Community Forum: the Indian Ocean Tsunami and the North Coast

On Monday, January 24, four members of Humboldt State University Geology Department hosted a Community Forum on the December earthquake and tsunami in Indonesia, and how it relates to what has occurred on the North Coast in the past, and is likely to happen again.

A capacity crowd in the Kate Buchanan room on the HSU campus gathered at 5 p.m. The lights remained dim for much of the next two hours, as the quietly attentive audience saw slides of maps and diagrams, photographs and even some video concerning the December 26 earthquake and resulting tsunami, as well as similar events in Hawaii, Alaska and here on the North Coast.

After thanking several scholars and organizations(including the National Oceanic and Atmospheric Administration and NASA) for their contributions to the presentations, Professor of Geology and HSU Geology Department chair Sue Cashman began by outlining what happened in South Asia.

The epicenter of the December 26 earthquake was off the west coast of Sumatra, on the east side of the Indian Island basin. At magnitude 9.0, it was the fourth largest earthquake recorded since instruments were first used to measure seismic events about a century ago. Nearly 300,000 people are now believed to have perished.

"This was an area known to have earthquakes," Cashman said. "It was so deadly because it generated a tsunami that radiated out in all directions across the Indian Ocean." But this was the only seismically active area in the Indian Ocean, so people at some distance from it may not have been aware of it. This may have contributed to the high number of fatalities. People did not know what an earthquake in the ocean could mean.

She gave a quick course in earthquake science. About a dozen huge plates ride the earth's mantle, under all the land and water on the planet. These plates have been moving slowly but inexorably for millennia, and they are still moving. Earthquakes and volcanoes occur in a zone where two or more plates meet.

Plates that are moving apart from each other cause seismic motion called "spreading." Plates sliding past each other create "transform" faults, like the famous San Andreas. One plate moving under another creates "subduction" zones. About 75% of the world's earthquakes occur from subduction zones, and they generate about 90% of the seismic energy released worldwide.
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