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Why Wasn't There a Larger Tsunami from the Magnitude 8.7 March 28, 2005, Sumatra Earthquake?

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Regional map of the area affected by the December and March earthquakes and tsunamis.
Above: Regional map of the area affected by the December and March earthquakes and tsunamis. Stars indicate epicenters for both earthquakes. Ovals indicate approximate areas of energy concentration (high slip) for the two earthquakes (gray, December; yellow stripes, March). Large arrows indicate approximate directions of tsunami-wave focusing for the two events. Thick red line shows sea-floor trace of the interplate thrust fault at the Sumatra-Andaman subduction zone. [larger version]

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:

  1. The first factor, obviously, is that the March earthquake was smaller in magnitude than the December earthquake. The magnitude of an earthquake is a function of the rupture area and the average amount of slip throughout the rupture. The December earthquake had a higher average amount of slip and ruptured a longer segment of the interplate thrust fault than the March earthquake.
  2. The primary-slip patch for the December earthquake (nearest to the epicenter) occurred beneath water depths of approximately 2 to 4 km, in contrast to the primary-slip patch for the March earthquake (nearest to the epicenter), which occurred beneath the island shelf at water depths of less than 1 km. This difference in water depth over areas of greatest energy release produced a difference in amplification as the tsunami traveled from the source region toward the shore.
  3. In addition, the primary-slip patch for the December earthquake was closer to the sea floor ("updip" along the ruptured fault) than the primary-slip patch for the March earthquake. This factor, along with the difference in magnitude, resulted in greater vertical movement of the sea floor in December. As indicated by the finite fault models on the USGS National Earthquake Information Center's event pages for the December 26 earthquake and the March 28 earthquake, the peak vertical displacements of the sea floor were approximately 5 m for the December earthquake and 3 m for the March earthquake.
  4. Finally, there is a significant difference between the two events in the primary direction of tsunami-wave focusing, which affects the distant or "far field" impact of the tsunami. In December, the tsunami energy was focused to the west, toward Sri Lanka and India, and to the east, toward Thailand. In March, the tsunami energy was focused to the southwest, away from any nearby landmasses (excluding Sumatra itself). The island of Sumatra blocked significant wave activity toward Thailand and Malaysia (see map).

Snapshots of computer animations of the March 28 tsunami (left) and the December 26 tsunami (right) 60 minutes after the earthquakes that generated them.
Above: Snapshots of computer animations of the March 28 tsunami (left) and the December 26 tsunami (right) 60 minutes after the earthquakes that generated them. Views northwestward. In the March 28 snapshot, Nias is the farthest visible island to the north. The Banyak Islands and Simeulue, north and northwest of Nias, respectively, are hidden by the waves. Both topography and wave heights are vertically exaggerated (at different scales). [larger version]

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.

Related Sound Waves Stories
Second Tsunami Causes Damage in Indonesia—USGS Scientists Post Observations on the World Wide Web
April 2005
Assessing Tsunami Impacts in the Republic of Maldives
April 2005
Astonishing Wave Heights Among the Findings of an International Tsunami Survey Team on Sumatra
March 2005
USGS Scientists Study Sediment Deposited by 2004 Indian Ocean Tsunami
February 2005
Indian Ocean Earthquake Triggers Deadly Tsunami
Dec. 2004 / Jan. 2005
Could It Happen Here?
Dec. 2004 / Jan. 2005

Related Web Sites
University of Hawai'i Sea Level Center
University of Hawai'i
U.S. Geological Survey Earthquake Hazards Program's Earthquake News & Highlights Archives
U.S. Geological Survey (USGS)
Notes From the Field...USGS Scientists in Sumatra Studying Recent Tsunamis
U.S. Geological Survey (USGS)
Tsunamis and Earthquakes
U.S. Geological Survey (USGS)
The December 26, 2004 Indian Ocean Tsunami: Initial Findings on Tsunami Sand Deposits, Damage, and Inundation in Sri Lanka
U.S. Geological Survey (USGS)
Tsunami Generation from the 2004 M=9.0 Sumatra Earthquake
U.S. Geological Survey (USGS)

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Second Tsunami Causes Damage in Indonesia

Why Wasn't the Second Tsunami Larger?

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Updated December 02, 2016 @ 12:09 PM (JSS)