What is ‘The Big One’ and when will it happen?

You may see it in the news from time to time - headlines about the ‘Big One.’ The Big One, in this case, refers to a powerful earthquake that will rock the Pacific Northwest of the United States. Recently it’s been the focus of many news articles and broadcasts. If the Big One is fated to happen, do we have any idea when it will hit? The short answer is maybe within our lifetimes, but the long answer is a bit more complicated.

What is the ‘Big One’?

Many people using the term the ‘Big One’ are referring to what is known as a Megathrust Earthquake. These types of earthquakes can be as powerful as a 9 on the Richter scale (what scientists use to measure earthquake strength). The scale ranges from 1 to 10, with the highest ever recorded being a 9.5. Megathrust Earthquakes can only happen in specific types of places on the earth's crust known as subduction zones.

Let’s zoom out a bit. The Earth’s crust is made of huge sections that we call plates. These plates come together in different areas and sometimes create subduction zones. In these zones, one plate moves under the other, which can lead to the creation of things like mountains or volcanoes. It can also cause earthquakes. For the Pacific Northwest, this zone is formed by the Juan de Fuca plate sliding underneath the North America plate.

As one slides beneath the other, the plates can get stuck, leading to a buildup of tension. Eventually the tension becomes too much for the plates to take. When that happens, the tension may be released all at once, in a powerful Megathrust Earthquake. These quakes can also cause massive tsunamis, giant waves that destroy most things in their path. This one-two hit of an earthquake and a tsunami can make these types of earthquakes extra dangerous. It’s also why people are nervous about the next one.

How do we know this powerful earthquake will happen?

The simple answer is that it's happened before, and there are both geological and human records of it. Hundreds of years ago, many First Nation peoples living in the Pacific Northwest experienced a big earthquake. Their accounts speak of shaking that destroyed houses and caused landslides. Next came a massive wall of water that swept entire villages and forests away. Across the Pacific, there are written records of a tsunami hitting Japan seemingly out of nowhere. When the timings of these accounts were compared, both lined up to the January of 1700, about 325 years ago.

Events like this also leave physical evidence in the form of rocks, sand, and other landmarks. Let’s take a look at what the geological record says. The same defined layer of sand can be found in rocks across the Pacific Northwest. This is a sign of past tsunamis, kind of like a signature. The tsunami brings sand from the ocean with it, leaving that sand behind even after the water levels go back down. In addition, various ghost forests can be found along the coasts of Washington and Oregon. When a Megathrust Earthquake happens, some coastal lands can sink several feet. These forests sank into the ocean where the salt water killed the trees and preserved them to this day. When dated, these ghost forests also date back to the year 1700. All of these signs point to this happening in the past, meaning it could happen again.

When will the next 'Big One' happen?

The timing of the Big One really depends on how lucky we are. Research shows that these earthquakes happen once every 400 to 600 years. The last one happened about 325 years ago. According to the Oregon Department of Energy Management, within the next 50 years there is about a 37% chance of a Megathrust Earthquake. This means there is a very real possibility that many of those reading this will live to see it happen. But, it is also not a guarantee. Many geological events are spoken about like they are due to happen any day, but events like this don't have a due date. Will the ‘Big One’ happen eventually? Yes. Is it due to happen any day now? No. It's important to be aware of big events like this, but also not to live in fear over maybes. Be ready, but not afraid.