WL residents near Bolton Fault 'live on the edge'
South of the city of Portland, in the hills of West Linn, you can catch stunning views of Mount Hood from the windows of homes perched on the ridge above Willamette Drive (Highway 43).
The slopes are steep enough that your downhill neighbor's house may not block your view of the Willamette River valley below.
Hidden Springs Road is just one of many throughways that wind down this ridge. Slopes with up to a 20 percent grade can make the area nearly impassable during the winter months when snow and ice descend upon us.
This is life "living on the edge," or perhaps more correctly "living on the fault-edge."
The Bolton Fault creates this ridge that forms the backbone of West Linn, including neighborhoods such as Hidden Springs and Rosemont Summit. Homeowners living on the fault-edge carry some extra risk of property damage from both earthquakes and landslides.
A fault is a boundary between two pieces of earth that have moved in opposite directions. In the case of the Bolton Fault, the piece of earth forming the ridge above Willamette Drive moved up relative to the road.
Faults and their associated earthquakes are a fact of life in the Pacific Northwest. The entire landscape of greater Portland has been shaped by seismic activity and faulting over millions of years.
It is easy to appreciate the unique topography this provides, but this topography also leads to problems for homeowners.
Portland's most densely populated area is heavily influenced by three main faults that run parallel to the Willamette River. These faults include the Oatfield Fault, the Portland Hills Fault and the East Bank Fault.
The Portland Hills fault is responsible for the steep change in elevation between Washington Park and the city center. The Portland Hills and Oatfield faults are estimated to be up to 30 miles long. The uplifted area between these two faults forms the forested, northwest trending ridge that we call the Tualatin Mountains, also known as the West Hills of Portland.
These Portland faults and the Bolton Fault are known as near-surface faults and are different in many ways from the more famous Cascadia fault, which lies beneath the Pacific Ocean off the Oregon Coast.
The Cascadia Fault extends much deeper below the surface of the earth than the Portland and West Linn area faults.
Near-surface does not necessarily mean less dangerous. Major slippage on the Portland Hills fault or Bolton Fault could be just as damaging locally as an earthquake caused by the Cascadia Fault.
Dangers posed by earthquakes on these near-surface faults can be divided into three major categories: shake damage, liquefaction and landslides.
I have not mentioned the tsunami risk that all Oregon coastal travelers are aware of from the tsunami zone warning signs.
If there is an earthquake on the Cascadia fault, then the resulting tsunami is one of the biggest threats to Oregon homeowners along the coast. The West Linn and Portland faults are not below the ocean and would not create a tsunami if they move.
Shake damage means exactly what it says, damage to your home from shaking caused by an earthquake. It can crack foundations, topple homes, and destroy belongings.
Liquefaction is a "quicksand" effect that can be triggered by tremors in the ground. When soils that are saturated with groundwater start to shake, it causes the soil particles to disaggregate and become suspended in the water. Your foundation soils become like quicksand or mud and your home, which is resting on those soils, will start to sink and tilt.
The third risk, landslides, may represent the biggest risk for many West Linn homeowners. Landslides include processes ranging from slumping of soils to collapse of entire slopes.
Landslides around Portland do not always occur as a catastrophic or sudden failure. Rather you will start to see cracks in the soil as parts of the groundcover slip slowly down the slope.
The problem for homeowners is that it doesn't take much soil slippage to crack walls and foundations. Shaking from an earthquake can exacerbate an existing landslide problem and increase the chances of a catastrophic slope failure.
Now that we've got the bad news out of the way, we can focus on the positive. Major earthquakes don't occur very often in the Portland region.
Movement on the 600-mile-long Cascadia Fault Zone is estimated to create major earthquakes every 350 to 600 years, with the last one being in the year 1700.
The Portland and West Linn faults are estimated to have a major earthquake every 10,000 years or so.
Predicting these earthquakes is not a precise science, and the best we can assume is that sometime between now and the year 2300 we would expect another major earthquake.
So, when someone tells you that we are "due" for another major earthquake, you can put that comment into the proper time perspective.
Of course, this doesn't mean you shouldn't be prepared for a large earthquake. It is one of those hazards with a low chance of occurring in any given year, but the life-threatening damage it can cause demands preparation and a good plan on your part.
The best way to prepare for any natural disaster is to keep yourself informed. The West Linn City website contains interactive maps that allow you to see risks and hazards in your neighborhood.
The maps are a great source for understanding how your lifestyle in West Linn interacts with the natural world around you (westlinnoregon.gov/maps/natural-hazards-mitigation-maps).
You can see from these maps that around the Bolton Fault the earthquake shake risk is moderate, but the landslide risk is high.
Good preparation includes many things, and part of that preparation should be a talk with your insurance agent since most basic homeowner's insurance does not cover earthquake or landslide damage.
Protect yourself physically with a family disaster plan, but also protect yourself financially. Then relax and enjoy those views.
William House is a West Linn