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

Tectonics Observatory, California Institute of Technology
(Page prepared by Elizabeth Nadin)
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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 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.

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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. how land level changes
  2. when a historic earthquake happened

Context for Use

This exercise is appropriate for mid- to upper-level class in sedimentology or in paleoseismology/geodesy. It may also be appropriate for a longer (1–2 weeks) exercise in introductory oceanography. There is a shorter version of this lesson for an introductory geology or oceanography class at http://serc.carleton.edu/margins/minilessons/32975.html.

TOOLS
  1. Figures 1 through 6 (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
You may also wish to add a discussion at the end of global context. Where else in the world could you apply the technique?
Use tidal gauge data from different places around the world. Is sea level rising or falling globally? How might the Gulf coast differ from Alaska? Does sea level appear to be falling in tectonically active regions? For a sed/strat or geomorphology class, this exercise would be a terrific match to a terrace-level assignment.

Another idea is to get GPS data from stations on Sumatra and compare modern horizontal motions with the historical vertical motions.

Description and Teaching Materials

Please see attached file assignment handout with questions (Acrobat (PDF) 889kB Apr26 11)
Figure 1 -- Location Map (Acrobat (PDF) 2.4MB Apr13 09)
Figure 2 -- Head Bai (Acrobat (PDF) 293kB May29 09)
Figure 3 -- Coral Unconformities Example (Acrobat (PDF) 518kB Apr13 09)
Figure 4 -- Head Tb1 (Acrobat (PDF) 278kB Apr13 09)
Figure 5 -- Head Tb2 (Acrobat (PDF) 839kB Apr21 09)
Figure 6 -- Comparison of Tb Corals (Acrobat (PDF) 540kB May28 09)

Teaching Notes and Tips

It may prove useful to go over part I in class together, explaining how corals grow and how the growth rings record inter- and co- seismic motions.
Teachers can contact Laurie Kovalenko (lauriek@gps.caltech.edu) for a full answer key.

Assessment

I plan to make this an assignment to begin in class, and students would be allowed to work in pairs or in groups. I will walk around the room and see where they need extra help and then make clarifying remarks for the whole class. For this advanced-level version, students should be able to finish the assignment 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. A final discussion will see if students can evaluate global vs. local contexts.

References and Resources