Lee Siebert’s research while affiliated with Smithsonian Institution and other places

The 1980 eruption of Mount St. Helens was instrumental in advancing understanding of how volcanoes work. Lateral edifice collapses and the generation of volcanic debris avalanches were not widely recognized prior to that eruption, making assessment of their hazards and risks challenging. The proliferation of studies since 1980 on resulting deposits and evaluation of processes leading to their generation has built on the insights from the 1980 eruption. Volcano-related destabilizing phenomena, such as strength reduction by hydrothermal alteration, deformation and structural modifications from shallow magma intrusion, and thermal pressurization of pore fluids supplement those factors also affecting nonvolcanic slopes and can lead to larger failures. Remote and ground-based monitoring techniques can aid in detecting potentially destabilizing dynamic processes and in forecasting the size and location of future large lateral collapses, although forecasting remains a topic of investigation. More than a thousand large lateral collapse events likely ≥ 0.01 km ³ in volume have now been identified from deposits or inferred from source area morphology, leading to a recognition of their importance in the evolution of volcanoes and the hazards they pose. Criteria for recognition of debris-avalanche deposits include morphological factors and textural characteristics from outcrop to microscopic scale, allowing discrimination from other volcaniclastic deposits. Lateral edifice failure impacts a broad spectrum of volcanic structures in diverse tectonic settings and can occur multiple times during the evolution of individual volcanoes. Globally, collapses ≥ 0.1 km ³ in volume have been documented 5–6 times per century since 1500 CE, with about one per century having a volume ≥ 1 km ³ . Smaller events < 0.1 km ³ are underrepresented in the earlier record but also have high hazard impact.

In the four decades since the 1980 eruption of Mount St. Helens, debris-avalanche deposits generated by gravitational lateral collapse of volcanoes have become widely recognized. Selected regionally sequenced case studies highlight the evolution of thought regarding these events prior to 1980 in contrast to subsequent research with benefit of insights from the events of May 18, 1980. These typically hummocky deposits, of volcanic materials but lying far beyond volcanoes, had puzzled geologists for more than a century and been interpreted as a wide range of primary and secondary volcanic or non-volcanic features. Contrary to general perception, however, the volcanological literature contained multiple accounts prior to 1980 that recognized the landslide origin of some of these deposits, albeit mostly in regional publications not widely known. The burst of interest in lateral-collapse events after 1980 has led to an average of one regional or global debris-avalanche inventory annually in terrestrial or submarine settings and the recognition of a thousand events from nearly 600 volcanoes. The last major volcaniclastic process to be widely recognized and understood, large-volume debris avalanches originating from lateral collapse of volcanic edifices have been found to be a relatively common occurrence across a wide spectrum of volcanic features and settings.

A new database of volcanic debris avalanche deposits (VDADs) from 594 volcanoes in 52 countries has been compiled based on published inventories and our own unpublished data. This work presents an overview of the distribution of VDADs around the world, their sizes, recurrence intervals, and a quantitative characterization of their controlling parameters. Around 50% of the catalogued deposits are located in Japan, the Americas and Russia. Multiple lateral collapses are common at a single volcanic edifice, with a dozen or more events at some volcanoes. The deposits of volcanic debris avalanches are similar to those of non-volcanic rock avalanches (RAs) in terms of shape, deposit morphology and texture. A comparative analysis of the deposit parameters volume, area, drop height and runout length, however, shows that their main difference lies in the maximum volume RAs can typically reach. This is mainly due to differences in source scar geometry. At the same drop height, VDAs can produce larger volumes because their scars are deeper-seated, more bowl-shaped, and extend along longer slopes than is common for rock slopes. Although the new database is still incomplete, this compilation shows great potential for future analyses and emphasizes the importance of strengthening such inventories with more well-studied cases for sound scientific studies and hazard assessments.

We present varve chronologies for sediments from two maar lakes in the Valle de Santiago region (Central Mexico): Hoya La Alberca (AD 1852-1973) and Hoya Rincn de Parangueo (AD 1839-1943). These are the first varve chronologies for Mexican lakes. The varved sections were anchored with tephras from Colima (1913) and Paricutin (1943/1944) and (210)Pb ages. We compare the sequences using the thickness of seasonal laminae and element counts (Al, Si, S, Cl, K, Ti, Mn, Fe, and Sr) determined by micro X-ray fluorescence spectrometry. The formation of the varve sublaminae is attributed to the strongly seasonal climate regime. Limited rainfall and high evaporation rates in winter and spring induce precipitation of carbonates (high Ca, Sr) enriched in (13)C and (18)O, whereas rainfall in summer increases organic and clastic input (plagioclase, quartz) with high counts of lithogenic elements (K, Al, Ti, and Si). Eolian input of Ti occurs also in the dry season. Moving correlations (5-yr windows) of the Ca and Ti counts show similar development in both sequences until the 1930s. Positive correlations indicate mixing of allochthonous Ti and autochthonous Ca, while negative correlations indicate their separation in sublaminae. Negative excursions in the correlations correspond with historic and reconstructed droughts, El Nio events, and positive SST anomalies. Based on our data, droughts (3-7 year duration) were severe and centred around the following years: the early 1850s, 1865, 1880, 1895, 1905, 1915 and the late 1920s with continuation into the 1930s. The latter dry period brought both lake systems into a critical state making them susceptible to further drying. Groundwater overexploitation due to the expansion of irrigation agriculture in the region after 1940 induced the transition from calcite to aragonite precipitation in Alberca and halite infiltration in Rincn. The proxy data indicate a faster response to increased evaporation for Rincn, the lake with the larger maar dimensions, solar radiation receipt and higher conductivity, whereas the smaller, steeper Alberca maar responded rapidly to increased precipitation.

Large magnitude explosive eruptions in Japan were compiled for the Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database. Here we use this dataset to investigate the under-recording of Japanese explosive eruptions. We identify under-recording of Volcanic Explosivity Index (VEI) 4–5 eruptions on two timescales. Model fitting and Akaike’s information criterion (AIC and AICc) model selection suggest that these trends can be represented with the double exponential decay model, reflecting geologic processes. The time series of the recording rate of larger eruptions (VEI 6 and 7) show a slowly decreasing trend in comparison to smaller eruptions. These time series can be represented with the single exponential decay model. The percentages of missing eruptions are estimated from the fitted models. Our results show an inverse correlation between VEI and degree of under-reporting suggesting that even larger VEI eruptions are under-recorded in the Quaternary. For example, 89 % of VEI 4 events, 65–66 % of VEI 5 events, 46–49 % of VEI 6 events and 36–39 % of VEI 7 events are missing from the record at 100 ka, 200 ka, 300 ka, and 500 ka, respectively. Comparison of frequencies of Japanese and global eruptions suggests that under-recording of the global database is 7.9–8.7 times larger than in the Japanese dataset. Therefore, under-recording of events must be taken into account in estimating recurrence rates of explosive eruptions using the geologic record.

The Large Magnitude Explosive Volcanic Eruptions (LaMEVE) database contains data on 1,883 Quaternary eruption records of magnitude (M) 4 and above and is publically accessible online via the British Geological Survey. Spatial and temporal analysis of the data indicates that the record is incomplete and is thus biased. The recorded distribution of volcanoes is variable on a global scale, with three-quarters of all volcanoes with M≥4 Quaternary activity located in the northern hemisphere and a quarter within Japan alone. The distribution of recorded eruptions does not strictly follow the spatial distribution of volcanoes and has distinct intra-regional variability, with about 40% of all recorded eruptions having occurred in Japan, reflecting in part the country’s efforts devoted to comprehensive volcanic studies. The number of eruptions in LaMEVE decreases with increasing age, exemplified by the recording of 50% of all known Quaternary eruptions during the last 20,000 years. Historical dating is prevalent from 1450 AD to the present day, substantially improving record completeness. The completeness of the record also improves as magnitude increases. This is demonstrated by the calculation of the median time, T50, for eruptions within given magnitude intervals, where 50% of eruptions are older than T50: T50 ranges from 5,070 years for M4-4.9 eruptions to 935,000 years for M≥8 eruptions. T50 follows a power law fit, suggesting a quantifiable relationship between eruption size and preservation potential of eruptive products. Several geographic regions have T50 ages of <250 years for the smallest (~M4) eruptions reflecting substantial levels of under-recording. There is evidence for latitudinal variation in eruptive activity, possibly due to the effects of glaciation. A peak in recorded activity is identified at 11 to 9 ka in high-latitude glaciated regions. This is absent in non-glaciated regions, supporting the hypothesis of increased volcanism due to ice unloading around this time. Record completeness and consequent interpretation of record limitations are important in understanding volcanism on global to local scales and must be considered during rigorous volcanic hazard and risk assessments. The study also indicates that there need to be improvements in the quality of data, including assessment of uncertainties in volume estimates.

A new database of volcanic fatalities is presented and analysed, covering the period 1600 to 2010 AD. Data are from four sources: the Smithsonian Institution, Witham (2005), CRED EM-DAT and Munich RE. The data were combined and formatted, with a weighted average fatality figure used where more than one source reports an event; the former two databases were weighted twice as strongly as the latter two. More fatal incidents are contained within our database than similar previous works; approximately 46% of the fatal incidents are listed in only one of the four sources, and fewer than 10% are in all four. 278,880 fatalities are recorded in the database, resultant from 533 fatal incidents. The fatality count is dominated by a handful of disasters, though the majority of fatal incidents have caused fewer than ten fatalities. Number and empirical probability of fatalities are broadly correlated with VEI, but are more strongly influenced by population density around volcanoes and the occurrence and extent of lahars (mudflows) and pyroclastic density currents, which have caused 50% of fatalities. Indonesia, the Philippines, and the West Indies dominate the spatial distribution of fatalities, and there is some negative correlation between regional development and number of fatalities. With the largest disasters removed, over 90% of fatalities occurred between 5 km and 30 km from volcanoes, though the most devastating eruptions impacted far beyond these distances. A new measure, the Volcano Fatality Index, is defined to explore temporal changes in societal vulnerability to volcanic hazards. The measure incorporates population growth and recording improvements with the fatality data, and shows prima facie evidence that vulnerability to volcanic hazards has fallen during the last two centuries. Results and interpretations are limited in scope by the underlying fatalities data, which are affected by under-recording, uncertainty, and bias. Attempts have been made to estimate the extent of these issues, and to remove their effects where possible. The data analysed here are provided as supplementary material. An updated version of the Smithsonian fatality database fully integrated with this database will be publicly available in the near future and subsequently incorporate new data.

Geologic mapping at the base of Volcán Barú, Panama, characterizes two large andesitic volcanic debris avalanche deposits attributed to sector collapse at Volcán Barú. The older Caisán debris avalanche deposit is at or beyond the radiocarbon dating range, >43,500 yr B.P., whereas the younger Barriles debris avalanche deposit is constrained by two radiocarbon ages that are ca. 9000 yr B.P. The total runout length of the Caisán deposit was ~50 km, covering nearly 1200 km2. The Barriles deposit extended to ~45 km and covered >990 km2, overlapping most of the Caisán. Over 4000 hummocks from these deposits were digitized, and statistical analysis of hummock location and geometry depicts fl ow patterns of highly fragmented material affected by underlying topography and also helps to defi ne the shorter runout limit of the Barriles deposit. The Barriles and Caisán deposits are primarily unconfi ned deposits that are among the world’s most voluminous subaerial debris avalanche deposits. Two different geospatial procedures, utilizing deposit thicknesses and edifi ce reconstruction, yield calculated volumes ~30 km3 and larger for both deposits. Subaerial deposits of comparable scale include those from Mount Shasta, Socompa, and Shiveluch. Currently, the modern edifi ce is 200–400 m lower than the estimated precollapse Barriles and Caisán summits, and only 16%–25% of the former edifi ce has been replaced since the last failure. The ~10 km3 postcollapse lava-dome complex, however, implies a Holocene magma production rate of 1.1 km3/k.y., comparable to elevated eruptive pulses documented at other stratovolcanoes, underscoring the importance of hazards assessment and monitoring of this active volcano.

To facilitate the assessment of hazards and risk from volcanoes, we have created a comprehensive global database of Quaternary Large Magnitude Explosive Volcanic Eruptions (LaMEVE). This forms part of the larger Volcanic Global Risk Identification and Analysis Project (VOGRIPA), and also forms part of the Global Volcano Model (GVM) initiative (http://www.globalvolcanomodel.org). A flexible search tool allows users to select data on a global, regional or local scale; the selected data can be downloaded into a spreadsheet. The database is publically available online at http://www.bgs.ac.uk/vogripa and currently contains information on nearly 3,000 volcanoes and over 1,800 Quaternary eruption records. Not all volcanoes currently have eruptions associated with them but have been included to allow for easy expansion of the database as more data are found. Data fields include: magnitude, Volcanic Explosivity Index (VEI), deposit volumes, eruption dates, and rock type. The scientific community is invited to contribute new data and also alert the database manager to potentially incorrect data. Whilst the database currently focuses only on large magnitude eruptions, it will be expanded to include data specifically relating to the principal volcanic hazards (e.g. pyroclastic flows, tephra fall, lahars, debris avalanches, ballistics), as well as vulnerability (e.g. population figures, building type) to facilitate risk assessments of future eruptions.