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New evidence of recent earthquakes geologically near Portland, Oregon metro area – ScienceDaily

An ancient ancient trench dug across the Galis Creek Fault, located about 35 kilometers (about 22 miles) west of Portland, Oregon, documents evidence of three superficial rupture earthquakes that occurred about 8,800, 4,200 and 1,000 years ago.

The results, published in the Bulletin of the American Seismological Society, indicate that earthquakes occur roughly every 4,000 years due to the fault. If the 73-kilometer (45-mile) fault ruptures completely, the result could be a 7.1 to 7.4 magnitude earthquake that would pose a major seismic hazard to the Portland metro area, according to Alison Horst and colleagues.

In comparison, the 1993 Scotts Mills earthquake about 50 kilometers (31 miles) south of Portland was a magnitude 5.7 earthquake, and caused damage totaling about $ 30 million, the researchers note.

The region is part of the seismically active Cascadia subduction zone, where the Juan de Fuca tectonic plate bends below the North American plate. The Gales Creek Fault lies within the Cascadia forearc, the land sandwiched between the oceanic trench where the Juan de Fuca bend and line of the Cascadia volcanoes in Washington State and Oregon fed by the slip plate.

Said Horst, a paleontologist who previously worked at Portland State University and now in the Washington State Department of Resources.

Damage mapping and analysis in the Pacific Northwest may be difficult, since fault surface effects are often covered by urban development and dense forests, or are difficult to access in mountainous areas. To learn more about possible recent seismic activity along these forearm faults, Horst and her colleagues dug a trench across the Gallis Creek fault, which was previously designated and is being investigated by the US Bureau of Reclamation and the US Geological Survey along a portion of the fault caused by the Skogens Dam in County Washington, Oregon

The reclamation project showed evidence of surface deformation along the fault in sediments from a more recent geological time period, called the Holocene. After digging a trench across the fault – first by hand and then with a backhoe – the researchers searched for evidence of previous earthquakes in the rock layers, and determined an estimated date of each earthquake using radiocarbon analysis of the coal in the layers.

Trench digging showed strong evidence of at least three surface-rupture earthquakes from the Holocene epoch along the fault, with some weaker signs of one potential earthquake that occurred 1,000 years later, and one that occurred 8,800 years ago.

The researchers also estimated the magnitude of the earthquake that would rupture the full length of the Gales Creek fault, assuming the full length rupture in one go and the rupture event did not extend across multiple faults.

“The link between the rupture on the Galis Creek fault and adjacent faults is still unknown, as there are no other ancient earthquake studies of earthquake ages for adjacent faults and as a result there is no indication of older earthquakes with overlapping age estimates on adjacent faults,” he explained. “Future work on faults in the region could allow us to improve our understanding of rupture connectivity on these low slip faults, and long redundancy errors.”

The researchers concluded that the results indicate that other defects within the Oregon portion of the Cascadia forearc should be studied for signs of Holocene earthquakes.

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Material provided by the American Seismological Society. Note: The content can be modified according to the style and length.

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