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Why Wasn't There a Larger Tsunami from the Magnitude 8.7 March 28, 2005, Sumatra Earthquake?
The question many people asked after the magnitude 8.7 March 28, 2005, earthquake was "Why wasn't there a tsunami?" To start with, there was a tsunami generated by this earthquake, which is clearly evident from most tide-gauge stations throughout the Indian Ocean (see compilation at University of Hawai'i Sea Level Center). For example, a tide gauge at Malé in the Republic of Maldives, south of India, recorded a tsunami about 20 cm high (from crest to trough) after the March 28 earthquake; the same site recorded a tsunami about 2 m high after the December 26, 2004, earthquake. A damaging local tsunami was also produced by the March 28 earthquake, as indicated by reports from coastal areas near its epicenter and data collected by an international tsunami survey team that includes U.S. Geological Survey scientists (see related article "Second Tsunami Causes Damage in Indonesia—USGS Scientists Post Observations on the World Wide Web" and reports from the field). At the time of this writing, the exact intensity of the local tsunami generated by the March earthquake had not been determined. Gathering data to assist in that determination was one of the goals of the survey team, which spent most of April conducting fieldwork along Sumatra's west coast. So, a more appropriate question would be "Why was the March tsunami smaller than the December tsunami?" We are learning more and more details about both the magnitude 9.0 December earthquake and the magnitude 8.7 March earthquake that will help us answer this question. Rather than thinking of fault slip during an earthquake as starting from the hypocenter—the point within the Earth where the rock begins to break—and proceeding uniformly along the fault, earthquake rupture commonly is strongly heterogeneous, with some patches of the fault rupturing more than others. A rough schematic of high-slip patches for the December (gray) and March (yellow stripes) earthquakes is shown on the map accompanying this article. The epicenters for both earthquakes are shown by stars. At present, there appear to be four primary factors about the triggering earthquakes that help explain the differences between the two tsunamis:
Knowledge gained from these important earthquakes will help us to provide more detailed tsunami-propagation simulations. Furthermore, analysis of these two earthquakes and comparison with historical earthquakes of similar magnitude (M > 8.5) will allow us to better forecast future variations in tsunami runup. This improved understanding will greatly aid global efforts to provide accurate tsunami warnings and hazard assessments. The ultimate goal is to save lives and reduce property damage from future tsunami disasters.
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in this issue:
Why Wasn't the Second Tsunami Larger? Assessing Tsunami Impacts in the Maldives
Mountain Beaver Population Slow to Recover After Wildfire
USGS Participates in "Spoonbill Bowl" USGS Scientist Interviewed About Hurricane Research Researcher Shares Coral-Reef Expertise
New Nematode Named After USGS Scientist USGS Biologist Recognized by National Park Service
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U. S. Department of the Interior | U.S. Geological Survey
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