buildings were destroyed or ruined.

Numerous aftershocks were felt by local residents. Preliminary data indicated at least 33 aftershocks registered M5.0 or stronger, including three in the M6 range: an M6.3 a little more than a half an hour after the main shock, an M7.2 at 21:37, and the largest, an M6.8, at 04:18:32 on June 26. It was the largest earthquake in Peru since a M7.7 struck about 200 miles to the northwest on November 12, 1996. It was the seventh M7+ earthquake to strike the planet in 2001 and the largest since the intense Solomon Islands M8.2, M7.8 and M7.2 earthquake series in November of 2000.

Part 1: Locate the Epicenter 1. Using Figure 1, record the arrival times of the P and S waves at each station in Table 1. Read each arrival time as

precisely as possible (in minutes and estimated fraction of a minute). The first vertical line marks the P-wave arrival and the second vertical line marks the S-wave arrival time.

Table 1. SPB = Sao Paolo, Brazil; HDC = Heredia, Costa Rica; FDF = Fort de France, MartiniqueSeismograph

StationP-wave Arrival

(minutes)S-wave Arrival

(minutes)Time Difference(S - P; minutes)

Epicenter Distance (km)

SPB 5 mins 15 sec 9 mins 50 sec 4 mins 35 sec 2,800

HDC 6 mins 10 mins 10 sec 4 mins 10 sec 3,000

FDF 6 mins 30 sec 11 mins 50 sec 6 mins 20 sec 3,200

2. Determine the S-P interval by taking the S-wave arrival time minus the P-wave arrival time. Record your results in Table 1.

1

3. Use your S and P-wave arrival times and Figure 2 to determine the distance from each station to the epicenter. There are 3 ways you can do this.

1) Go to your P-wave arrival time along the vertical axis, move horizontally until you intersect the P line, and then go straight down to the horizontal axis and record the Distance to Epicenter.

2) Do the same thing using your S-wave arrival times and the S line. 3) Use your Time Difference, and find the place on the graph where the S and P lines are that amount of time

apart, then go straight down to the horizontal axis.

You should get approximately the same answer for all 3 of these methods. Don’t forget to multiply the numbers along the horizontal axis by 1,000 to get the correct distance!

4. Which station is located farthest from the epicenter? SPB HDC FDF

10 20 Time (min.)

Figure 1. Seismograms from three monitoring stations.

2

Figure 2. Arrival time curves for P and S waves. Note that the horizontal axis is 1000’s of km.

3

Distance to Epicenter (1000’s of km)

Arrival Time (min.)

5. On Map 1, the epicenter distances were used to draw circles around each monitoring station. Based on their geographic locations (Brazil, Costa Rica, Martinique), label each station on Map 1 with its abbreviated name (SPB, HDC, FDF).

6. The intersection of all 3 circles marks the earthquake epicenter. Drag this arrow to point at the epicenter on the map.

7. Why is it necessary to have seismograms from at least three monitoring stations to find the epicenter of the quake? It is nessesary because they locate the epicenter by triangulation. The distance is figured out by using the arrival times of seismic waves of all three monitoring stations.

Map 1. Monitoring stations and epicenter of the Peru earthquake

4

HDC

FDF

SPB

8. The earthquake occurred on a tectonic plate boundary. Refer to Map 2 to answer the following questions:a. Between which two plates did the earthquake occur?

South American and Nasca plate

b. Are the plates moving towards one another, away from one another, or sliding past one another?The plates are moving toward one another

c. What type of boundary is this?Convergent boundary

Map 2. Tectonic boundaries

5

Volcanoes

For each of the volcanoes below, choose its type and magma viscosity by making the text bold.

Volcano Type Magma Viscosity

9.

10.

11.

12. How does magma viscosity affect the steepness of a volcano? Choose the correct answer by making the text bold.

More viscous = more or less steep?

Less viscous = more or less steep?

6

Shield

Cinder cone

Stratovolcano

Shield

Cinder cone

Stratovolcano

High

Intermediate

Low

High

Intermediate

Low

Shield

Cinder cone

Stratovolcano

High

Intermediate

Low

Mount Saint Helens Eruptions

Figure 3. Satellite image of Mount St Helens before (top) and after (bottom) the 1980 eruption.7

N

N

13. Take a look at Figure 3. When the volcano erupted, in what direction did the majority of the debris go?North

14. When Mount St Helens erupted in 1980, red hot lava did not spew out of the volcano and pour down its flanks. Rather, Figure 4 shows the eruption and some of the destruction that occurred as a result of it. Based on this information, list three volcanic hazards that may occur from eruptions at Mount St Helens.

1. Ash2. Gases3. Pyroclastic flow

Figure 4. Destruction caused by 1980 eruption of Mount St Helens

15. The eruptive history of Mount St Helens since 1400 AD is shown in Figure 5. What is the average number of years that the volcano remains dormant between eruptions?

Plus or minus 120 years

16. Give a geologic explanation as to why the eruption cycle appears to be somewhat regular. (hint: refer to Predicting Volcanic Eruptions in Ch 9 lecture)

Since Mount St. Helens eruption cycle was so active within the 700+ years as shown in figure 5, there is substantial reason to believe that the volcano would be classified as active and therefore eruptions would occur on a “regular” schedule

17. Based on the average dormant interval, and the fact that Mount St Helens last erupted in 1980, estimate the year in which another major eruption is likely to occur. I would estimate that the next eruption would take place around 2080.

18. How accurate do you think such a prediction might be? Explain.I believe that this prediction is very accurate. Based on the data provided, an eruption takes placed every 100 years. The last eruption took place in 1980, therefore 100 years from that date would be 2080.

8

Figure 5. Eruptive history of Mount St Helens since 1400 AD

9