SLOPE STABILITY ANALYSIS OF MOUNT MEAGER, SOUTH-WESTERN BRITISH COLUMBIA, CANADA

The Mount Meager Volcanic Complex (MMVC) in south-western British Columbia is a potentially active, hydrothermally altered massif comprising a series of steep, glaciated peaks. Climatic conditions and glacial retreat has led to the further weathering, exposure and de-buttressing of steep slopes composed of weak, unconsolidated material. This has resulted in an increased frequency of landslide events over the past few decades, many of which have dammed the rivers bordering the Complex. The breach of these debris dams presents a risk of flooding to the downstream communities. Preliminary mapping showed there are numerous sites around the Complex where future failure could occur. Some of these areas are currently undergoing progressive slope movement and display features to support this such as anti-scarps and tension cracks. The effect of water infiltration on stability was modelled using the Rocscience program Slide 6.0. The main site of focus was Mount Meager in the south- east of the Complex where the most recent landslide took place. Two profiles through Mount Meager were analysed along with one other location in the northern section of the MMVC, where instability had been detected. The lowest Factor of Safety (FOS) for each profile was displayed and an estimate of the volume which could be generated was deduced. A hazard map showing the inundation zones for various volumes of debris flows was created from simulations using LAHARZ. Results showed the massif is unstable, even before infiltration. Varying the amount of infiltration appears to have no significant impact on the FOS annually implying that small changes of any kind could also trigger failure. Further modelling could be done to assess the impact of infiltration over shorter time scales. The Slide models show the volume of material that could be delivered to the Lillooet River Valley to be of the order of 109 m3 which, based on the LAHARZ simulations, would completely inundate the valley and communities downstream. A major hazard of this is that the removal of such a large amount of material has the potential to trigger an explosive eruption of the geothermal system and renew volcanic activity. Although events of this size are infrequent, there is a significant risk to the communities downstream of the complex.

DOING MORE WITH SHORT PERIOD DATA: DETERMINING MAGNITUDES FROM CLIPPED AND OVER-RUN SEISMIC DATA AT MOUNT ST. HELENS

How can we calculate earthquake magnitudes when the signal is clipped and over-run? When a volcano is very active, the seismic record may saturate (i.e., the full amplitude of the signal is not recorded) or be over-run (i.e., the end of one event is covered by the start of a new event). The duration, and sometimes the amplitude, of an earthquake signal are necessary for determining event magnitudes; thus, it may be impossible to calculate earthquake magnitudes when a volcano is very active. This problem is most likely to occur at volcanoes with limited networks of short period seismometers. This study outlines two methods for calculating earthquake magnitudes when events are clipped and over-run. The first method entails modeling the shape of earthquake codas as a power law function and extrapolating duration from the decay of the function. The second method draws relations between clipped duration (i.e., the length of time a signal is clipped) and the full duration. These methods allow for magnitudes to be determined within 0.2 to 0.4 units of magnitude. This error is within the range of analyst hand-picks and is within the acceptable limits of uncertainty when quickly quantifying volcanic energy release during volcanic crises. Most importantly, these estimates can be made when data are clipped or over-run. These methods were developed with data from the initial stages of the 2004-2008 eruption at Mount St. Helens. Mount St. Helens is a well-studied volcano with many instruments placed at varying distances from the vent. This fact makes the 2004-2008 eruption a good place to calibrate and refine methodologies that can be applied to volcanoes with limited networks.

Numerical and granulometric approaches to geophysical granular flows

Volcanic ash clouds can be fed by an upward-directed eruption column (Plinian column) or by elutriation from extensive pyroclastic-flows (coignimbrite cloud). For large-scale eruptions, there is considerable uncertainty about which mechanism is dominant. Here we analyze in a novel way a comprehensive grainsize database for pyroclastic deposits. We demonstrate that the Mount Pinatubo climactic eruption deposits were substantially derived from coignimbrite clouds, and not only by a Plinian cloud as generally thought. Coignimbrite ash-fall deposits are much richer in breathable <10 m ash (5–25 wt%) than pure Plinian ash at most distances from the source volcano. We also show that coignimbrite ash clouds, as at Pinatubo, are expected to be more water rich than Plinian clouds, leading to removal of more HCl prior to stratospheric injection, thereby reducing their atmospheric impact.

Comparing satellite and ground-based observations of paroxysmal degassing events at Etna volcano, Italy

Mount Etna, Italy, is one of the most active volcanoes in the world, and is also regarded as one of the strongest volcanic sources of sulfur dioxide (SO2) emissions to the atmosphere. Since October 2004, an automated ultraviolet (UV) spectrometer network (FLAME) has provided ground-based SO2 measurements with high temporal resolution, providing an opportunity to validate satellite SO2 measurements at Etna. The Ozone Monitoring Instrument (OMI) on the NASA Aura satellite, which makes global daily measurements of trace gases in the atmosphere, was used to compare SO2 amount released by the volcano during paroxysmal lava-fountaining events from 2004 to present. We present the first comparison between SO2 emission rates and SO2 burdens obtained by the OMI transect technique and OMI Normalized Cloud-Mass (NCM) technique and the ground-based FLAME Mini-DOAS measurements. In spite of a good data set from the FLAME network, finding coincident OMI and FLAME measurements proved challenging and only one paroxysmal event provided a good validation for OMI. Another goal of this work was to assess the efficacy of the FLAME network in capturing paroxysmal SO2 emissions from Etna, given that the FLAME network is only operational during daylight hours and some paroxysms occur at night. OMI measurements are advantageous since SO2 emissions from nighttime paroxysms can often be quantified on the following day, providing improved constraints on Etna’s SO2 budget.

Recurrent voluminous sector collapses at Volcán Barú, Panama

Two volcanic debris avalanche deposits (VDADs), both attributed to sector collapse at Volcán Barú, Panama, have been identified after an investigation of deposits that covered more than a thousand square kilometers. The younger Barriles Deposit is constrained by two radiocarbon ages that are ~9 ka; the older Caisán Deposit is at or beyond the radiocarbon range, >43,500 ybp. The total runout length of the Caisán Deposit was ~50 km and it covers 1190 km2. The Barriles Deposit extended to about 45 km and covered an area of 966 km2, overlapping most of the Caisán. The VDADs are blanketed by pyroclastic deposits and contain a predominance of andesitic material likely representing volcanic dome rock which accumulated above the active vent at Barú before collapsing. Despite heavy vegetation in the field area, over 4000 individual hummocks were digitized from aerial photography. Statistical analysis of hummock locations and geometries depict flow patterns of highly- fragmented material reflecting the effects of underlying topography and also help to define the limit of Barriles’ shorter termination. Barriles and Caisán are primarily unconfined, subaerial volcanic deposits that are among the world’s most voluminous. Calculated through two different geospatial processes, thickness values from field measurements and inferences yield volumes >30 km23 for both deposits. VDADs of comparable scale come from Mount Shasta, USA; Socompa, Chile/Argentina; and Shiveluch, Russia. Currently, the modern edifice is 200-400m lower than the pre-collapse Barriles and Caisán summits and only 16-25% of the former edifice has been replaced since the last failure.