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Geology Rocks Day 4: Beauty and the Beast, On the Trail of The Big One

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Figure 1. Overview of the Day 4 route. Red areas are landslides recently mapped using lidar, a high-accuracy-laser scanning technique.

The layover day ride down Highway 101 to Brookings is one of the most beautiful stretches of coast in the world. You should try to ride at least a part of it. Steep rugged mountains drop into the sea, and clean empty beaches are interspersed with thick forest, sheer cliffs, sea stacks, and arches. For those in need of a rest before the Day 5 climb, a jet-boat ride up the Rogue River, one of the premier fishing and whitewater rivers in the west, is almost as beautiful.

The rocks continue to be a variety of exotic terranes, made largely of mélange, folded and broken sandstone, and serpentine. You will see patches of bright green serpentine just south of Gold Beach after you cross Hunter Creek, just south of Cape Sebastian, and between Myers Creek and Pistol River. In many places you may see large knockers standing out on the open grassy slopes. The highly varied geology of mélange and knockers is largely responsible for the spectacular coastal scenery, as the waves accentuate the chaotic nature of the bedrock to produce a dramatic coastline.

You may have a harder time noticing that the slopes above, and sometimes beneath, the highway are riddled with hundreds of landslides. Using new laser-scanned topographic maps, geologists have identified several thousand ancient landslides lurking along the slopes of the coastal mountains between Gold Beach and Brookings, making this area one of the most landslide-rich in the country.

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Figure 2. The broken rocks of the exotic terranes make for spectacular coastal scenery with many cliffs and offshore rocks and islands. The orange line marks the route.

The force behind this landscape, both the 200 million-year-old mélange and the very recent landslides, is subduction. Subduction happens where an oceanic plate and continental plate collide, and the oceanic plate slides slowly beneath the edge of the continent. The boundary between the plates is a fault zone, typically hundreds of miles long, along which the ocean plate drives beneath the continent in a series of 50-foot jerks, separated by hundreds of years of steady buildup of pressure.

These jerks are called subduction megathrust earthquakes, and are typically magnitude 8 to 9. The magnitude-9 earthquake in Tohoku, Japan in 2011 is the most memorable recent example. During each earthquake, the rocks along the fault get broken and mixed, and over millions of years, after thousands of earthquakes, a thick layer of broken rock forms around the fault. This is how the mélange we see in the hills is formed, and in the case of these exotic terranes, the earthquakes that formed them occurred hundreds of millions of years ago, along a subduction zone that died 150 million years ago.

However, a new subduction zone started up along the Pacific Northwest coast perhaps 50 million years ago. It is still active, and is slowly building up stress today, preparing for the next megathrust earthquake. In between earthquakes, the fault is locked and both plates move together. In fact, an ultra-precise GPS station at Cape Blanco, our Day 3 option destination, is currently moving towards Alberta, Canada at the rate of a little over a half inch a year. At that rate it has moved, along with all of western Oregon and Washington, about 15 feet since the last Cascadia megathrust earthquake in 1700 AD. At the scale of tectonic plates, the crust of the earth is flexible, and it can bend like a bow as shown in Figure 3. As the oceanic plate pushes against the continent along the locked fault, the rocks along the coast not only move steadily to the NE, they also rise as the continent flexes under the strain. When the earthquake finally occurs, the flex relaxes and the whole edge of the continent lurches 15-30 ft back towards the southwest, and the rising areas along the coast subside as the flex relaxes. Fifty miles offshore, the leading edge of the continent rises abruptly, displacing a huge amount of water and producing a bulge on the surface of the ocean. As the bulge of water collapses, it sends out huge tsunami waves, which arrive on the coast in as little as 15 minutes.

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Figure 3. The subduction earthquake cycle. The colliding plates remain locked for centuries, slowly bending as stress builds up. When the earthquake occurs, the bent plate snaps back, and the tip of the overriding plate flips up, making a huge bulge in the ocean surface, which turns into a devastating tsunami.

Computer models of potential future waves have been completed for the entire Oregon coast, and in many parts of coast you are riding today, waves as high as 80 to 100 feet above sea level are possible. The tsunami in Japan in 2011 killed over 16,000 people who were unable to evacuate in time, so it is important to know how to evacuate safely in the unlikely event of a Cascadia tsunami. You can download an app for your phone to show the way to safety or view evacuation maps for Oregon coastal communities here.

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An incredible job of geologic sleuthing has shown that the Cascadia subduction zone has produced 42 megathrust earthquakes in the last 10,000 years, 19 of which were magnitude 9 to 9.2, and another 23 of “only” magnitude 8 to 8.7. The thousands of landslides along the coast are largely the result of this violent history.

In addition to generating a tsunami, the next Cascadia earthquake will cause very strong shaking along the coast for up to five minutes, which in turn will trigger landslides to move on the steep coastal slopes made up of sheared and shattered mélange rock. Geologists expect to see thousands of landslides occur during the next Cascade earthquake, and Highway 101 may be blocked for years.

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Figure 4. Looking east at the coast and ocean floor, you can see where the route lies in relation to the Cascade Subduction Zone. The giant fault slopes back beneath the land, and the red line on the ocean floor marks the point where it reaches the surface of the earth. This red line corresponds to the purple line on the first panel of Figure 2.

The giant fault that produced all of these earthquakes lies just a few miles off the coast along the ride, and because the fault slopes gently to the east underground, it is only 15 miles away from the route, straight down. Don’t worry: The odds of an earthquake occurring on the day you ride the coast are about 1 in 50,000! This Pulitzer-prize-winning article about the impact of the next Cascadia earthquake is worth a read.

Ian Madin is the Chief Scientist for the Oregon Department of Geology and Mineral Industries and in his spare time provides geology blogs for the Cycle Oregon. He will be providing commentary during the evening program and will be a available to answer questions during the ride.


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