Hi All. Think some of you might be interested in this. It seems the rupture of the entire Cascadia fault may be the exception rather then the norm. Take Care…Don in creepy town

“When studying Cascadia, Harvey Kelsey, a geologist at Humboldt State University in California, and colleagues began with Bradley Lake, a small lake about 500 meters inland on the southern Oregon coast. They chose Bradley, after a year-long reconnaissance of lakes from Washington to California, because for the past 7,000 years, it has recorded Cascadia-generated tsunamis, but not farther-traveling ones, as the lake is too high and too far inland. What they found there were two major observations: “One is that the entire margin of Cascadia does not move during an earthquake, and second is that tsunamis occur in a cluster-gap-cluster pattern,” Kelsey says.”

“Reporting in the July/August Geological Society of America Bulletin, they determined that tsunamis entered the lake on average once every 390 years, whereas thelocal record of Cascadia earthquakes near Bradley shows an interval of one every 500 years. “If the entire Cascadia margin moved, then we would see a one-to-one match,” Kelsey says.”

“The research team also assembled a detailed record of when tsunamis hit Bradley. The data reveal that tsunamis occur in 250- to 400-year clusters followed by gaps of 700 to 1,300 years, with the earthquake in 1700 possibly beginning a new cluster of quakes. On Jan. 26, 1700, a magnitude-9.0 earthquake ruptured the entire length of the subduction zone and generated tsunami deposits from California to Canada (see story, above). This “megathrust” event was the last major movement on the Cascadia fault system.”

“Their work is meticulous. They have put together an excellent standard for dating along the Oregon coast,” says Brian Atwater, a U.S. Geological Survey (USGS) geologist at the University of Washington who specializes in Cascadia tsunami research. Previously, geologists thought that the 1700 event, which broke along the entire margin, represented the “norm,” Atwater says, but Kelsey’s team’s work “brings back to light earlier observations that Cascadia has a variable rupture mode. This is a really important finding.”

http://www.geotimes.org/sept05/geophen.html

• Cascadia Fault Ruptures