SEMI-AUTOMATIC, ALS-BASED GEOMORPHOLOGICAL CHARACTERIZATION OF THE SHASTA VALLEY DEBRIS AVALANCHE DEPOSIT, NORTHERN CALIFORNIA

The failure of destabilized volcano flanks is a likely occurrence during the lifetime of a stratovolcano, and can generate large debris avalanches. The significant hazards associated with these events in the Cascade range were demonstrated by the collapse of Mt St Helens (WA), which triggered its devastating explosive eruption in 1980.

Mt Shasta is a 4,317 m high, snow-capped, steep-sloped stratovolcano located in Northern California. The current edifice began forming on the remnants of an ancestral Mt Shasta that collapsed ~300-380k years ago producing one of the largest debris avalanches known on Earth. The debris avalanche deposit (DAD) covers a surface of ~450 km² across the Shasta valley, with a volume of ~26 km³.

The recently acquired NCALM Aerial Laser Scanner (ALS) survey across the Shasta DAD provided a new topographic dataset of the area with unprecedented resolution. Statistical measures of the ALS-derived DTM will be used to characterize the hummock topography, differentiate between facies and geomorphic units, and extract the morphological parameters of the DAD. Specifically, the hummocks will be semi-automatically outlined using the NETVOLC code, and the morphological parameters of each hummock will be computed with the MORVOLC program. Given the limited extent of the ALS survey (i.e. 40 km²), the high-resolution dataset will be used for validation of the morphological parameters extracted from freely available, broader coverage DTMs such as the National Elevation Dataset (NED). Preliminary results show an overall underestimation of the geomorphological parameters using the NED-derived DTM compared to ALS. In addition, the ALS dataset permits the identification of subtle topographic features not apparent in the field or in coarser resolution datasets, including a previously unmapped fault. We will show an example and comparison with the NED-derived DTM.

This work aims to improve our understanding of the Shasta DAD morphology and dynamics, and provide insight into the cause, timing of events and mode of emplacement of the DAD. The Cascade range includes numerous large extinct, dormant or active stratovolcanoes. Knowledge of the link between basement structures and the Shasta DAD will elucidate the causes of edifice instability and may be used to target priority areas for volcanic hazard mapping.