Co- and post-seismic debris flows triggered by the 2015 Gorkha Earthquake

Maxwell P. Dahlquist, Josh West, & Julie Martinez

Submitted August 15, 2017, SCEC Contribution #7707, 2017 SCEC Annual Meeting Poster #135

Debris flows are a primary driver of erosion and a major geologic hazard in many steep landscapes,
particularly near the headwaters of rivers, and are generated in large numbers by extreme events. The
2015 M w 7.8 Gorkha Earthquake triggered ~25,000 coseismic landslides in central Nepal. During the ensuing monsoon, sediment delivered to channels by landslides was mobilized in the heavy rains, and new postseismic landslides were triggered in rock weakened by the shaking. These coseismic and
postseismic landslide-generated debris flows form a useful dataset for studying the impact and behavior
of debris flows on one of the most active landscapes on Earth. Debris flow-dominated channel reaches
are generally understood to have a topographic signature recognizable in slope-area plots and distinct
from fluvial channels, but in examining debris flows associated with the Gorkha earthquake we find they frequently extend into reaches with geometry typically associated with streamflow. We examine a
dataset of these debris flows, considering whether they are generated by coseismic or postseismic landslides, whether they are likely to be driving active incision into bedrock, and whether their channels
correspond with those typically associated with debris flows. Preliminary analysis of debris flow
channels in Nepal suggests there may be systematic differences in the geometry of channels containing
debris flows triggered by coseismic versus postseismic landslides, which potentially holds implications for hazard analyses and the mechanics behind the different debris flow types.

Key Words
Debris flows, landslides, hazards, landscape evolution, geomorphology

Dahlquist, M. P., West, J., & Martinez, J. (2017, 08). Co- and post-seismic debris flows triggered by the 2015 Gorkha Earthquake. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology