The rift near Portland may cause a major earthquake

There’s a bug near Portland, Oregon, that has the potential to cause strong vibration in the area – and that happened as recently as 1,000 years ago.

New research at the Gallis Creek Fault, located 22 miles (35 kilometers) west of Portland, reveals that earthquakes on the fault have torn the surface three times in the last 9,000 years. Today, the flaw is capable of triggering a magnitude 7.1 to 7.4 earthquake, which could cause extremely strong shaking, damage property and possibly even threaten life in the Portland metro area.

Fortunately, large earthquakes on the fault are rare, the researchers reported Oct.20 in the Bulletin of the American Seismological Society. They are repeated every 4,000 years or so, as a rough average, and there is no evidence that the fault is currently at risk of rupture. However, the new findings indicate the need to study other adjacent faults for signs of relatively recent earthquakes. These investigations could reveal more about the risks these breakdowns pose to Portland and other populated areas of Oregon.

Related: The 10 Greatest Earthquakes in History

“Many errors in the area are of significance based on its proximity to population centers,” said Alison Hurst, a paleontologist at Portland State University and now at the Washington State Department of Resources, in a statement.

Signs of earthquakes

Portland is located in the Cascadia subduction zone, the Pacific Northwest region where the Juan de Fuca tectonic plate sinks under the North American plate. This subduction of the plates leads to earthquakes and volcanic activity.

Gales Creek is a ship’s bow fault, which means that it is located in the area between the spot where the plates meet and the volcanic chain link to the subduction zone. It extends for 45 miles (73 km) in a northwest direction in a fertile forested area in the mountains of the Coast Range north of the Willamette River. The vegetation makes it difficult to see evidence of previous earthquakes on the fault, such as the cliff-like cliffs that form when the Earth’s crust is broken.

However, mapping efforts using a technique called lidar (light detection and range identification) indicate that the fault showed the fingerprints of past earthquakes. Lidar uses laser pulses sent from a drone or plane to detect changes in terrain, effectively erasing obstacles such as vegetation. For example, the paths of some flows along the fault have been offset, as if they had suddenly shifted to one side.

Ancient earthquakes

To trace the history of the Gallis Creek fault, Horst and her colleagues dug a group of trenches up to 4.9 feet (1.5 meters) deep using shovels and a bulldozer. Within these trenches, they were able to see layers of sediments deposited over thousands of years, including streaks of dark, coal-rich soil left over from long-ago floods. Fractured portions of these layers showed evidence of past earthquakes. Since charcoal is an organic matter, researchers can use radiocarbon dating, which is based on the rate at which radioactive forms of carbon decompose, to determine when the layers were laid.

This geological history showed three earthquakes large enough to disturb the Earth’s surface rift. The oldest dates back about 8,800 years, 4,200 years later, and the last about 1,000 years ago. This was an average of this large earthquake every 4,000 years on the Galis Creek fault.

Given the length of the rift, the researchers calculated that if the Gales Creek fault slid completely today, it could cause an earthquake of between 7.1 and 7.4 degrees, depending on the depth of the earthquake’s origin. However, the actual risk of a full rift occurring in the near future is unknown. The authors write that the last earthquake that occurred along a similar fault in the region was a magnitude 5.7 earthquake in March 1993 south of Portland, and caused $ 30 million in damage.

The next step, Horst said in the statement, is to study other faults in the regions for signs of ancient earthquakes. Learning of other faults could reveal links between faults, explaining earthquake hazards around Portland.

Originally published on Live Science.

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