How earthquake-prone is Portland?

An expert from the Oregon Department of Geology spoke at Portland State Wednesday about the fault lines in Portland being a possible earthquake threat to the city.

Although the majority of Portland’s faults are minor dead faults, it is uncertain which of the minor faults could be an earthquake threat to Portland, according to Ian Madin, chief scientist at the Oregon Department of Geology and Mineral Industries.

Faults, fractures in rock surfaces, are the locations where most earthquake activity occurs, and Madin said Portland is riddled with fault lines.

“The jury is still out on what is happening in downtown Portland with the faults,” he said. His presentation was a part the PSU Geology Department’s Winter Seminar Series.

A fault in downtown Portland believed to run along both sides of the downtown hills, known as the Portland Hills Fault, is a main focus of study in Portland. Geologists using sets of “indirect techniques” have found evidence of folding on the Portland Hills Fault caused by earthquakes.

Because visual surface clues provide little information about faults in Portland, geologists like Madin must rely on the use of a combination of indirect geophysics techniques on the Portland Hills Fault.

“It’s a real uphill battle to convince people that this is evidence of past earthquakes,” Madin said.

He added that he has uncovered indirect evidence that a fault runs under the line of the new aerial tram. Engineers remain unconvinced that there is a need to design buildings and infrastructure for the possibility of an earthquake.

Next year Madin will study the Portland Hills Fault and look for more evidence of earthquake activity. A major challenge he faces when studying the city’s faults is uncovering what lies beneath Portland’s urbanization.

The type of data on local earthquake faults that Madin and other geologists have collected isn’t convincing enough for some engineers. Some old downtown buildings have been retrofitted for earthquake resistance, but Madin said that the minimum protection for occupants is a way to evacuate the building in the event of an earthquake, with no continued function of the building.

Little is known about the faults that exist under Portland and what sort of earthquake threats the city faces. Geologists are still unsure what fault caused a large earthquake in the spring of 1993, and faults are more difficult to map in Oregon than other notable earthquake-prone areas such as California because the Portland fault lines are not visible with satellites images.

“We have a hard time seeing even the major faults on the ground,” said Madin. He said the lack of visibility of the fault lines makes it hard for geologists like him to convince engineers that earthquake-prone buildings are needed.

Madin’s lifelong ambition is to map faults in Portland and learn about the city’s earthquake hazards. He received a master’s in geology from Oregon State University. Some Portland State students are currently working with Madin, and work to identify Portland’s faults was done by PSU many years ago.

For additional information about Portland’s faults and earthquakes, visit www.oregongeology.com.

The Geology Winter Seminar Series continues Wednesday, Feb. 7, with “Temperature and Suspended Sediment Modeling of Detroit Lake, Cascade Mountains” presented by Stuart Rounds of the U.S. Geological Survey Portland. The seminar will be held in the Cramer Hall sub-basement, Room 17, at 3:30 p.m. For more information, contact Scott Burns at 503-725-3389 or [email protected].