Geologists piece together history, future of earthquakes, tsunamis

31 January 2005 | 11:39 Code : 4700 Geoscience events
A panel of HSU geologists presented Monday evening more unsettling evidence that a large magnitude earthquake is on the way for the North Coast.
A panel of HSU geologists presented Monday evening more unsettling evidence that a large magnitude earthquake is on the way for the North Coast.
The geologists met in HSU’s Kate Buchanan room to discuss to a group of more than 100 students, scientists and concerned area residents why the Dec. 26 Indonesian subduction earthquake and tsunami were so deadly.
HSU geologist Sue Cashman said the magnitude 9 quake was the fourth-largest instrumentally measured earthquake of all time and the Eurasian plate’s largest ever. The devastating earthquake and tsunami killed more than 200,000 people, making it the fifth-deadliest earthquake/tsunami event recorded.
Subduction-zone earthquakes are caused when one plate of the earth’s crust slides underneath another plate. The Cascadia subduction zone is the area where the Juan de Fuca and Gorda plates are moving beneath the North American plate, Cashman said.
Showing slides, video clips and 3-D animations, the scientists compared the recent event to past Cascadia subduction zone earthquakes and explained what the implications would likely be for this area’s next significant subduction zone rupture.
The presentation at times seemed like a CSI episode with the HSU geologists piecing together a several-thousand-year history of earthquakes and tsunamis with written historical records, tree ring data and sediment core samples of freshwater lakes from several sites along the Pacific Northwest coast.
HSU geologist Harvey Kelsey presented evidence he gathered from core samples taken in Southern Oregon’s Bradley Lake that indicates that about seven magnitude 9 Cascadia subduction zone earthquakes have occurred in the past 6,000 years. Each of those earthquakes resulted in a roughly 18-foot tsunami or larger, he said.
Kelsey said the Cascadia subduction zone earthquakes and their subsequent tsunamis appear to have occurred in clusters — several magnitude 9 earthquakes during a few hundred years followed by a gap of roughly 1,000 years. He said a nearly 700-year gap preceded the last Cascadia subduction zone earthquake that occurred Jan. 26, 1700, which sent a tsunami hurtling into Japan that was recorded in writing by Japanese merchants.
Kelsey said that he thought that the magnitude-9 earthquake 300 years ago was probably the beginning of the next cluster of big Cascadia subduction zone quakes that he said is more likely to occur in the next few decades to hundreds of years.

“Our point here is not to terrify you,” HSU geologist Lori Dengler said. “There is absolutely no reason why we cannot live with subduction-zone earthquakes.”

Dengler stressed that there was plenty of good news mixed with the bad.

Most North Coast residents live well outside of tsunami hazard areas, including nearly all of Eureka and Arcata, she said. Some low-lying areas such as King Salmon, however, are in particular danger because of their close proximity to the bay’s entrance — a straight shot for a tsunami’s wave energy.
Dengler also said that there was no historical evidence that the dunes north of Samoa have ever been “over-topped” by a tsunami and the highest dunes between Samoa and Manila would likely be a safe refuge from even the largest predicted tsunamis.

“For most of us, it’s the earthquake that we are going to need to be concerned with,” Dengler said.

She said that people can find general earthquake information and check out where the relative tsunami hazards are along the North Coast by visiting the HSU geology Web site at

Dengler said knowing where the tsunami hazards are and are not is critical. Evacuating from areas outside a tsunami’s reach might be very dangerous with large aftershocks that are likely to follow a major subduction zone earthquake, she said.

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