The Mossyrock Dam in seen in this aerial file photo taken on May 28, 2015, before Tacoma Power reduced the water level at the dam.
In the first week of the new year there have been more than 50 earthquakes reported near Mount St. Helens, including a 3.9 magnitude earthquake that struck about 23 miles south of Morton on Wednesday. That earthquake was the second largest since 1981 in the Mount St. Helens vicinity. The scores of accompanying rumbles are occuring at a rate that’s about four times more frequent than normal.
All those tremors have renewed concerns over the structural stability of the Mossyrock and Mayfield dams on the Cowlitz River in East Lewis County. Representatives from Tacoma Power, which owns and operates the hydroelectric dams, say that neither dam has been put in jeopardy by the recent seismic activity.
However, they are well aware of the inherent dangers of one or multiple dam failures that could be put in motion by earthquakes in the region.
In early 2017, Tacoma Power announced plans to keep the water level at Riffe Lake below 750 feet, which is about 30 feet lower than normal during spring and summer, for the foreseeable future. That announcement came on the heels of concerns raised in a study conducted by the U.S. Geological Survey. That study noted the spillway piers on the upriver side of Mossyrock Dam are susceptible to failing and breaking off during a large scale earthquake of magnitude 7.5 or greater in the vicinity of the dam. At full capacity, that type of failure would result in an uncontrolled flow of 230,000 cubic feet per second that would inundate communities downstream.
During the winter, the lake is generally kept at a level of 745 feet, except when extra water is temporarily held during heavy rainfall or rapid snowmelt events. On Friday, the water level at Riffe Lake was reported at just 722 feet. Former Tacoma Power generation manager Pat McCarty previously told The Chronicle that the lower water level, which he called “the new normal,” will be maintained until seismic retrofits can be applied to Mossyrock Dam.
A similar study at Mayfield Dam found that the bridge deck does not meet safety standards for a similar worst case scenario seismic event. Last February, McCarty noted that no action had yet been taken in regard to the Mayfield Dam issue, but he insisted that a plan was being developed in order to update the suspect infrastructure.
In an email to The Chronicle on Friday, Monika Sundbaum, a public information representative for Tacoma Power, wrote, “I spoke with our chief dam safety engineer about the earthquake. Thankfully, this event did not impact us. He said the 3.9 magnitude quake occurred more than 20 miles away from Mossyrock Dam and even further away from Mayfield Dam. Although our employees did not feel the earthquake when it occurred, as soon as they learned of it, they followed the protocol of checking the magnitude, as well as distance from the dams; The combination of these two factors determines if any further action is needed.”
Sundbaum noted that those emergency procedures are prepared in accordance with the Federal Energy Regulatory Commission. According to those guidelines, an earthquake with a magnitude less than 4.0 would have to occur within 3.1 miles of the dam in order to trigger a formal evaluation. According to the Cascades Volcano Observatory, the earthquake occurred at 12:36 a.m. Wednesday morning about 5 miles north of Mount St. Helens and at a depth of 6 miles. That event was categorized as a tectonic earthquake that aligned with normal regional stress and faulting. Scientists say that the quake was not associated with volcanic activity. Following that earthquake, at least 15 aftershocks occured. Those quakes ranged from magnitude 0.6 to 2.7 and were located south-southeast of Morton. The 3.9 magnitude quake was felt as far away is Portland and Puget Sound. The strongest earthquake on record near Mount St. Helens since 1981 was a magnitude 4.2 at Johnston Ridge Observatory on Valentine’s Day, 2011.
Steve Mansfield, Lewis County Emergency Management Director, said that the swarm of earthquakes near Mount St. Helens in recent days should serve as a warning to local residents.
“As far as my concerns, they haven’t changed. We have this Cascadia Fault thing sitting off our coast that people need to at least be aware of and at least start talking about what they need to do,” said Mansfield. “The dams today are the same risk today that they were last week, six months ago, 10 years ago or 20 years ago for me … We need to be talking about it, be aware of it and prepare for it.”
Each year, Tacoma Power and Lewis County Emergency Management conduct a test of the emergency sirens around the Cowlitz River dams. Last year, officials conducted a full scale mock-emergency response in the valleys below the dams. Mansfield noted that additional siren testing will likely be conducted around April of this year.
However, Mansfield stressed the need for additional Lewis County residents to sign up for the Lewis County Alert system. He said some warning calls can be placed using reverse-911 technology to ring home telephones, but he noted that most people do not have landlines any longer. By signing up for Lewis County Alert, users can be alerted to developing emergency situations by cellphone, text or email.
“I can alert certain segments of our population, but I need you to go in and opt into the system,” explained Mansfield.
While this week’s earthquakes have not resulted in widespread chaos or catastrophe, Mansfield said that it is never a bad time to prepare for the worst.
“We have put the preparedness message out there and it is all about personal preparedness,” said Manfield.
He noted that emergency situations can take many forms and strike at any time, so folks should be prepared with storage of extra water, food, evacuation routes and a plan for reuniting with family.
“In my position I need to always feel that we’re not as prepared as we need to be, because that’s what drives me. Steve Mansfield is never going to be able to stop all these things from happening, but I can make sure that we are as prepared as possible,” said Mansfield.
Learn more about Lewis County Alert at lewiscountywa.gov/em/lewis-county-alert.
