Dynamics and kinematics of eruptive activity at Fuego volcano, Guatemala 2005-2009

Volcanoes are the surficial expressions of complex pathways that vent magma and gasses generated deep in the Earth. Geophysical data record at least the partial history of magma and gas movement in the conduit and venting to the atmosphere. This work focuses on developing a more comprehensive understanding of explosive degassing at Fuego volcano, Guatemala through observations and analysis of geophysical data collected in 2005 – 2009. A pattern of eruptive activity was observed during 2005 – 2007 and quantified with seismic and infrasound, satellite thermal and gas measurements, and lava flow lengths. Eruptive styles are related to variable magma flux and accumulation of gas. Explosive degassing was recorded on broadband seismic and infrasound sensors in 2008 and 2009. Explosion energy partitioning between the ground and the atmosphere shows an increase in acoustic energy from 2008 to 2009, indicating a shift toward increased gas pressure in the conduit. Very-long-period (VLP) seismic signals are associated with the strongest explosions recorded in 2009 and waveform modeling in the 10 – 30 s band produces a best-fit source location 300 m west and 300 m below the summit crater. The calculated moment tensor indicates a volumetric source, which is modeled as a dike feeding a SW-dipping (35°) sill. The sill is the dominant component and its projection to the surface nearly intersects the summit crater. The deformation history of the sill is interpreted as: 1) an initial inflation due to pressurization, followed by 2) a rapid deflation as overpressure is explosively release, and finally 3) a reinflation as fresh magma flows into the sill and degasses. Tilt signals are derived from the horizontal components of the seismometer and show repetitive inflation deflation cycles with a 20 minute period coincident with strong explosions. These cycles represent the pressurization of the shallow conduit and explosive venting of overpressure that develops beneath a partially crystallized plug of magma. The energy released during the strong explosions has allowed for imaging of Fuego’s shallow conduit, which appears to have migrated west of the summit crater. In summary, Fuego is becoming more gas charged and its summit centered vent is shifting to the west - serious hazard consequences are likely.

Evaluating stress patterns for Plio-Quaternary brittle deformation along the Calama-Olacapato-El Toro fault zone, Central Andes

Understanding the geometry and kinematics of the major structures of an orogen is important to elucidate its style of deformation, as well as its tectonic evolution. We describe the temporal and spatial changes in the state of stress of the trans-orogen area of the Calama-Olacapato-El Toro (COT) Fault Zone in the Central Andes, at about 24°S within the northern portion of the Puna Plateau between the Argentina-Chile border. The importance of the COT derives principally from the Quaternary-Holocene activity recognized on some segments, which may shed new light on its possible control on Quaternary volcanism and on the seismic hazard evaluation of the area. Field geological surveys along with kinematic analysis and numerical inversion of ∼140 new fault-slip measurements have revealed that this portion of the COT zone, previously considered a continuous, long-lived lineament, in reality has been subjected to three different kinematic regimes: 1) a Miocene transpressional phase with the maximum principal stress (σ1) chiefly trending NNE-SSW; 2) an extensional phase that started by 9 Ma, with a horizontal NW-SE-striking minimum principal stress (σ3) – permutations between σ2 and σ3 axes have been recognized at two sites – and 3) a left-lateral strike-slip phase with a horizontal ∼E-W &sigma1 and ∼N-S σ3 dating to the Late Pliocene-Quaternary. Spatially, in the Quaternary, the left-lateral component decreases toward the westernmost tip of the COT, where it transitions to extension; this produced to a N-S horst and graben structure. Hence, even if transcurrence is still active in the eastern portion of the COT, as focal mechanisms of crustal earthquakes indicate, our study demonstrates that extension is becoming the predominant structural style of deformation, at least in the western region. These major temporal and spatial changes in the tectonic regimes are attributed in part to changes in the magnitude of the boundary forces due to subduction processes. The overall orogen-perpendicular extension might be the result of vertical stress larger than both the horizontal stresses induced by gravitational effect of a thickened crust.

EVALUATION OF THE EVOLVING STRESS FIELD OF THE YELLOWSTONE VOLCANIC PLATEAU FROM 1988 TO 2010 FROM EARTHQUAKE FIRST MOTIONS INVERSION

Within the Yellowstone National Park, Wyoming, the silicic Yellowstone volcanic field is one of the most active volcanic systems all over the world. Although the last rhyolite eruption occurred around 70,000 years ago, Yellowstone is still believed to be volcanically active, due to high hydrothermal and seismic activity. The earthquake data used in this study cover the period of time between 1988 and 2010. Earthquake relocations and a set of 369 well-constrained, double-couple, focal mechanism solutions were computed. Events were grouped according to location and time to investigate trends in faulting. The majority of the events has oblique, normal-faulting solutions. The overall direction of extension throughout the 0.64 Ma Yellowstone caldera looks nearly ENE, consistently with the direction of alignments of volcanic vents within the caldera, but detailed study revealed spatial and temporal variations. Stress-field solutions for different areas and time periods were calculated from earthquake focal mechanism inversion. A well-resolved rotation of σ3 was found, from NNE-SSW near the Hebgen Lake fault zone, to ENE-WSW near Norris Junction. In particular, the σ3 direction changed throughout the years in the Norris Junction area, from being ENE-WSW, as calculated in the study by Waite and Smith (2004), to NNE-SSW, while the other σ3 directions are mostly unchanged over time. The Yellowstone caldera was subject to periods of net uplift and subsidence over the past century, explained in previous studies as caused by expanding or contracting sills, at different depths. Based on the models used to explain these deformation periods, we investigated the relationship between variability in aseismic deformation and seismic activity and faulting styles. Focal mechanisms and P and T axes were divided into temporal and depth intervals, in order to identify spatial or temporal trends in deformation. The presence of “chocolate tablet” structures, with composite dilational faults, was identified in many stages of the deformation history both in the Norris Geyser Basin area and inside the caldera. Strike-slip component movement was found in a depth interval below a contracting sill, indicating the movement of magma towards the caldera.