Gunnar B. Gudmundsson

Gradual caldera collapse at Bárdarbunga volcano, Iceland, regulated by lateral magma outflow

Driven to collapse Volcanic eruptions occur frequently, but only rarely are they large enough to cause the top of the mountain to collapse and form a caldera. Gudmundsson et al. used a variety of geophysical tools to monitor the caldera formation that accompanied the 2014 Bárdarbunga volcanic eruption in Iceland. The volcanic edifice became unstable as magma from beneath Bárdarbunga spilled out into the nearby Holuhraun lava field. The timing of the gradual collapse revealed that it is the eruption that drives caldera formation and not the other way around. Science, this issue p. 262 Magma flow from under the Bárdarbunga volcano drove caldera collapse during the 2014 eruption. INTRODUCTION The Bárdarbunga caldera volcano in central Iceland collapsed from August 2014 to February 2015 during the largest eruption in Europe since 1784. An ice-filled subsidence bowl, 110 square kilometers (km2) in area and up to 65 meters (m) deep developed, while magma drained laterally for 48 km along a subterranean path and erupted as a major lava flow northeast of the volcano. Our data provide unprecedented insight into the workings of a collapsing caldera. RATIONALE Collapses of caldera volcanoes are, fortunately, not very frequent, because they are often associated with very large volcanic eruptions. On the other hand, the rarity of caldera collapses limits insight into this major geological hazard. Since the formation of Katmai caldera in 1912, during the 20th century’s largest eruption, only five caldera collapses are known to have occurred before that at Bárdarbunga. We used aircraft-based altimetry, satellite photogrammetry, radar interferometry, ground-based GPS, evolution of seismicity, radio-echo soundings of ice thickness, ice flow modeling, and geobarometry to describe and analyze the evolving subsidence geometry, its underlying cause, the amount of magma erupted, the geometry of the subsurface caldera ring faults, and the moment tensor solutions of the collapse-related earthquakes. RESULTS After initial lateral withdrawal of magma for some days though a magma-filled fracture propagating through Earth’s upper crust, preexisting ring faults under the volcano were reactivated over the period 20 to 24 August, marking the onset of collapse. On 31 August, the eruption started, and it terminated when the collapse stopped, having produced 1.5 km of basaltic lava. The subsidence of the caldera declined with time in a near-exponential manner, in phase with the lava flow rate. The volume of the subsidence bowl was about 1.8 km3. Using radio-echo soundings, we find that the subglacial bedrock surface after the collapse is down-sagged, with no indications of steep fault escarpments. Using geobarometry, we determined the depth of magma reservoir to be ~12 km, and modeling of geodetic observations gives a similar result. High-precision earthquake locations and moment tensor analysis of the remarkable magnitude M5 earthquake series are consistent with steeply dipping ring faults. Statistical analysis of seismicity reveals communication over tens of kilometers between the caldera and the dike. CONCLUSION We conclude that interaction between the pressure exerted by the subsiding reservoir roof and the physical properties of the subsurface flow path explain the gradual near-exponential decline of both the collapse rate and the intensity of the 180-day-long eruption. By combining our various data sets, we show that the onset of collapse was caused by outflow of magma from underneath the caldera when 12 to 20% of the total magma intruded and erupted had flowed from the magma reservoir. However, the continued subsidence was driven by a feedback between the pressure of the piston-like block overlying the reservoir and the 48-km-long magma outflow path. Our data provide better constraints on caldera mechanisms than previously available, demonstrating what caused the onset and how both the roof overburden and the flow path properties regulate the collapse. The Bárdarbunga caldera and the lateral magma flow path to the Holuhraun eruption site. (A) Aerial view of the ice-filled Bárdarbunga caldera on 24 October 2014, view from the north. (B) The effusive eruption in Holuhraun, about 40 km to the northeast of the caldera

Crustal deformation measured by GPS in the South Iceland Seismic Zone due to two large earthquakes in June 2000

Two large earthquakes struck the South Iceland Seismic Zone in June 2000, the first on June 17 (MW =6.5) and the second on June 21 (MW =6.4). These are the largest earthquakes in the area in the past 88 years. A network of GPS stations was remeasured followingthe earthquakes. The whole network was last measured in 1995, and partly in 1999. We correct for the interseismic motion from 1995 to 2000, to obtain the coseismic displacements. The largest co- seismic motion we observe is about 0.55 m in the epicentral area of the June 17 event. We model the surface deforma- tion for the two earthquakes usingrectang ular dislocations in an elastic half space. Best fit uniform slip models indi- cate that the events occurred on two parallel, N-S vertical faults, with right-lateral strike slip motion. This is the same style of faultingbelieved to have occurred in larg e historical earthquake sequences in South Iceland.

Detailed distribution of microearthquakes along the northern Reykjanes Ridge, off SW‐Iceland

We have obtained a snapshot image of the earthquake activity along the northern Reykjanes Ridge (RR) using a dense ocean bottom seismometer (OBS) array. The earthquakes are narrowly concentrated along the NE-SW trending ridge crest. The seismicity is segmented into clusters by the en-echelon NNE-SSW trending axial volcanic systems which characterize the obliquely spreading ridge. The hypocenters are concentrated at somewhat greater depths (3-7 km) than on the Reykjanes Peninsula (RP), where the crust is thicker. Normal faulting dominates at the ridge crest. The focal planes are oriented more NE-SW than NNE-SSW, i.e., parallel to the ridge crest and its peripheral faults rather than the trend of individual axial volcanic systems and associated faults.

海底地震観測による Tjörnes Fracture Zone の地震活動

We present a detailed seismicity in and around the Tjornes Fracture Zone (TFZ) on the northern insular shelf of Iceland, based on the experiment conducted there in August 1991 using 18 ocean bottom seismographs (OBSs) and 24 land seismographs. The TFZ is a broad zone (75km x 100km) that connects the Northern Volcanic Zone (NVZ), the northern part of the neovolcanic zone that is thought to be a portion of the Mid-Atlantic Ridge (MAR) within Iceland, with the Kolbeinsey Ridge, a segment of the MAR 100km to the west. The aim of this experiment, which consisted of a seismic refraction survey and a microearthquake observation, was to understand the detailed structure of TFZ. Though seismicity in the TFZ was extremely low during the observation period judging from the permanent land station data (Icelandic Seismological Station Network, ISSN), the OBS array could