Abstract A short, intense sequence of volcano‐tectonic earthquakes preceded a period of strong volcanic tremor at White Island volcano, New Zealand, in July—September 1991. The tremor was initially harmonic with clear higher harmonics, but after 3 days was gradually replaced by broadband non‐harmonic tremor. Good examples of both harmonic and non‐harmonic tremor were recorded. Shock waves were observed in the eruption column of May 91 vent from early August, coinciding with the period of non‐harmonic tremor. The harmonic tremor is interpreted to have been due to a standing wave vibration in vesicular magma in the conduit beneath May 91 Vent, and the non‐harmonic tremor to open‐vent degassing activity near the top of the vent.
A major source of error in forecasting where airborne volcanic ash will travel and land is the wind pattern above and around the volcano. GNS Science, in conjunction with MetService, is seeking to move its routine ash forecasts from using the ASHFALL program, which cannot allow for horizontal variations in the wind pattern, to HYSPLIT, which uses a full 4-D atmospheric model. This has required some extensions to the standard version of the HYSPLIT program, both to get appropriate source terms and to handle the fall velocities of ash particles larger than 100 microns. Application of the modified HYSPLIT to ash from the Te Maari eruption of 6 August 2012 from Tongariro volcano gives results similar to the observed ash distribution. However, it was also apparent that the high precision of these results could be misleading in actual forecasting situations, and there needs to be ways in which the likely errors in atmospheric model winds can be incorporated into ash models, to show all the areas in which there is a significant likelihood of deposited ash with each particular volcanic eruption model.
At approximately 09:36 UTC on 27 April 2016, a phreatic eruption occurred on Whakaari Island (White Island) producing an eruption sequence that contained multiple eruptive pulses determined to have occurred over the first 30 min, with a continuing tremor signal lasting ~ 2 h after the pulsing sequence. To investigate the eruption dynamics, we used a combination of cross-correlation and coherence methods with acoustic data. To estimate locations for the eruptive pulses, seismic data were collected and eruption vent locations were inferred through the use of an amplitude source location method. We also investigated volcanic acoustic–seismic ratios for comparing inferred initiation depths of each pulse. Initial results show vent locations for the eruptive pulses were found to have possibly come from two separate locations only ~ 50 m apart. These results compare favorably with acoustic lag time analysis. After error analysis, eruption sources are shown to conceivably come from a single vent, and differences in vent locations may not be constrained. Both vent location scenarios show that the eruption pulses gradually increase in strength with time, and that pulses 1, 3, 4, and 5 possibly came from a deeper source than pulses 2 and 6. We show herein that the characteristics and locations of volcanic eruptions can be better understood through joint analysis combining data from several data sources.
Abstract Volcanic ashfall forecasts are highly dependent on eruption parameters and synoptic weather conditions at the time and location of the eruption. In Aotearoa, New Zealand, MetService and GNS Science have been jointly developing an ashfall forecast system that incorporates 4D high-resolution numerical weather prediction (NWP) and eruption parameters into the HYSPLIT model, a state-of-the art hybrid Eulerian and Lagrangian dispersion model widely used for volcanic ash. However, these forecasts are based on discrete eruption parameters combined with a deterministic weather forecast and thus provide no information on output uncertainty. This shortcoming hinders stakeholder decision making, particularly near the geographical margin of forecasted ashfall and in areas with large gradients in forecasted ash deposition. This study presents a new approach that incorporates uncertainty from both eruptive and meteorologic inputs to deliver uncertainty in the model output. To this end, we developed probabilistic density functions (PDFs) for the three key eruption parameters (plume height, mass eruption rate, eruption duration) tailored to Aotearoa’s volcanoes, and combine them with NWP ensemble datasets to generate probabilistic ashfall forecasts using the HYSPLIT model. We show that the Latin Hypercube Sampling (LHS) technique can be used to representatively span this high-dimensional parameter space with fewer model runs than Monte Carlo techniques, thus allowing this methodology to be used in near real-time forecast systems. We also propose new probabilistic summary products such as hazard matrices, probability of exceedance of cumulative ashfall, and arrival time forecasting, which together support public information and emergency responders decision making.
Inferno Crater Lake, Waimangu, one of the largest hot springs in New Zealand, displays vigorous cyclic behavior in lake level and temperature. It provides a natural small‐scale laboratory for investigating the geo‐electrical signature of fluid flows. We measured self‐potential and electrical resistivity to see whether the huge variations of fluid volume, approximately 60,000 m 3 during a mean cycle period of 40 days, could be detected. Electrical resistivity measurements revealed spectacular changes over time, with the medium becoming more conductive as the lake receded. This result is consistent with analog models, where the vapor phase is replaced by liquid at recession. The self‐potential survey did not detect temporal changes related to fluid movements. This can be explained by the pH of the pore water (∼2.3), which is close to the point of zero charge of silica.
While volcanic events are commonly characterized by multiple eruptive stages, most probabilistic tephra hazard analyses only simulate the major (paroxysmal) stage. In this study, we reconsider this simplified treatment by comparing hazard outcomes from simulated single‐ and multistage eruption sequences, using the Okataina Volcanic Center (OVC) in New Zealand as a case study. Our study draws upon geological evidence particular to the OVC as well as generalized patterns of eruptive behavior from other analogous volcanic centers. Exceedance probabilities of simulated tephra thickness, the cumulative duration of explosive behavior, and the duration of the entire eruptive sequence were all compared. Multistage simulations show an increased hazard with the greatest differences lying close to the vent for long duration and high thickness thresholds and at intermediate distances between the vent and the maximum extent of the deposit for lower thickness and duration thresholds. Multiple explosive stages increase the likelihood of an event lasting longer than 1 month by up to sevenfold and, for given low‐probability events, accumulated tephra thicknesses in some locations may increase by 1 order of magnitude and impact up to 22% more of New Zealand's North Island. Given our understanding of the eruptive history of the Okataina Volcanic Center, multistage simulations provide a better understanding of the potential hazard from this source.
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