Does a series of small earthquakes mean the ‘big one’ is more likely? We asked WA experts

BELLINGHAM – Since Feb. 18, Washington has seen seven earthquakes with a magnitude above 2.5, including the strongest earthquake to hit the state in five years, a magnitude 4.5 that struck Orcas Island early in the morning Monday. Two more earthquakes, a 4.8 and a 3.7, have hit southern British Columbia since mid-February.

A string of earthquakes is sometimes interpreted as a sign that a more significant earthquake is on the way.

Washington hasn’t seen a major earthquake since the 6.8 magnitude Nisqually earthquake in 2001. The Cascadia Subduction Zone, capable of producing a 9.0 magnitude earthquake, last produced a major quake in 1700. The fault goes an average of 535 years between major quakes, according to the Washington State Department of Natural Resources, although the amount of time between major tremors ranges from 200 years to 100 years.

But according to Harold Tobin, the chair of seismology and geohazards at University of Washington and director of the Pacific Northwest Seismic Network, any concerns that the series of small earthquakes could be a sign of the “big one” are misguided.

“The bottom line is, it is a myth,” Tobin said in an interview. “If we see earthquakes of this size, and we are seeing, certainly, the last few weeks in the Puget Sound region, we’ve seen more than in a normal week and so people are standing up and noticing. But if you look back over decades, we see periods like this that come and go and earthquakes are distributed pretty randomly through time.”

In fact, when a series of earthquakes occurs in a row, there’s usually no connection between them, according to Tobin.

“This has been a grouping of events in a short time, but it’s not showing us that we’re leading up to something bigger,” Tobin said. “They’re not on the same fault systems. Individually, they’re kind of typical seismic activity for our region. The ebb and flow is that sometimes it’s quieter and sometimes it’s a little noisier.”

Instead, when you have several earthquakes in a short time, Tobin said it’s typically just a product of chance.

“Sometimes in a random sequence, some things will happen close together,” Tobin said.

Do small earthquakes release pressure?

Another common misconception is that a series of smaller earthquakes means a large earthquake is less likely, according to Tobin. The myth holds that small earthquakes relieve stress from a fault to the point that it prevents a larger earthquake from happening in the future. Tobin said that’s not how faults work.

“That’s also not true, and it’s just that a fault that’s building up stress over time that could have a larger earthquake will also have smaller ones,” Tobin said.

Additionally, the way earthquakes are measured on a logarithmic scale, meaning that what appears to be a small increase in magnitude is really a much more significant one. As a result, what might seem like a big enough earthquake to relieve stress from the fault is likely insignificant, according to Tobin.

“You have a magnitude 4.0 earthquake, that’s 10 times smaller seismic waves than a magnitude 5.0 earthquake. Turns out it’s actually 30 times smaller in terms of how much energy is released,” Tobin said. “And then that 5.0 is 30 times smaller than a 6.0, so a 4.0 is actually (almost) 1,000 times smaller than a 6.0. Another way to think of that is that to release the stress of one 6.0 would take 1,000 magnitude 4.0 earthquakes in the same fault.”

Are small earthquakes good or bad?

There is one exception to the idea that a series of earthquakes aren’t connected, though: aftershocks.

“We think of aftershocks (as) a really big earthquake happens, and then there are aftershocks,” Tobin said. “Even relatively smaller earthquakes can have aftershocks. So the 4.5 from Monday, for example, has produced some aftershocks. Now they’re way, way smaller. In fact, they’re so small that no one could possibly have been feeling them.”

Aftershocks, however, aren’t always smaller than the initial tremor. The initial earthquake is then retroactively labeled as a foreshock. In these cases, there is some truth to the claim that a small one is a sign of larger one to come, Tobin said.

“Occasionally, an earthquake will produce, let’s call it an aftershock that’s bigger than the first earthquake. And so when that happens then we, in hindsight, can define the first earthquake as a foreshock,” Tobin said.

But the odds of that occurring are relatively low. The United States Geological Survey puts the odds of any earthquake being a foreshock at 5%, while Tobin said that a smaller earthquake typically has around one in 50 odds of leading to a significantly larger earthquake, and that number drops significantly after the first few days.

“The 4.5, based on our observations of earthquakes through lots and lots of time, only has something between a 2% and 5% chance of being a foreshock to a significantly larger earthquake,” Tobin said. “So any earthquake could be a foreshock to something bigger, but it’s very rare.”