MITIGATING LAHAR HAZARDS: THE RAINIER LAHAR DETECTION SYSTEM (Invited Presentation)

Mount Rainier (Washington) is an andesitic stratovolcano that has erupted over 40 times in the Holocene, most recently ~1,000 years ago. Lahars are the most significant volcanic hazard associated with Rainier. At least nine large lahars have reached into now heavily populated areas in the last 5600 years, most recently the Electron Mudflow in ~1500 A.D. Although most have occurred in association with eruptions, geologic evidence indicates that the Electron Mudflow was initiated by a large landslide with no associated eruptive activity. An area of weak, hydrothermally altered rock remains in the landslide scar of the Electron Mudflow. Although eruptions remain the most likely mechanism for generating future large lahars, the spontaneous failure of this rock also has the potential to generate a large lahar similar to the Electron Mudflow event. In response to this hazard, the U. S. Geological Survey (USGS) installed a lahar detection system in 1998 along drainages vulnerable to a future spontaneous-failure-caused lahar. The system was designed to automatically alert State and County emergency managers and provide enough time to evacuate people in the lahar’s path. Starting in 2016, the USGS has been working with partners to upgrade and expand the Rainier Lahar Detection System (RLDS) to include modern instrumentation, real-time telemetry, an expanded range of sensor types, a 10x increase in the number of stations, and improved lahar-detection algorithms. Due to the paucity of recordings of lahars and debris flows worldwide, the USGS is engaging in a range of studies that will aid in our understanding of how to improve rapid automated detection of such events. These studies include deployments of temporary seismic and infrasound sensors in drainages likely to have debris flows, experiments in a USGS-operated large-scale flume, and numerical simulations. Given the low recurrence interval of large lahars (9 in the last 5,600 years), we are dependent on recordings of smaller events at Mount Rainier and elsewhere to improve the ability of the RLDS to automatically detect, localize, and characterize large lahars.