Benjamin Hunter Mackey

Sediment yield, spatial characteristics, and the long-term evolution of active earthflows determined from airborne LiDAR and historical aerial photographs, Eel River, California

In mountainous landscapes with weak, fine-grained rocks, earthflows can dominate erosion and landscape evolution by supplying sediment to channels and controlling hillslope morphology. To estimate the contribution of earthflows to regional sediment budgets and identify patterns of landslide activity, earthflow movement needs to be quantified over significant spatial and temporal scales. Presently, there is a paucity of data that can be used to predict earthflow behavior beyond the seasonal scale or over spatially extensive study areas. Across 226 km^2 of rapidly eroding Franciscan Complex rocks of the Eel River catchment, northern California, we used a combination of LiDAR (light detection and ranging) and orthorectified historical aerial photographs to objectively map earthflow movement between 1944 and 2006. By tracking the displacement of trees growing on earthflow surfaces, we find that 7.3% of the study area experienced movement over this 62 yr interval, preferentially in sheared argillaceous lithology. This movement is distributed across 122 earthflow features that have intricate, elongate planform shapes, a preferred south-southwesterly aspect, and a mean longitudinal slope of 31%. The distribution of mapped earthflow areas is well-approximated by a lognormal distribution with a median size of 36,500 m^2. Approximately 6% of the study area is composed of earthflows that connect to major channels; these flows generated an average sediment yield of 19,000 t km^(−2) yr^(−1) (rock erosion rate of ∼7.6 mm/yr) over the 62 yr study period, equating to a regional yield of 1100 t km^(−2) yr^(−1) (∼0.45 mm/yr) if distributed across the study area. As such, a small fraction of the landscape can account for half of the regional denudation rate estimated from suspended sediment records (2200 t km^(−2) yr^(−1) or ∼0.9 mm/yr). We propose a conceptual model for long-term earthflow evolution wherein earthflows experience intermittent activity and long periods of dormancy when limited by the availability of readily mobilized sediment on upper slopes. Ultimately, high-order river channels and ephemeral gully networks may serve to destabilize hillslopes, controlling the evolution of earthflow-prone terrain.

Long-term kinematics and sediment flux of an active earthflow, Eel River, California

Although earthfl ows are the dominant erosion mechanism in many mountainous landscapes, estimates of long-term earthfl ow-driven sediment fl ux remain elusive because landslide displacement data are typically limited to contemporary time periods. Combining high-resolution topography from airborne LiDAR (light detection and ranging), total station surveying, orthorectifi ed historical aerial photographs, and inventories of meteoric 10 Be in soil pits, we quantifi ed ~150 years of slope movement on a 1.5-km-long earthfl ow in the Eel River catchment, northern California, United States. Using LiDAR-derived topography, we mapped the upper half of the earthfl ow into three distinct kinematic zones: an upslope source area, a long narrow transport zone, and a mid-slope compressional zone. From our air photo analysis (1944‐2006), average velocities are fastest in the transport zone (1.7 m/a), slowest in the source zone (<1 m/a), and decrease monotonically over the past 30 years in all three zones. Meteoric 10 Be inventories systematically increase with distance downslope of the source area, consistent with the notion that the elongate transport zone acts like a relatively undeformed soil conveyor that can be used to quantify long-term displacement. Because our 10 Be-derived transport zone velocity of 2.1 m/a averages over the past 150 years, pre-1944 velocities likely approached 2.5 m/a, suggesting that twentieth century land-use practices have not increased rates of sliding. Although our results reveal a progressive decline in velocity that may refl ect exhaustion of readily mobilized source material, velocities temporarily increased in the midtwentieth century due to major hydrologic events. Given an average velocity of 2 m/a, the Kekawaka earthfl ow is defl ating its source area over 20 times faster than the regional erosion rate, emphasizing the localized and vigorous role of active earthfl ows in landscape evolution.

Landslide-dammed paleolake perturbs marine sedimentation and drives genetic change in anadromous fish

Large bedrock landslides have been shown to modulate rates and processes of river activity by forming dams, forcing upstream aggradation of water and sediment, and generating catastrophic outburst floods. Less apparent is the effect of large landslide dams on river ecosystems and marine sedimentation. Combining analyses of 1-m resolution topographic data (acquired via airborne laser mapping) and field investigation, we present evidence for a large, landslide-dammed paleolake along the Eel River, CA. The landslide mass initiated from a high-relief, resistant outcrop which failed catastrophically, blocking the Eel River with an approximately 130-m-tall dam. Support for the resulting 55-km-long, 1.3-km3 lake includes subtle shorelines cut into bounding terrain, deltas, and lacustrine sediments radiocarbon dated to 22.5 ka. The landslide provides an explanation for the recent genetic divergence of local anadromous (ocean-run) steelhead trout (Oncorhynchus mykiss) by blocking their migration route and causing gene flow between summer run and winter run reproductive ecotypes. Further, the dam arrested the prodigious flux of sediment down the Eel River; this cessation is recorded in marine sedimentary deposits as a 10-fold reduction in deposition rates of Eel-derived sediment and constitutes a rare example of a terrestrial event transmitted through the dispersal system and recorded offshore.

Strong proximal earthquakes revealed by cosmogenic 3He dating of prehistoric rockfalls, Christchurch, New Zealand

The 2011 rupture of previously undetected blind faults beneath Christchurch, New Zealand, in moment magnitude (M w ) 6.2 and 6.0 earthquakes triggered major rockfalls that caused fatalities and infrastructure damage. Here we use field, geospatial, seismologic, numerical modeling, and cosmogenic 3 He data to provide first evidence for prehistoric rockfall ca. 8– 6 ka, and a possible preceding event ca. 14– 13 ka, at a site where extensive rockfall occurred in the Christchurch earthquakes. The long (∼7 ± 1 k.y.) time intervals between successive rockfall events and the high peak ground velocity thresholds required for rockfall initiation at this site (∼20–30 cm/s) preclude earthquakes from major identified seismic sources, including the plate boundary Alpine fault, as likely rockfall triggering sources. Rockfalls were probably triggered by strong paleoearthquakes sourced from active faults proximal (i.e., <10–20 km) to Christchurch, including the sources of the 2011 Christchurch earthquakes and/or other currently unidentified faults. Given the inherent incompleteness of seismic source catalogues and challenges in obtaining earthquake chronologies for blind faults, high scientific priority should be given to the search for, and analysis of, geologic records of strong earthquake shaking near populated areas.

Historic drought puts the brakes on earthflows in Northern California

Californias ongoing, unprecedented drought is having profound impacts on the states resources. Here we assess its impact on 98 deep-seated, slow-moving landslides in Northern California. We used aerial photograph analysis, satellite interferometry, and satellite pixel tracking to measure earthflow velocities spanning 1944–2015 and compared these trends with the Palmer Drought Severity Index, a proxy for soil moisture and pore pressure that governs landslide motion. We find that earthflow velocities reached a historical low in the 2012–2015 drought, but that their deceleration began at the turn of the century in response to a longer-term moisture deficit. Our analysis implies depth-dependent sensitivity of earthflows to climate forcing, with thicker earthflows reflecting longer-term climate trends and thinner earthflows exhibiting less systematic velocity variations. These findings have implications for mechanical-hydrologic interactions that link landslide movement with climate change as well as sediment delivery in the region.

Synchronous late Pleistocene extensional faulting and basaltic volcanism at Four Craters Lava Field, central Oregon, USA

Central Oregon (northwestern USA), where northern Basin and Range extension diminishes in magnitude across the High Lava Plains, exhibits widespread extensional faulting and Quaternary volcanism, yet the relations between the processes are complex and chronology is poorly constrained. Here we use cosmogenic 3 He exposure dating of basalt lava flows to quantify the timing of normal faulting and emplacement of a lava field on the margins of pluvial Fort Rock Lake. The northwest-trending Christmas Valley fault system displaces High Lava Plains volcanic rocks, forming an ∼3-km-wide graben that transects the eastern Fort Rock Basin. A portion of the western edge of the graben is marked by a normal fault displaying flexural shear folding with a prominent vertical hinge crack, called Crack in the Ground. Lava flows of the Four Craters Lava Field flowed into this crack. Exposure dating of the Four Craters Lava Field, on the eastern flank of the older Green Mountain shield volcano, indicates an emplacement age of 14 ± 1 ka. We dated Green Mountain basalt exposed in the walls of the crack (the fault wall), which also yielded exposure ages of 14 ± 1 ka. The similar ages suggest that substantial crack opening occurred at about the same time the Four Craters lava was emplaced. These data indicate a period of synchronous normal faulting and dike-fed cinder cone activity emanating from a fault-parallel fissure ∼2 km northeast of the crack ca. 14 ka, with minimal displacement since.