Unusual earthquake activity is occurring at Mount Adams in Washington

Mount Adams, about 70 miles north of Portland, is one of the quietest active volcanoes in the Cascade Range. A small earthquake is typical every few years. But it has confounded expectations recently – there were six small quakes in September alone.

Jon Major, scientist-in-charge of the U.S. Geological Survey in Vancouver, said there was no cause for concern. He recently sat down with OPB Environmental Editor Courtney Sherwood to discuss the latest seismic readings, how they open the door to new research at Mount Adams — and why scientists are convinced there is no connection between the Cascade volcanic chain and a strange smell wafting down the Mount Adams Interstate 5 corridor.

Their conversation has been edited for length and clarity.

Courtney Sherwood: Could you give us a high-level review – what was observed at Mount Adams last month and how is that different from what would be normal there?

Jon Major: At Mount Adams we will typically experience a localized earthquake every few years. And this year we had nine earthquakes that we were actually able to locate at Mount Adams. In September we actually had six earthquakes that were located at Mount Adams.

Sherwood: Do we know what causes these quakes? Is this magma moving? Is this plate tectonics?

Significant: It's a little mysterious. One of our challenges is that there is only one seismic station within about seven miles of the volcano. So it's really difficult to get very precise locations and depths. In response, we are building some seismic stations to collect more data and get better information about the location, size and depth of these earthquakes.

Sherwood: The information USGS has posted online says there is no cause for concern. Can you explain how we know this?

Significant: We can put this into perspective a bit. Mount St. Helens and Mount Rainier will experience between 10 and 20 earthquakes per month.

The volcanoes in the Cascades are constantly talking to us. Mount Adams is generally pretty quiet. This time we decided to perk up a bit and talk to each other a bit more. We don't know exactly why it speaks to us.

These earthquakes are quite small. The magnitude of the earthquakes that we have actually been able to locate range from just under magnitude 1 to around magnitude 2. If a person had stood at Mount Adams, they would not have felt these earthquakes.

And we have no other signs that anything is going on at the volcano.

We don't have our own gas measuring station there. However, there are currently no signs that the volcano appears to be degassing in any way.

We don't currently have any GPS stations out there. We have no ground instruments that can detect ground deformation. But we have satellite images that can detect some ground deformation. And at least in the limited time windows in which satellite images can provide accurate data, no ground deformation can currently be detected.

Sherwood: Do we know why Mount Adams is typically quiet compared to some other Cascades volcanoes?

Significant: If we look a little at the history of Mount Adams, Mount Adams is largely a volcano that spews flows of lava. We call this effusive volcanism versus explosive volcanism. Mount Adams has rarely been explosive in its history.

The last eruption from the volcano's summit is believed to have been more than 10,000 years ago. And for the past 10,000 years, Mount Adams' eruptions have occurred around the lower flank of the volcano. They were cinder cone-like eruptions and small lava flows that traveled perhaps a few miles from the base of the volcano, a bit more like the volcanism we might see over in Indian Heaven or down in central Oregon.

It is not a volcano that was very active.

This is not intended to diminish the risks posed by Mount Adams. There are communities downstream of Mount Adams – Trout Lake – and one of the dangers that Mount Adams poses to the communities are, we call them lahars, these volcanic mud flows.

Over the past 6,000 years, Mount Adams has experienced several large volcanic mudflows that reached Trout Lake and beyond. That's why we take it seriously. Given these earthquakes, this is a good opportunity for us to learn a little more about this volcano. There is no sign that this indicates an outbreak, but we are keeping an eye on it and just trying to gather more information.

Sherwood: If it looks like something is going to happen here, are you pretty confident that there will be enough warnings and opportunities to let people know about it? It sounds like you're not saying that would happen, but rather that things would change.

Significant: Secure. When magma pushes to the surface, it will do a few things. It has to break rock to get to the surface. And when it breaks rock, it will create earthquakes. These earthquakes will increase in frequency, become larger, and become shallower over time.

So that's one thing that we look for in each of our volcanoes. What is the type and type of earthquakes? Are they changing? Are they getting bigger? Are they becoming more common?

The second thing that magma does when it pushes to the surface is that it deforms the ground. Imagine a water balloon that you inflate with water, right? The more water you pump in, the bigger the balloon becomes. Well, the same thing happens with volcanoes.

When we start to see obvious ground deformation, when we start to see gas releases, that will tell us something.

Any eruption in one of our volcanoes in the Cascades will signal quite loudly that something is moving within the volcano. And the development of technologies that we have undergone over the last 40 years, since the eruption of Mount St. Helens, gives us a much greater ability to detect such things early.

Sherwood: OK, I just have one last question and I know this is a little strange. About a week ago there was news about this mysterious smell wafting south from southwest Washington along the I-5 corridor. Some people thought it smelled like sulfur. There was speculation online. Is there a possibility that this is related?

Significant: No, I think we can pretty confidently rule this out for several reasons.

First let’s start with Mount St. Helens. So the obvious suspect as far as the volcano is concerned is probably Mount St. Helens, and we have a gas monitoring station at the top of the crater. When these odors were reported, our gas monitoring station showed that virtually no sulfur-containing gases were being released, neither sulfur dioxide nor hydrogen sulfide, which cause the rotten egg smell.

Related: A strange smell is reportedly wafting south toward Portland along the I-5 corridor

Over at Mount Adams, there is currently no fumarole vent that would obviously leak gases.

As gases escape from volcanoes and drift in the wind, they become rarefied in the atmosphere. And since I'm so far from that area – I don't know how far as the crow flies, but it's pretty far from the Portland-Vancouver area – for such a strong smell to come from Mount Adams, you'd really have to do a lot expel gas.

And then gases become trapped in the magma. If magma were to push up to the surface where gases could escape, we would see earthquakes that would indicate magma pushing upward, and we don't see that. So I think we can pretty much rule out Mount Adams as well.