Dacula, Georgia Earthquake Intensity Exercise

Background information:

The area around Dacula, GA (a few miles east of Lawrenceville in Gwinnett County, Georgia) experienced a swarm of small earthquakes in Fall 1995 and Winter 1996.

The larger earthquakes consisted of a loud ka-BOOM, followed by shaking, which rattled windows and awakened sleeping persons. The earthquakes commonly occurred late at night or early in the morning and were initially thought to be illegal explosives. (See article on the Count at Dacula).

Objectives:

In this exercise, students will:

1. Use the Modified Mercalli Intensity (MMI) Scale to rank earthquakes according to their intensity
2. Plot earthquake intensity data on a map
3. Contour earthquake data on a map
4. Determine the approximate location of the epicenter
Students will use real data obtained by surveying persons who experienced the earthquakes.

Modified Mercalli Intensity Scale:

The Modified Mercalli Intensity Scale is used to rank earthquakes according to how they are experienced by people in the area where the earthquake is felt. The scale ranges from I to XII (always written in Roman numerals). On this scale, I is the mildest and XII is the most severe.

MMI Description
I
Not felt except by a few under especially favorable circumstances. Sound only; no vibrations.
II
Felt only by a few persons at rest, especially on upper floors of buildings.
III
Felt quite noticeably indoors, especially on upper floors of buildings, but many people do not recognize it as an earthquake. Hanging objects swing. Vibration like passing of trucks. Duration can be estimated.
IV
Hanging objects swing. Sensation like heavy truck striking building. Walls creak. At night some awakened.. Standing motor cars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes.
V
Felt by nearly everyone, many awakened. Felt outdoors; direction can be estimated. Liquids disturbed, some spilled. Small objects moved or overturned. Some dishes, windows, etc. broken. Doors swing open or closed. Pictures move on the wall. Pendulum clocks stop, start, change rate.
VI
Felt by all. Many frightened and run outdoors. Persons walk unsteadily. Windows, dishes, glassware broken. Small objects (knickknacks, books, etc.) fall off shelves. Pictures fall off walls. Some heavy furniture moved or overturned. Some fallen plaster, and masonry or chimneys cracked. Damage slight. Small bells ring (church, school). Trees, bushes shaken (visibly, or heard to rustle).
VII
Everybody runs outdoors. Difficult to stand. Noticed by drivers of motor cars. Hanging objects quiver. Furniture broken. Damage negligible in buildings of good design and construction; slight to moderate damage in well-built ordinary structures; considerable damage in poorly built or badly designed structures. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, etc. Waves on ponds; water becomes turbid with mud. Small slides and caving in along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged.
VIII
Damage slight in specially designed structures; damage considerable in ordinary substantial buildings with partial collapse; damage great in poorly designed structures. Steering of motor cars affected. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, columns, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes.
IX
General panic. Damage considerable in specially designed structures, sometimes with complete collapse. Wood frame houses, if not bolted, shifted off foundations. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluvial areas sand and mud ejected, earthquake fountains, sand blows, sand craters.
X
Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Railroad tracks bent slightly. Ground badly cracked.
XI
Few, if any masonry structures remain standing. Bridges destroyed. Railroad tracks bent greatly. Underground pipelines completely out of service. Broad cracks or fissures in ground.
XII
Damage total. Waves seen on ground surface. Objects thrown upward into the air. Large rock masses displaced.

Directions:

1. Print out (or have available) the map of the Dacula, GA area. You may need to print it in two parts, overlap them, trim the overlapping edge so the pattern matches evenly, and tape them together.
Top portion of the map of the Dacula, GA area.
Bottom portion of the map of the Dacula, GA area.
For best results (so that all data stations are on the printout), set your printer to print in "Landscape" mode.

2. Have available a copy of the Modified Mercalli Intensity (MMI) Scale (see above).

3. Using the the Modified Mercalli Scale and the descriptive data in the table below, rank the intensity of the earthquakes as they were felt at each data reporting station. PUT THE MMI NUMBER IN THE PROPER BLOCK IN THE TABLE BELOW.
(Note that assigning a MMI ranking is somewhat subjective. An earthquake report may not fit neatly into one category. You may rank an earthquake slightly differently than another student does. That is OK. BUT, you need to be consistent and follow the same procedure each time.)

4. Fill in the color which corresponds to the intensity of the earthquake in the appropriate column in the table below.

• White (no color) = earthquake not heard or felt
• Blue = earthquake was heard only. no shaking. (I and II)
• Green = vibration felt, but no dishes reported to rattle. (III)
• Yellow = strong vibration or shaking; like truck striking building. dishes or windows rattle. some awakened. (IV)
• Red = dishes or windows broken, cracked plaster, pictures move, objects overturned.
• Purple = anything more severe.

5. Locate the number on the map corresponding to each earthquake report.

6. Color in the white number on the map using the color which corresponds to the intensity of the earthquake.
(Colored pencil or crayon works better than markers on printouts.)

7. Contour the data on the map by drawing lines to separate the different colors. Be sure to indicate outer limits of area where earthquakes were experienced. (Note that there may be a few unusual data points here and there, due to differing home construction, or how close to bedrock their houses sit, etc.).

Your contour map should resemble a bull's eye pattern, with concentric circles of each color. The highest earthquake intensity will be in the center. (You may color in the entire area between lines with the appropriate color, once you are certain your lines are drawn in the right place.)

Descriptive Data from persons who experienced the Dacula, Georgia earthquake swarm:

The following data are based on surveys with residents of the Dacula, GA area.

Data
point
MMI
number
(fill in)
Color
(fill in)
Earthquake report or description
1     Big jolt. Boom and immediate shaking. Rattles windows and dishes. Awakened. Walls do not creak. First felt like car hit brick pillar on porch. Mostly felt between midnight and 7 am. Three large ones on Dec 12.

2     Started November 10. Continuing and intensifying. Strongest was Dec. 1. Started at 10:30 the night before. Definitely awakened. Moved bed back and forth. Windows rattled. Like dynamite blast. Big one on Dec 6. A lot on Dec 10 and 11. Jolt like truck hitting the building. Nothing broken.

3     Two small pictures fell off the wall. Dishes, windows, everything shakes and rattles. Little things fall. Sometimes have sound only. Sometimes whole house shakes. Felt best in Kitchen, which rests on bedrock. Like an explosion. Boom. Started noticing Nov 10, a few per week. More frequent since Dec 6; 14-20 per day. Some times written down.

4     Like a sonic boom followed by ground shaking. Thought it was space shuttle going over. Awakened 2:30, 4 am. Some very loud, some minor. Could tell it was coming from underground, not the air. Began calling police to look for blasting. There is a quarry in Grayson, but no action there at those times. Called congressman. Fire marshal said it was "systematic displacement". Maybe due to the creek water seeping into the ground. Sometimes a minor pop and a little shake or just noise. No dishes rattle, but their neighbors' have.

5     Started about end of Nov or first of Dec. 11:15 pm heard booms, lasted until 3 am. 4 or 5 times. Same the next night. Very loud sounds. Two sounds: KA-BOOM. The first sound is low and muffled, followed by a big BOOM. Awakened at 5:45 am. They don't occur just one at a time. 3 - 6 at a time ofer 2-3 hours. Called police and newspaper and fire marshal. One was so strong it shook dishes, china, crystal. Whole house shook. Others a like a soft thud, like a soft sound against a wall.

6     Heard but not felt. Like blasting or thunder. No shaking.

7     Sounded like loud explosion, hanging objects swung out of place. Windows rattled.

8     Sounded like loud explosion. Small objects moved or overturned. Building shook slightly. We were awakened. Windows rattled. Felt or heard 5 of them.
9     Sounded like explosion off in distance, hanging objects swing out of place, building shook strongly, awakened, dishes and windows rattled. A window cracked in the front of our house due to the earthquake. We have a lot - every other day. Very large one on 1-16-96 at 3 am.
10     Sounded like loud explosion, small objects moved or overturned. Building shook slightly. dishes and windows rattled. no damage but small stuff overturned, scared me. I didn't know what was happening. I was just lying on my bed and all the sudden I felt the bed shake. whew. I didn't like it at all. Only felt one but my Mom feels one little one every day in the morning about 5:45 while I'm in the shower.
11     Heard sound only, no vibration. Like explosion - off in distance. 3 or 4 times lately.
12     Awakened. Sounded off in distance. Windows rattled. Have heard many over several years. Most were daytime. Have felt a few at night.
13     Sounded like loud explosion, off in distance. The center wall inside and part of ceiling is cracked.
14     Sounded like loud explosion, nearby. Building shook slightly. No damage. Had 4 or 5.
15     Building shook slightly. Felt and heard less than 5. One time in early December at 1:30 at night, also 10 am one morning.
16     Had some a few years ago. early morning. Heard a few nights when husband was at the fire station. No windows rattle. Little tremble and sound soft, then LOUDER. Not awakened.
17     Sounded like loud explosion, off in distance. Building shook slightly. Awakened. Windows rattled. Felt 3. (Nov-Dec 95).
18     None heard or felt.
19     None heard or felt.
20     None heard or felt.
21     Sounded like loud explosion. Building shook slightly. Windows rattled. Lasted about 2 minths (Nov-Dec 95), sometimes several in one day. Thought it was dynamite. No damage. Heard/felt many.
22     Sounded like loud explosion. Had two over 2-3 months.
23     Sounded like loud explosion. Building shook slightly. Awakened. Dishes and windows rattled. Had many.
24     Sounded like loud explosion. Building shook slightly. Awakened. Dishes and windows rattled. Felt ground shaking. Had a lot.
25     Felt vibration. Heard sound. Building shook slightly. Awakened. Windows rattled. No damage. Had approximately 12.
26     Not heard or felt.
27     Sounded like loud explosion off in distance. Building shook slightly. Windows rattled. Thought it was like a big bomb. They quit the second week of March. Nothing in the 6 weeks since then.
28     Sound only, no vibrations. Sounded off in distance. Building shook slightly. Windows rattled. Had three.

This exercise created by Pamela J. W. Gore, Georgia Perimeter College, Clarkston, GA, in the Atlanta, GA area.
Copyright 1998, 2009 Pamela J. W. Gore