Lahar Warning System

Upgrade and Expansion Projects

There are currently five AFM sites on both the Carbon and Puyallup Rivers, plus two radio repeaters. The United States Geological Survey (USGS), in coordination with Pierce County Emergency Management (PCEM), has made plans to upgrade the Acoustic Flow Monitors (AFM) detection technology. The USGS is working on a multiyear project to enhance the system and create new sites called Lahar Monitoring Stations (LMS) with new software, more solar panels, and some cameras. The upgrade project will focus first on the Puyallup drainage. Both systems will run in tandem to ensure operability before the old AFMs are removed.

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PCEM is also in the midst of an expansion project that will increase the number and reach of the sirens. PCEM currently maintains thirty-four sirens in the Puyallup River Valley, with the plan to add several more within the next year. Once the project is complete, PCEM will be able to use voice warnings that include alternate languages, integrate flashing lights to reach additional vulnerable populations, and expand the network to the Nisqually River Valley. The sirens can then be used to warn about all hazards. The sound of the monthly test may change as a result of this expansion project.

Click on the video to hear what the siren sounds like.


One of the natural hazards facing residents of Pierce County is volcanic activity from Mt. Rainier. Experts consider it one of the most dangerous volcanoes in the world, largely because of the threat of a lahar. A lahar is a large volcanic mudslide, stemming from snow and ice melt; given the sheer number and volume of glaciers on Mt Rainier, and proximity of residents to the mountain, the damage could be catastrophic.

The Puyallup River Valley is at greatest risk. Tens of thousands of people live in areas that may have as little as 40 minutes to as much as 3 hours to move to safety once a large lahar is detected, so the system is robust, warnings are disseminated promptly and widely, and people in harm’s way are taught how to prepare and take protective actions.

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In response to this hazard, Pierce County Emergency Management (PCEM) has worked closely with our partners to create an integrated lahar warning system. The Mt. Rainier Lahar Warning System consists of two separate components, operating in tandem: Acoustic Flow Monitors (AFM) and the All Hazard Alert Broadcast (AHAB) sirens. Residents will also receive warning via PCALERT and other alerting mechanisms.

System Components

Acoustic Flow Monitors (AFM): An automated system detects lahar flows by using a network of small sensors called acoustic flow monitors (AFMs) embedded underground to measure ground vibrations made by passing lahars. Computer base stations located in the Washington State Emergency Operations Center (EOC) and South Sound 9-1-1 continuously analyze signals from the field stations. Upon detection of a lahar, the computer alerts the 24 hour emergency monitoring and notification centers, who then initiate the warning component of the system. Warning messages would trigger immediate, preplanned emergency response actions.

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All Hazard Alert Broadcast (AHAB) Sirens: The siren component is one aspect of that pre-planned emergency response. When imminent danger threatens, the outdoor sirens will be activated. Citizens will be advised to evacuate the valley floor and head to higher ground. The sirens are made for outdoor warning and used to alert residents in the Puyallup River Valley of the need to evacuate due to a lahar or other volcanic event. The outdoor sirens are tested at noon on the first Monday of every month. The outdoor sirens wail for approximately five minutes during the testing. During a real event, the outdoor sirens will continuously wail until the batteries die or they are destroyed.

Lahar Detection Instrumentation Package

PCEM works in partnership with the United States Geological Survey (USGS), the Pacific Northwest Seismic Network (PNSN), Washington Military Department’s Emergency Management Division, and South Sound 9-1-1 to monitor and operate the systems.

The AFM component was developed by the USGS in 1998 and is now maintained by PCEM, while the sirens are operated and maintained by PCEM. Both components are in the midst of upgrade and expansion projects.

The outdoor sirens have been placed throughout the valley in close partnership with cities and towns, to include Orting, Puyallup, Sumner, Fife, and the Port of Tacoma. PCEM is in the midst of an expansion project that will increase the number and reach of the sirens. That expansion will include towns in the Nisqually River Valley, and outreach to partners there has already begun.