Halldor Geirsson

Intrusion triggering of the 2010 Eyjafjallajökull explosive eruption

Gradual inflation of magma chambers often precedes eruptions at highly active volcanoes. During such eruptions, rapid deflation occurs as magma flows out and pressure is reduced. Less is known about the deformation style at moderately active volcanoes, such as Eyjafjallajökull, Iceland, where an explosive summit eruption of trachyandesite beginning on 14 April 2010 caused exceptional disruption to air traffic, closing airspace over much of Europe for days. This eruption was preceded by an effusive flank eruption of basalt from 20 March to 12 April 2010. The 2010 eruptions are the culmination of 18u2009years of intermittent volcanic unrest. Here we show that deformation associated with the eruptions was unusual because it did not relate to pressure changes within a single magma chamber. Deformation was rapid before the first eruption (>5u2009mm per day after 4 March), but negligible during it. Lack of distinct co-eruptive deflation indicates that the net volume of magma drained from shallow depth during this eruption was small; rather, magma flowed from considerable depth. Before the eruption, a ∼0.05u2009km3 magmatic intrusion grew over a period of three months, in a temporally and spatially complex manner, as revealed by GPS (Global Positioning System) geodetic measurements and interferometric analysis of satellite radar images. The second eruption occurred within the ice-capped caldera of the volcano, with explosivity amplified by magma–ice interaction. Gradual contraction of a source, distinct from the pre-eruptive inflation sources, is evident from geodetic data. Eyjafjallajökull’s behaviour can be attributed to its off-rift setting with a ‘cold’ subsurface structure and limited magma at shallow depth, as may be typical for moderately active volcanoes. Clear signs of volcanic unrest signals over years to weeks may indicate reawakening of such volcanoes, whereas immediate short-term eruption precursors may be subtle and difficult to detect.

Volcano deformation at active plate boundaries: Deep magma accumulation at Hekla volcano and plate boundary deformation in south Iceland

Volcano deformation at active plate boundaries: Deep magma accumulation at Hekla volcano and plate boundary deformation in south Iceland

Kinematics of the western Caribbean: Collision of the Cocos Ridge and upper plate deformation

Subduction of the Cocos plate and collision of the Cocos Ridge have profound effects on the kinematics of the western Caribbean, including crustal shortening, segmentation of the overriding plate, and tectonic escape of the Central American fore arc (CAFA). Tectonic models of the Panama Region (PR) have ranged from a rigid block to a deforming plate boundary zone. Recent expansion of GPS networks in Panama, Costa Rica, and Colombia makes it possible to constrain the kinematics of the PR. We present an improved kinematic block model for the western Caribbean, using this improved GPS network to test a suite of tectonic models describing the kinematics of this region. The best fit model predicts an Euler vector for the counterclockwise rotation of the CAFA relative to the Caribbean plate at 89.10°W, 7.74°N, 1.193° Ma−1, which is expressed as northwest-directed relative block rates of 11.3u2009±u20091.0–16.5u2009±u20091.1 mm a−1 from northern Costa Rica to Guatemala. This model also predicts high coupling along the Nicoya and Osa segments of the Middle American subduction zone. Our models demonstrate that the PR acts as a single tectonic block, the Panama block, with a predicted Euler vector of 107.65°W, 26.50°N, 0.133° Ma−1. This rotation manifests as northeast migration of the Panama block at rates of 6.9u2009±u20094.0–7.8u2009±u20094.8 mm a−1 from southern Costa Rica to eastern Panama. We interpret this motion as tectonic escape from Cocos Ridge collision, redirected by collision with the North Andes block, which migrates to the northwest at 12.2u2009±u20091.2 mm a−1.

Tectonic inversion in the Caribbean‐South American plate boundary: GPS geodesy, seismology, and tectonics of the Mw 6.7 22 April 1997 Tobago earthquake

On 22 April 1997 the largest earthquake recorded in the Trinidad-Tobago segment of the Caribbean-South American plate boundary zone (Mw 6.7) ruptured a shallow (~9u2009km), ENE striking (~250° azimuth), shallowly dipping (~28°) dextral-normal fault ~10u2009km south of Tobago. In this study, we describe this earthquake and related foreshock and aftershock seismicity, derive coseismic offsets using GPS data, and model the fault plane and magnitude of slip for this earthquake. Coseismic slip estimated at our episodic GPS sites indicates movement of Tobago 135u2009±u20096 to 68u2009±u20096u2009mm NNE and subsidence of 7u2009±u20099 to 0u2009mm. This earthquake was anomalous and is of interest because (1) its large component of normal slip and ENE strike are unexpected given the active E-W dextral shearing across the Caribbean-South American plate boundary zone, (2) it ruptured a normal fault plane with a low (~28°) dip angle, and (3) it reactivated and inverted the preexisting Tobago terrrane-South America ocean-continent (thrust) boundary that formed during early Tertiary oblique plate convergence.

Magma Movements in Volcanic Plumbing Systems and their Associated Ground Deformation and Seismic Patterns

Abstract Improving our understanding of volcanic hazards requires better knowledge of the location, volume and properties of magma bodies in the roots of active volcanoes, as well as information on melt supply and magma transfer. This requires a good understanding of both the geometric structure of the volcanic and igneous plumbing system, as well as observations of sub-surface magma movements and their interpretation. Arrival of new magma in volcano roots often causes volcanic unrest expressed by one or more of the following: increased seismicity, ground deformation, volcanic gas release and ground temperature changes. Recent eruptions and magmatic events in Iceland have provided opportunities to apply repeated geodetic observations at volcanoes to measure ground deformation and interpret these measurements together with seismic observations in terms of subsurface magmatic processes.

Single-Station Seismo-Acoustic Monitoring of Nyiragongo's Lava Lake Activity (D.R. Congo)

Since its last effusive eruption in 2002, Nyiragongo has been an open-vent volcano characterized by the worlds largest persistent lava lake. This lava lake provides a unique opportunity to detect pressure change in the magmatic system by analyzing its level fluctuations. We demonstrate that this information is contained in the seismic and infrasound signals generated by the lava lake’s activity. The continuous seismo-acoustic monitoring permits quantification of lava lake dynamics, which is analyzed retrospectively to identify periods of volcanic unrest. Synchronous, high-resolution satellite SAR (Synthetic Aperture Radar) images are used to constrain lava lake level by measuring the length of the SAR shadow cast by the rim of the pit crater where the lava lake is located. Seventy-two estimations of the lava lake level were obtained with this technique between August 2016 and November 2017. These sporadic measurements allow for a better interpretation of the continuous infrasound and seismic data recorded at the closest station (~6 km from the crater). Jointly analyzed seismo-acoustic and SAR data reveal that slight changes in the spectral properties of the continuous cross-correlated low-frequency seismo-acoustic records (and not solely the single LP events) can be used to track fluctuations of the lava lake level on a daily and hourly basis. We observe that drops of the lava lake and the appearance of significant LP “lava lake” events are a consequence of deep magma intrusion, which induces changes in the shallow magmatic system. This study highlights the potential to continuously monitor Nyiragongo’s lava lake activity (and subsequent information about pressure changes within the magmatic system) using a single seismo-acoustic station located several kilometers from the vent.