Mount Rainier, a significant active stratovolcano in the Cascade Range of Washington state, has been dormant for the past thousand years. Nevertheless, the nearby communities of Tacoma and South Seattle have a constant reminder of the threat they live under—they are built on ancient mudflows created by past volcanic eruptions.
Geologists are particularly concerned about lahars, a dangerous mixture of water and volcanic lava that accumulates debris as it flows down drainage channels and valleys. The rapid melting of ice or snow following an eruption is the primary cause of these volcanic formations, posing a significant threat to the surrounding areas.
Over the past 6,000 years, geologists have recorded eleven significant lahars from Mount Rainier that have affected the Puget Lowlands.
In 2022, scientists tested Mount Rainier’s lahar detection technology under two extreme conditions. In the first scenario, a 260 million cubic meters deep and four-meter thick debris flow might reach the densely populated lowlands of Orting, Washington, about one hour after an eruption. Another area with a “pronounced hazard” was the Nisqually River Valley. In this area, there is a significant risk that a massive lahar might remove enough water from Alder Lake to collapse the 100-meter-tall Alder Dam.
Forty years ago, Mount St. Helens created a devastating lahar in the southern Cascade Range; nonetheless, densely populated areas were unaffected.
Since its initial installation in 1998 by the US Geological Survey, Mount Rainier’s lahar detection system has been upgraded and improved upon. At around twenty spots along the two lahar-prone paths and the volcano’s slopes, there are broadband seismometers, trip wires, infrasound sensors, cameras, and GPS receivers. In real-time, these sensors transmit data.
Because technology was so limited in the 1990s, the first system had very low bandwidth requirements and very low power consumption. A more comprehensive network of sensors would improve the ability to distinguish between seismic signals detected by different stations and those caused by debris flows, earthquakes, and eruptions.
In March, the biggest lahar evacuation drill ever conducted included over 50,000 students from six different Washington school districts: Puyallup, Sumner-Bonney Lake, Orting, White River, and Carbonado.
