The Seismic Signature of Debris Flows: Flow Mechanics and Early Warning at Montecito, California

Debris flows are concentrated slurries of water and sediment that shape the landscape and pose a major hazard to human life and infrastructure. Seismic ground motion-based observations promise to provide new, remote constraints on debris flow physics, but the lack of data and a theoretical basis for interpreting them hinders progress. Here we present a new mechanistic physical model for the seismic ground motion of debris flows and apply this to the devastating debris flows in Montecito, California on 9 January 2018. The amplitude and frequency characteristics of the seismic data can distinguish debris flows from other seismic sources and enable the estimation of debris-flow speed, width, boulder sizes, and location. Results suggest that present instrumentation could have provided 5 min of early warning over limited areas, whereas a seismic array designed for debris flows would have provided 10 min of warning for most of the city. Plain Language Summary Rainwater carries mud and rocks down hillsides, forming debris flows. Debris flows not only shape the landscape but also pose a major hazard to human life and infrastructure, as exemplified by what occurred in Montecito, California on 9 January 2018. An effective early warning system for debris flows would accurately determine the location and magnitude of debris flow events and could help save lives. Progress has been made in using seismic ground motions produced by debris flows for this purpose. However, the development of such early warning methods depends on effective collection of data as well as fundamental understanding of the underlying mechanics, both of which have been lacking. Here we discovered that the ground motion caused by debris flows is well recorded with current technologies and has unique characteristics that can be understood through a mechanistic model. These unique characteristics, together with the new mechanistic model, allow us to estimate key information about the debris flows, including the speed, width, rock sizes, and location. With present instrumentation, early warning could have provided 5 min of warning over limited areas. Implementation of a seismic array designed for debris flows would have provided 10 min of warning for most of the city.