Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning

High dispersion near-infrared spectra have been taken of seven highly-evolved, variable, intermediate-mass (4-6 Msun) AGB stars in the LMC and SMC in order to look for C, N and O variations that are expected to arise from third dredge-up and hot-bottom burning. The pulsation of the objects has been modelled, yielding stellar masses, and spectral synthesis calculations have been performed in order to derive abundances from the observed spectra. For two stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared with the abundances predicted by detailed AGB models. Both stars show very large N enhancements and C deficiencies. These results provide the first observational confirmation of the long-predicted production of primary nitrogen by the combination of third dredge-up and hot-bottom burning in intermediate-mass AGB stars. It was not possible to derive abundances for the remaining five stars: three were too cool to model, while another two had strong shocks in their atmospheres which caused strong emission to fill the line cores and made abundance determination impossible. The latter occurrence allows us to predict the pulsation phase interval during which observations should be made if successful abundance analysis is to be possible.Comment: Accepted for publication in MNRA

Dynamic liquefaction of shear zones in intact loess during simulated earthquake loading

The 2010-2011 Canterbury earthquake sequence in New Zealand exposed loess-mantled slopes in the area to very high levels of seismic excitation (locally measured as >2 g). Few loess slopes showed permanent local downslope deformation, and most of these showed only limited accumulated displacement. A series of innovative dynamic back pressured shear-box tests were undertaken on intact and remoulded loess samples collected from one of the recently active slopes replicating field conditions under different simplified horizontal seismic excitations. During each test, the strength reduction and excess pore water pressures generated were measured as the sample failed. Test results suggest that although dynamic liquefaction could have occurred, a key factor was likely to have been that the loess was largely unsaturated at the times of the large earthquake events. The failure of intact loess samples in the tests was complex and variable due to the highly variable geotechnical characteristics of the material. Some loess samples failed rapidly as a result of dynamic liquefaction as seismic excitation generated an increase in pore-water pressure, triggering rapid loss of strength and thus of shear resistance. Following initial failure, pore pressure dissipated with continued seismic excitation and the sample consolidated, resulting in partial shear-strength recovery. Once excess pore-water pressures had dissipated, deformation continued in a critical effective stress state with no further change in volume. Remoulded and weaker samples, however, did not liquefy, and instead immediately reduced in volume with an accompanying slower and more sustained increase in pore pressure as the sample consolidated. Thereafter excess pressures dissipated and deformation continued at a critical state. The complex behaviour explained why, despite exceptionally strong ground shaking, there was only limited displacement and lack of run-out: dynamic liquefaction was unlikely to occur in the freely draining slopes. Dynamic liquefaction however remained a plausible mechanism to explain loess failure in some of the low-angle toe slopes, where a permanent water table was present in the loess

The January 2013 Wanganui River debris flood resulting from a large rock avalanche from Mt Evans, Westland, New Zealand

At 9:16 a.m. on 2 January 2013, a flood in Wanganui River, South Westland, severed a fibre-optic communications cable and the road link to south Westland (State Highway 6) at the northern approach to the bridge. This flood occurred during heavy rain which triggered both a rapid rise in stream flows and caused several large landslides in the headwaters of Wanganui River. The flood also inundated farmland in the lower valley. Flows in other South Westland rivers at the time reached high but not flood levels. The flood was caused by a rock avalanche from a 4.5–5.5 Mm3 collapse of the west ridge of Mt Evans (2620 m a.s.l) which fell onto both Evans Glacier and County Glacier in the headwaters of different catchments. One extremely rapid landslide (&gt;35 m/s) swept down Evans Glacier, eroding snow and ice, and then slowed as it descended a gentle river flat eroding flood water, river alluvium and underlying glacial sediments. After traversing the river flat, the water-laden front of the rapid (~35 m/s) rock avalanche evolved into a debris flow in a steeper reach of the stream channel. The debris flow bulked up further with water and boulders from the stream bed and became a debris-laden flood, in places up to about 400 m wide and 80 m deep. The flow deposited large boulders, caused considerable erosion and triggered further landslides that extended up to 120 m above the river channel up to 12 km downstream from the source on Mt Evans. A much smaller rock avalanche on County Glacier did not reach County Stream and had no visible effect on its downstream catchment, Waitaha River.</p

Laboratory simulation of a slow landslide mechanism

Understanding how slow landslides accelerate and decelerate and under what circumstances they catastrophically reactivate are important for both hazard management and implementing appropriate landslide mitigation. Our study used novel laboratory testing of intact landslide materials in a Dynamic Back-Pressured Shearbox (DBPSB) to study how the speed of a slow landslide varies in response to pore water pressure changes at a landslide shear surface. The DBPSB is based on a direct shear device, modified to allow measurement and control of pore water pressure and the dynamic application of normal and shear stresses. It is capable of carrying out static direct shear testing on soils whilst controlling back pressure and measuring pore water pressure in the sample. We used the DBPSB to replicate loading conditions on a landslide shear plane using intact samples of the basal shear zone of the Utiku landslide complex, New Zealand. During each test we measured the deformation of the landslide shear surface to different patterns of pore water pressure increase and decrease. The results have been compared with high resolution slope movement, pore water pressure and rainfall records available for the landslide since 2008. Relating the laboratory measurements with movement pattern records from the site provides new ability to quantitatively examine landslide movement mechanisms, their causes and controls.</p

Slow episodic movement driven by elevated pore-fluid pressures in shallow subaqueous slopes

Subaqueous slopes are susceptible to a broad range of failure mechanisms and deformation styles, many of which are not well characterised. We undertook novel laboratory-based testing using a Dynamic Back-Pressured Shearbox on samples collected from an area subject to ongoing slope failures, situated on the upper slope of New Zealand's Hikurangi Margin, to determine how increases in pore water and gas pressures generate shallow mass movement. Using both water and nitrogen gas we observed similar responses in both cases, indicating that behaviour is dominated by the normal effective stress state regardless of pore-fluid phase. Shear-strain accumulation, representing landslide movement, shows a slow episodic pattern, in common with many shallow terrestrial landslides. Our results are relevant for landslides occurring in shallow near surface sedimentary sequences but have implications for deep-seated landslide behaviour. They suggest that once movement initiates at a critical effective stress, its rate is regulated through dilation and pore expansion within the shear zone, temporarily increasing effective stress within a narrow shear band and suppressing rapid shear. Consequently, under certain conditions, shallow submarine landslides (e.g. spreading failures) can undergo slow episodic movement which allows them to accumulate large shear strains without accelerating to catastrophic movement even when they are unconstrained