Historical Earthquakes and Uplift/Subsidence of Sumatra from Coral Growth Rings - Introductory Version

Tectonics Observatory, California Institute of Technology
(Page prepared by Elisabeth Nadin)
Author Profile

Summary

The Sumatra region is prone to earthquakes because it lies at the boundary of two of Earth's shifting tectonic plates—the Indian Ocean crust is creeping steadily northeast and subducting beneath Sumatra. The steady horizontal movements, and pulses of faster horizontal motion that occur during earthquakes, are recorded by GPS stations on the islands. But GPS has only been around since the 1980s, so it can't tell us about land motions associated with large earthquakes that happened in Sumatra long ago. GPS also lacks precise information on vertical motions, and cannot be installed under water, so GPS measurements of coastal land movements are incomplete.
In order to figure out how often large earthquakes happen in the Sumatra region, scientists have turned to coral micro-atolls. They also use coral records to reconstruct progressive sea level changes. In this lab, students will use data from real corals collected in Sumatra to track the sea-level and earthquake record of the region over the past century.

Learning Goals

When studying earthquakes, scientists often concentrate on coseismic displacement of land along faults. In recent years, we have learned that there are also interseismic land level shifts in earthquake-prone regions. In this assignment, you will use coral microatolls to measure both coseismic and interseismic land-level changes. You will learn that corals provide precise data on
1) relative sea-level changes in a tectonically active region
2) when a historic earthquake happened

Context for Use

This lesson is appropriate for introductory classes in geology and oceanography. It should be associated with a lesson on earthquakes. Begin with an introduction of atolls, unconformities, and the terms "coseismic" and "interseismic." The graphing concepts might be difficult for some students, and this lesson may take an hour in class plus an hour of at-home work.

There is a lengthier version here:
http://serc.carleton.edu/margins/minilessons/31965.html
for use in an advanced class in oceanography or in paleoseismology/seismology. It may also be done over a longer time (1-2 weeks) for an intro oceanography class.

There is an optional discussion suggestion at the end of this mini-lesson. You can take advantage of tidal gauge data from around the world to discuss global sea-level changes that may be related to climate change, and compare these to local sea-level changes that are related to tectonic activity.

Description and Teaching Materials

Please see the attached file of the assignment, plus the attached figures (you may want to print out multiple copies of the coral figures 2 and 3 so students can hand in a clean final project).
Tools needed:
1) Figures 1–4 (including a location map) are included with this lab. You will refer to and annotate the coral figures to answer the following questions.
2) Colored pencils, ruler, and calculator

Assignment handout with overview and questions (Acrobat (PDF) 884kB Apr27 11)
Figure 1 -- Location Map -- Sumatra (Acrobat (PDF) 2.4MB May28 09)
Figure 2 -- Coral Head Bai (Acrobat (PDF) 293kB May29 09)
Figure 3 -- Coral Unconformities (Acrobat (PDF) 518kB May29 09)
Figure 4 -- Two Corals (Acrobat (PDF) 540kB May28 09)

Teaching Notes and Tips

Use part I in class as an introduction, explaining how corals grow and how the growth rings record inter- and co- seismic motions.
Please contact Laurie Kovalenko (lauriek@gps.caltech.edu) for a full answer key.

Assessment

I plan to give an introduction to this assignment in class, and leave them on their own to start graphing. I will walk around the room and see where they need extra help and then make clarifying remarks for the whole class. Students should be able to finish the assignment either in class or on their own after class time. I plan to weight the interpretation questions more heavily, because the graphs are sure to vary from student to student. As long as the general graph appearance is fine (trends are consistent with the "answer"), then it should be considered correct.
Also, I think the optional discussion at then end will provide a means of seeing if students grasp global vs. local records. Where else in the world could this type of earthquake measurement be done? (where do corals like this grow?)

References and Resources