Long Valley Caldera Research Articles

The Economic Effects of Volcanic Alerts-A Case Study of High-Threat U.S. Volcanoes.

A common concern about volcanic unrest is that the communication of information about increasing volcanic alert levels (VALs) to the public could cause serious social and economic impacts even if an eruption does not occur. To test this statement, this study examined housing prices and business patterns from 1974-2016 in volcanic regions with "very-high" threat designations from the U.S. Geological Survey (USGS)-Long Valley Caldera (LVC), CA (caldera); Mount St. Helens (MSH), Washington (stratovolcano); and Kīlauea, Hawai'i (shield volcano). To compare economic trends in nonvolcanic regions that are economically dependent on tourism, Steamboat Springs, CO, served as a control as it is a ski-tourism community much like Mammoth Lakes in LVC. Autoregressive distributed lag (ARDL) models predicted that housing prices were negatively affected by VALs at LVC from 1982-1983 and 1991-1997. While VALs associated with unrest and eruptions included in this study both had short-term indirect effects on housing prices and business indicators (e.g., number of establishments, employment, and salary), these notifications were not strong predictors of long-term economic trends. Our findings suggest that these indirect effects result from both eruptions with higher level VALs and from unrest involving lower-level VAL notifications that communicate a change in volcanic activity but do not indicate that an eruption is imminent or underway. This provides evidence concerning a systemic issue in disaster resilience. While disaster relief is provided by the U.S. federal government for direct impacts associated with disaster events that result in presidential major disaster declarations, there is limited or no assistance for indirect effects to businesses and homeowners that may follow volcanic unrest with no resulting direct physical losses. The fact that periods of volcanic unrest preceding eruption are often protracted in comparison to precursory periods for other hazardous events (e.g., earthquakes, hurricanes, flooding) makes the issue of indirect effects particularly important in regions susceptible to volcanic activity.

A Novel Multidisciplinary Approach for the Thermo‐Rheological Study of Volcanic Areas: The Case Study of Long Valley Caldera

AbstractWe propose a novel multidisciplinary approach to image the thermo‐rheological stratification beneath active volcanic areas, such as Long Valley Caldera (LVC). We performed a thermal fluid dynamic model via optimization procedure of the thermal conditions of the crust. We characterize the topology of the hot magmatic bodies and the hot fluid circulation (the permeable fault‐zones), using both a novel imaging of the a and b parameters of the Gutenberg‐Richter law and an innovative procedure analysis of P‐wave tomographic models. The optimization procedure provides the permeability of a reservoir (5.0 × 10−14 m2) and of the fault‐zone (5.0 · 10−14 – 1.0 × 10−13 m2), as well as the temperature of the magma body (750–800°C). The imaging of the rheological properties of the crust indicates that the brittle/ductile transition occurs about 5 km b.s.l. depth, beneath the resurgent dome. There are again deeper brittle conditions about 15 km b.s.l., agreeing with the previous observations. The comparison between the conductive and the conductive‐convective heat transfer models highlights that the deeper fluid circulation efficiently cools the volumes above the magmatic body, transferring the heat to the shallow geothermal system. This process has a significant impact on the rheological properties of the upper crust as the migration of the B/D transition. Our findings show an active magmatic system (6–10 km deep) and confirm that LVC is a long‐life silicic caldera system.

Drought‐Triggered Magmatic Inflation, Crustal Strain, and Seismicity Near the Long Valley Caldera, Central Walker Lane

AbstractWe use GPS data to show synchronization between the 2011 and 2016 drought cycle in California, accelerated uplift of the Sierra Nevada Mountains, and enhanced magmatic inflation of the Long Valley Caldera (LVC) magmatic system. The drought period coincided with faster uplift rate, changes in gravity seen in the Gravity Recovery and Climate Experiment (GRACE), and changes in standardized relative climate dryness index. These observations together suggest that the Sierra Nevada elevation is sensitive to changes in hydrological loading conditions, which subsequently influences the LVC magmatic system. We use robust imaging of horizontal GPS velocities to derive time‐variable shear and dilatational strain rates in a region with highly variable station distribution. The results show that the highest strain rates are near the eastern margin of the Sierra Nevada and western edge of the Central Walker Lane (CWL) passing directly through LVC. The drought period saw geographic shifts in the distribution in active shear strain in the CWL more than 60 km from the LVC, delineating the minimum extent over which the active magmatic system affects the CWL tectonic environment. We analyze declustered seismicity data to show that locations with higher seismicity rates tend to be (1) areas with higher strain rates and (2) areas in which strain rates increased during drought‐enhanced inflation. We hypothesize that drought conditions reduce vertical surface mass loading, which decreases pressure at depth in the LVC system, in turn enhances magmatic inflation, and drives horizontal elastic stress changes that redistribute active CWL strain and modulate seismicity.

Seasonal seismicity at western United States volcanic centers

We examine 20-yr data sets of seismic activity from 10 volcanic areas in the western United States for annual periodic signals (seasonality), focusing on large calderas (Long Valley caldera and Yellowstone) and stratovolcanoes (Cascade Range). We apply several statistical methods to test for seasonality in the seismic catalogs. In 4 of the 10 regions, statistically significant seasonal modulation of seismicity (N90% probability) occurs, such that there is an increase in the monthly seismicity during a given portion of the year. In five regions, seasonal seismicity is significant in the upper 3 km of the crust. Peak seismicity occurs in the summer and autumn in Mt. St. Helens, Hebgen Lake/Madison Valley, Yellowstone Lake, and Mammoth Mountain. In the eastern south moat of Long Valley caldera (LVC) peak seismicity occurs in the winter and spring. We quantify the possible external forcing mechanisms that could modulate seasonal seismicity. Both snow unloading and groundwater recharge can generate large stress changes of N5 kPa at seismogenic depths and may thus contribute to seasonality. D 2005 Elsevier B.V. All rights reserved.

Near‐source velocity structure and isotropic moment tensors: A case study of the Long Valley Caldera

The effect of near‐source velocity structure on the recovery of the isotropic component in moment tensor inversions is explored using a finite‐difference method. Synthetic data generated using a 3D Long Valley Caldera (LVC) velocity model (Vp +/− 20%) were inverted for the full moment tensor using a linear time‐domain scheme utilizing Green's functions calculated from 1D models. While inversions of synthetic data with input isotropic components recovered isotropic components with 95% significance according to an F‐test relative to deviatoric inversions (isotropic component constrained to zero), inversions of synthetic data with no input isotropic component recovered only nominal isotropic components with less than 75% significance. This study demonstrates near‐source structure does not appear to falsely produce significant isotropic components of moment tensor inversions in the passband typically employed by regional inversion methods.

On fluid flow and mineral alteration in fractured caprock of magmatic hydrothermal systems

Geochemical evolution in hydrothermal fractured rock systems occurs through a complex interplay of multiphase fluid and heat flow and chemical transport processes.Building on previous work, we present here simulations of reactive hydrothermal flow that include (1) detailed fracture‐matrix interaction for fluid, heat, and chemical constituents, (2) gas‐phase participation in multiphase fluid flow and geochemical reactions, (3) the kinetics of fluid‐rock chemical interaction, and (4) heat effects on thermophysical and chemical properties and processes. The present study uses, as an example, water and gas chemistry data as well as caprock mineral composition from the hydrothermal system in Long Valley Caldera (LVC), California. The flow system studied is intended to capture realistic features of fractured magmatic hydrothermal systems. The purpose of this “numerical experiment” is to gain useful insight into process mechanisms such as fracture‐matrix interaction, liquid‐gas phase partitioning, and conditions and parameters controlling water‐gas‐rock interactions in a hydrothermal setting. Simulation results indicate that almost all CO2 is transported through the fracture. Cooling and condensation results in an elevated CO2 partial pressure. The CO2 is the dominant gas‐phase constituent close to the land surface. Close to the heat source, dissolution dominates over precipitation. Away from the heat source precipitation dominates because chemical constituents, transported from the bottom, precipitate in a lower‐temperature environment. Mixing with cold meteoric water enhances mineral dissolution and precipitation effects. The rock alteration pattern is sensitive to reaction kinetics. The predicted alteration of primary rock minerals and the development of secondary mineral assemblages are generally consistent with field observations in the LVC. The observed sequence of argillic alteration in the LVC consists of an upper zone with smectite and kaolinite (in the lower‐temperature region), a lower illite zone, and an intermediate mixed illite and smectite zone. The sequence is reasonably well reproduced in the numerical simulation. In addition, calcite and chlorite precipitation in the hot region coincides with the observations. Using the reactive geochemical transport model, we have successfully simulated spatial distribution of the three argillic alteration zones.

The helium and carbon isotope systematics of a continental geothermal system: results from monitoring studies at Long Valley caldera (California, U.S.A.)

This study reports fumarole and hot spring gas chemistry of a 3-year monitoring programme (1986–1988) at the seismically-active Long Valley caldera (LVC) in the Sierra Nevada, eastern California. The focus is on helium and carbon dioxide (isotopes and concentrations) and their variation in response to seismic activity in the region. Within the caldera, both species are predominantly magmatic in origin but their isotopic and elemental characteristics appear to be established prior to shallow-level intrusion and/or are influenced by pre-eruptive degassing. In response to intra-caldera and regional seismicity over the monitoring period, CO 2 and helium show markedly different behaviour: the greatest change in various carbon-related parameters (CO 2%; δ 13C(CO 2; CO 2 3 He ) occurred in 1986 and were most likely related to regional seismicity in the nearby Chalfont Valley. Helium did not respond to these events. The largest change (up to ∼ 25%) in 3 He 4 He ratios was seen in 1987 with the occurrence of both increases and decreases relative to the almost constant values observed in 1988. The increases are consistent with magma intrusion occurring within the caldera in 1987 whilst the decreases occurred significantly later (> 6 months) than any seismic activity. It is suggested that decreases in 3 He 4 He are related to the regional seismicity and that the hydrothermal system exerts a (temporal) control on the release of near-surface 4He. At LVC this is related to the timing of the late spring thaw. Results from previous monitoring programmes (when the level of seismicity in the caldera was higher) are evaluated against variations in the present work. There appears to be a convincing link between higher 3 He 4 He values and the level of seismicity in the caldera although factors related to location, magnitude and frequency of seismic events are difficult to quantify. Because of the inferred small isotopic contrast between new magma and presently-degassing magma at LVC, it is anticipated that large variations in 3 He 4 He within the central caldera are unlikely to occur until the magmatic volatile signal wanes as a function of degassing and time. Alternatively, only at those localities situated at significant distances from the region of magma intrusion (i.e. away from the resurgent dome vicinity) are helium and/or carbon likely to respond dramatically to intra-caldera seismicity.