Estimating rates of debris flow entrainment from ground vibrations

Debris flows generate seismic waves as they travel downslope and can become more dangerous
as they entrain sediment along their path. We present field observations that show a systematic relation
between the magnitude of seismic waves and the amount of erodible sediment beneath the flow. Specifically,
we observe that a debris flow traveling along a channel filled initially with sediment 0.34m thick generates
about 2 orders of magnitude less spectral power than a similar-sized flow over the same channel without
sediment fill.We adapt amodel fromfluvial seismology to explain this observation and then invert it to estimate
the level of bed sediment (and rate of entrainment) beneath a passing series of surges. Our estimates compare
favorably with previous direct measurements of entrainment rates at the site, suggesting the approach may be
a new indirect way to obtain rare field constraints needed to test models of debris flow entrainment.