Charles B. Connor

Three nonhomogeneous Poisson models for the probability of basaltic volcanism: Application to the Yucca Mountain region, Nevada

The distribution and timing of areal basaltic volcanism are modeled using three nonhomogeneous methods: spatio-temporal nearest neighbor, kernel, and nearest-neighbor kernel. These models give nonparametric estimates of spatial or spatio-temporal recurrence rate based on the positions and ages of cinder cones and related vent structures and can account for migration and shifts in locus, volcano clustering, and development of regional vent alignments. The three methods are advantageous because (1) recurrence rate and probability maps can be made, facilitating comparison with other geological information; (2) the need to define areas or zones of volcanic activity, required in homogeneous approaches, is eliminated; and (3) the impact of uncertainty in the timing and distribution of individual events is particularly easy to assess. The models are applied to the Yucca Mountain region (YMR), Nevada, the site of a proposed high-level radioactive waste repository. Application of the Hopkins F test, Clark-Evans test, and K function indicates volcanoes cluster in the YMR at the >95% confidence level. Weighted-centroid cluster analysis indicates that Plio-Quaternary volcanoes are distributed in four clusters: three of these clusters include cinder cones formed <1 Ma. Probability of disruption within the 8 km2 area of the proposed repository by formation of a new basaltic vent is calculated to be between 1 × 10−4 and 5 × 10−4 in 104 years (the kernel and nearest-neighbor kernel methods give a maximum probability of 5 × 10−4 in 104 years), assuming regional recurrence rates of 5–10 volcanoes/m.y. An additional finding, illustrating the strength of nonhomogeneous methods, is that maps of the probability of volcanic eruption for the YMR indicate the proposed repository lies on a steep probability gradient: volcanism recurrence rate varies by more than 2 orders of magnitude within 20 km. Insight into this spatial scale of probability variation is a distinct benefit of application of these methods to hazard analysis in areal volcanic fields.

Geologic factors controlling patterns of small‐volume basaltic volcanism: Application to a volcanic hazards assessment at Yucca Mountain, Nevada

The proposed high-level radioactive waste repository at Yucca Mountain, Nevada, is located within an active volcanic field. Probabilistic volcanic hazard models for future eruptions through the proposed repository depend heavily on our understanding of the spatial controls on volcano distribution at a variety of scales. On regional scales, Pliocene-Quaternary volcano clusters are located east of the Bare Mountain fault. Extension has resulted in large-scale crustal density contrast across the fault, and vents are restricted to low-density areas of the hanging wall. Finite element modeling indicates that this crustal density contrast can result in transient pressure changes of up to 7 MPa at 40 km depth, providing a mechanism to generate partial melts in areas where mantle rocks are already close to their solidus. On subregional scales, vent alignments, including one alignment newly recognized by ground magnetic mapping, parallel the trends of high-dilation tendency faults in the Yucca Mountain region (YMR). Forty percent of vents in the YMR are part of vent alignments that vary in length from 2 to 16 km. Locally, new geological and geophysical data show that individual vents and short vent alignments occur along and adjacent to faults, particularly at fault intersections, and left-stepping en echelon fault segments adjacent to Yucca Mountain. Conditions which formed these structures persist in the YMR today, indicating that volcanism will likely continue in the region and that the proposed repository site is within an area where future volcanism may occur. On the basis of these data the probability of volcanic disruptions of the proposed repository is estimated between 10−8/yr and 10−7/yr.

1995 ERUPTIONS OF CERRO NEGRO VOLCANO, NICARAGUA, AND RISK ASSESSMENT FOR FUTURE ERUPTIONS

Cerro Negro volcano, Nicaragua, continued a 147-yr-long duration of cinder-cone activity with a major eruption in 1995. Small, phreatically driven eruptions began in May 1995 and continued for 79 days. Following a 95 day repose, the main eruption produced 8 × 106 m3 of basalt from Cerro Negro over 13 days of activity and deposited 5 mm of ash in the city of Leon. Although the damage from the 1995 eruptions was fortunately minor, previous tephra falls from Cerro Negro have produced significant crop damage and multiple deaths through building collapse. In spite of its apparent longevity for a historically active cinder cone, Cerro Negro has mass-flow rates typical of arc-related basaltic cinder cone volcanoes. Volcanic hazards beyond 3 km from Cerro Negro consist of tephra falls. Few models are available to calculate tephra-fall risks from basaltic volcanoes such as Cerro Negro, and none have been applied to dispersive cinder cone eruptions. A convective-dispersive model of Suzuki is modified and evaluated using detailed data from the 1995 Cerro Negro eruption and is found to reasonably calculate tephra-fall thickness between 8 and 30 km from the vent. This model is used with detailed data from previous Cerro Negro eruptions in a tephra-fall hazard assessment. Cerro Negro also appears to have had a steady-state eruption rate since about A.D. 1900, which is used to estimate the timing of the next eruption as before A.D. 2006. The potential tephra fall from Cerro Negro in Leon, Nicaragua, is calculated as 2.2 mm/yr until 2006, with 95% confidence that deposits will be <11 cm thick.

Recurrence rates of volcanism in basaltic volcanic fields: An example from the Springerville volcanic field, Arizona

A spatio-temporal near-neighbor model is used to identify and map variations in the recurrence rate of volcanism in the Springerville volcanic field, Arizona, a large field on the Colorado Plateau boundary. Detailed mapping of individual lava flows and their associated vents, together with radiometric and paleomagnetic dating, demonstrates that 366 volcanic events have formed the Springerville volcanic field. These volcanic events consist of mapped units that erupted between 2.1 and 0.3 Ma over an area of 3000 km 2 . Cumulatively, the rate of vent formation in the Springerville field waxed prior to 1.5 Ma, was near steady state from 1.5 to 0.75 Ma, and has waned since 0.75 Ma. The increase in the rate of vent formation at ca. 1.5 Ma coincided with a shift in the locus of Springerville magmatism from west to east and an increase in the alkalic nature of the magma, including eruption of mugearites and benmoreites. The volume of flows, inferred from lava-flow areas, was steady state from 1.75 to 0.75 Ma. A near-neighbor spatio-temporal recurrence-rate model using seven near-neighbor volcanoes and a 0.5 m.y. time window reveals that (1) areas of waxing and waning magmatism in the Springerville volcanic field are much more localized and (2) volcanic activity within these areas is much more intense than implied by field-wide temporal trends. These volcano clusters are 10–20 km in diameter; they were commonly active for less than 0.25 m.y. Mugearites and benmoreites are limited to these areas of high recurrence rate. This clustered and petrologically distinctive, rather than distributed or random, volcanic activity suggests that individual source regions for the magma also are localized and short-lived compared with the area and longevity of the entire field. Because volcanic activity is spatially and temporally clustered, forecasting subsequent activity is more successful if the spatio-temporal recurrence-rate model is used, rather than the average recurrence rates. This success indicates that spatio-temporal recurrence-rate models are useful tools for the quantification of long-term volcanic hazards in basaltic volcanic fields.

Exploring Links Between Physical and Probabilistic Models of Volcanic Eruptions: The Soufriere Hills Volcano, Montserrat

(1) Probabilistic methods play an increasingly important role in volcanic hazards forecasts. Here we show that a probability distribution characterized by competing processes provides an excellent statistical fit (>99% confidence) to repose intervals between 75 vulcanian explosions of Soufriere Hills Volcano, Montserrat in September-October, 1997. The excellent fit is explained by a physical model in which there are competing processes operating in the upper volcano conduit on different time scales: pressurization due to rheological stiffening and gas exsolution, and depressurization due to development of permeability and gas escape. Our experience with the Soufriere Hills Volcano eruption sequence suggests that volcanic eruption forecasts are improved by accounting for these different conduit processes explicitly in a single probability model. INDEX TERMS: 8419 Volcanology: Eruption monitoring (7280); 8414 Volcanology: Eruption mechanisms; 8499 Volcanology: General or miscellaneous. Citation: Connor, C. B., R. S. J. Sparks, R. M. Mason, C. Bonadonna, and S. R. Young, Exploring links between physical and probabilistic models of volcanic eruptions: The Soufriere Hills Volcano, Montserrat, Geophys. Res. Lett., 30(13), 1701, doi:10.1029/2003GL017384, 2003.

Evidence of Regional Structural Controls on Vent Distribution: Springerville Volcanic Field, Arizona

Quantitative analysis of the geographic distribution of vents and comparison with regional structural, petrologic, and vent age data provide insight into the processes governing the emplacement of vents in the Springerville volcanic field, Arizona. A total of 409 vents in the Springerville volcanic field (SVF) have a mean distance to nearest neighbor vents of 955 m, a much closer spacing than is common in some platform-type volcanic fields. Based upon a cluster analysis search radius parameter of 4500 m, these vents comprise seven geographic clusters, with only five outlying vents occurring in the entire field. Cinder cone clusters in the western portion of the field are significantly older than clusters in the eastern portion of the field (p value of <0.001), and there is a tendency for cluster age to decrease to the east. This is particularly evident when mean cluster ages are calculated for tholeiite, alkaline olivine basalt, and evolved alkaline rock types independently. Application of the two-point azimuth and Hough transform methods demonstrates that regional cinder cone alignments transect these clusters. The most prominent of these alignments trend ENE in the eastern portion of the field and WNW in the western portion of the field, creating an overall arcuate pattern that is subparallel to the trend of the Mogollon Rim and the Colorado Plateau/Transition Zone boundary. These observations suggest that vents (and clusters) migrated from west to east in response to plate motion, but the general pattern of vent migration was complicated by regional structures, which enhanced the volume and duration of magmatism in some areas. The fractures or faults implied by vent alignments indicate that Shmin is oriented radial to the Colorado Plateau in the SVF. Preferred vent alignment orientations may be related to extension resulting from plateau uplift, and to a much smaller degree from a minor Basin and Range imprint. While regional in extent, the implied structures appear to differ significantly from some of those in several other plateau-marginal fields in that they cannot be related to major reactivated Precambrian structures. Our vent alignment data differ from those seen by other workers in the Zuni-Bandera and Mount Taylor fields, suggesting the stress field for the SVF is different from other fields in the proposed Jemez lineament. The stress field implied by vent alignment data, combined with structural data, suggests that the southwestern tectonic boundary of the Colorado Plateau of Brumbaugh (1987) should be extended southeastward to include the SVF at the plateaus southern boundary.

Inversion Is the Key to Dispersion: Understanding Eruption Dynamics by Inverting Tephra Fallout

Volcanologists increasingly rely on numerical simulations to better understand the dynamics of erupting volcanoes. Mathematical models are often used to explain the geologic processes responsible for eruption deposits found in the geologic record, and to better characterize possible hazards from future volcanic activity. Examples of models include the finite element flow and transport codes used to simulate pyroclastic flows, lahars, and volcanic debris avalanches (Iverson, 1997; Patra et al., 2005), analytical solutions or finite difference approximations to the advection-diffusion equation that are used to model tephra dispersion (see Bonadonna, this volume) and gas emissions from quiescent volcanoes (Delmelle et al., 2001), and cellular automata algorithms that model the advance of lava (Barca et al., 1994). Commonality among these examples involves the fact that we wish to estimate parameters related to the dynamics of volcanic activity directly from field observations. For instance, how well can we estimate the magnitude of an eruption from measurements of tephra deposits?

Recurrence rates of basaltic volcanism in SP cluster, San Francisco Volcanic Field, Arizona

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A 7000 yr perspective on volcanic ash clouds affecting northern Europe

The ash cloud resulting from the A.D. 2010 eruption of Eyjafjallajokull in Iceland caused severe disruption to air travel across Europe, but as a geological event it is not unprecedented. Analysis of peats and lake sediments from northern Europe has revealed the presence of microscopic layers of Icelandic volcanic ash (tephra). These sedimentary records, together with historical records of Holocene ash falls, demonstrate that Icelandic volcanoes have generated substantial ash clouds that reached northern Europe many times. Here we present the first comprehensive compilation of sedimentary and historical records of ash-fall events in northern Europe, spanning the past 7000 yr. Ash-fall events appear to have been more frequent in the past 1500 yr. It is unclear whether this reflects a true increase in eruption frequency or dispersal, or is an artifact of the records or the way in which they have been generated. In the past 1000 yr, volcanic ash clouds reached northern Europe with a mean return interval of 56 ± 9 yr (the range of return intervals is between 6 and 115 yr). Probabilistic modeling using the ash records for the last millennium indicates that for any 10 yr period there is a 16% probability of a tephra fallout event in northern Europe. These values must be considered as conservative estimates due to the nature of tephra capture and preservation in the sedimentary record.

Timing of basaltic volcanism along the Mesa Butte Fault in the San Francisco Volcanic Field, Arizona, from 40Ar/39Ar dates: Implications for longevity of cinder cone alignments

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