ALBERT

Description: In 2012, two phreatic eruptions occurred at New Zealand's Tongariro volcano for the first time in over a century. Interferometric Synthetic Aperture Radar (InSAR) data collected by the Italian Space Agency's COSMO-SkyMed satellite constellation, indicates up to 20 mm/yr of post eruptive subsidence focused across a 4 km 2 region in the vicinity of the eruption site. Modeling of the deformation data indicates a shallow source at ∼500 m depth (1100 m ASL) consistent with the depth of the hydrothermal system. We estimate an annual volume loss of 35000 m 3 , leading to a pressure loss of ∼0.09 MPa/yr. We suggest that fracturing associated with the eruptions has enabled the continued depressurization of the shallow hydrothermal system and that subsidence will continue until the fractures become resealed.

Description: The evaluation of seismic hazard along active faults requires knowledge of the long term slip rate, the portions of the fault which are accumulating strain and the magnitude, number and location of past earthquakes. Aseismic slip will lower the longterm seismic potential but the distribution of seismic versus aseismic slip along faults is often poorly constrained. Changes to the local stress field may cause the distribution of slip to vary over time periods shorter than the seismic cycle, making it difficult to assess. Using InSAR and GPS data, we present the co- and postseismic deformation associated with the 2007 Mw 6.8 George Sound (New Zealand) earthquake and the influence of the 2009 Mw 7.8 Dusky Sound earthquake on the postseismic deformation. Our results suggest that the stress change induced by the later magnitude 7.8 earthquake led to a period of reactivated aseismic slip along the Puysegur subduction interface which ruptured two years earlier.

Description: The 300-km-long Taupo Volcanic Zone (TVZ) formed as a result of backarc rifting associated with subduction of the Pacific plate beneath the Australian plate, with current extension rates of 8–15 mm yr −1 . Using GPS and Interferometric Synthetic Aperture Radar (InSAR) data, collected by the European (ESA) and Japanese (JAXA) space agencies, we present ground deformation observations from 2003 to 2011. Both InSAR and vertical GPS data show widespread subsidence across the central TVZ at rates of up to 20 mm yr −1 . Using simple elastic dislocation models to represent the contraction of a sill like body at 6 km depth we predict an annual volume change of 0.011-0.016 km 3 beneath the central TVZ. We suggest that the majority of the observed subsidence is a result of the cooling and subsequent contraction of magma within the shallow crust.

Description: We present crustal deformation results from a geodetic experiment (Retreating-Trench, Extension, and Accretion Tectonics (RETREAT)) focused on the northern Apennines orogen in Italy. The experiment centers on 33 benchmarks measured with GPS annually or more frequently between 2003 and 2007, supplemented by data from an additional older set of 6 campaign observations from stations in northern Croatia, and 187 continuous GPS stations within and around northern Italy. In an attempt to achieve the best possible estimates for rates and their uncertainties, we estimate and filter common mode signals and noise components using the continuous stations and apply these corrections to the entire data set, including the more temporally limited campaign time series. The filtered coordinate time series data are used to estimate site velocity. We also estimate spatially variable seasonal site motions for stations with sufficient data. The RMS scatter of residual time series are generally near 1 mm and 4 mm, horizontal and vertical, respectively, for continuous and most of the new campaign stations, but scatter is slightly higher for some of the older campaign data. Velocity uncertainties are below 1 mm/yr for all but one of the stations. Maximum rates of site motion within the orogen exceed 3 mm/yr (directed NE) relative to stable Eurasia. This motion is accommodated by extension within the southwestern and central portions of the orogen, and shortening across the foreland thrust belt to the northeast of the range. The data set is consistent with contemporaneous extension and shortening at nearly equal rates. The northern Apennines block moves northeast faster than the Northern Adria microplate. Convergence between the Northern Apennines block and the Northern Adria microplate is accommodated across a narrow zone that coincides with the northeastern Apennines range front. Extension occurs directly above an intact vertically dipping slab inferred by previous authors from seismic tomography. The observed crustal deformation is consistent with a buried dislocation model for crustal faulting, but associations between crustal motion and seismically imaged mantle structure may also provide new insights on mantle dynamics.

Description: The 2013 Cook Strait earthquake sequence began on the 18th July 2013 with two foreshocks of Mw 5.7 and Mw 5.8 and culminated in the Mw 6.6 Cook Strait and Lake Grassmere events on the 21st July and 16th August respectively. Located ∼ 50 km south of New Zealand's capital, Wellington, the earthquakes generated the most significant ground shaking in the Wellington and Marlborough regions in recent decades. During the first event, located under Cook Strait, continuously recording GPS instruments across central New Zealand recorded up to 5 cm of horizontal displacement. Modelling suggests that the rupture was 25 km long with up to 90 cm of dextral strike slip. The second event, located 20 km to the south west, caused displacements of up to 25 cm at GPS sites located around the Clifford bay area. In addition, two interferograms from RADARSAT-2 and TerraSAR-X showed up to 30 cm of Line-of-Sight displacement in the vicinity of Lake Grassmere. Modelling indicates predominantly dextral strike slip of up to 2.1 m. Coulomb Stress changes induced by the earlier foreshocks suggest that the Cook Strait event was triggered by the preceding events and that the Lake Grassmere event was subsequently triggered by the Cook Strait earthquake.

Description: The MW (moment magnitude) 7.9 Denali fault earthquake on 3 November 2002 was associated with 340 kilometers of surface rupture and was the largest strike-slip earthquake in North America in almost 150 years. It illuminates earthquake mechanics and hazards of large strike-slip faults. It began with thrusting on the previously unrecognized Susitna Glacier fault, continued with right-slip on the Denali fault, then took a right step and continued with right-slip on the Totschunda fault. There is good correlation between geologically observed and geophysically inferred moment release. The earthquake produced unusually strong distal effects in the rupture propagation direction, including triggered seismicity.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Eberhart-Phillips, Donna -- Haeussler, Peter J -- Freymueller, Jeffrey T -- Frankel, Arthur D -- Rubin, Charles M -- Craw, Patricia -- Ratchkovski, Natalia A -- Anderson, Greg -- Carver, Gary A -- Crone, Anthony J -- Dawson, Timothy E -- Fletcher, Hilary -- Hansen, Roger -- Harp, Edwin L -- Harris, Ruth A -- Hill, David P -- Hreinsdottir, Sigrun -- Jibson, Randall W -- Jones, Lucile M -- Kayen, Robert -- Keefer, David K -- Larsen, Christopher F -- Moran, Seth C -- Personius, Stephen F -- Plafker, George -- Sherrod, Brian -- Sieh, Kerry -- Sitar, Nicholas -- Wallace, Wesley K -- New York, N.Y. -- Science. 2003 May 16;300(5622):1113-8.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/12750512" target="_blank"〉PubMed〈/a〉

Description: 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 Eyjafjallajokull, 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 18 years 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 (〉5 mm 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 approximately 0.05 km(3) 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. Eyjafjallajokull'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.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Sigmundsson, Freysteinn -- Hreinsdottir, Sigrun -- Hooper, Andrew -- Arnadottir, Thora -- Pedersen, Rikke -- Roberts, Matthew J -- Oskarsson, Niels -- Auriac, Amandine -- Decriem, Judicael -- Einarsson, Pall -- Geirsson, Halldor -- Hensch, Martin -- Ofeigsson, Benedikt G -- Sturkell, Erik -- Sveinbjornsson, Hjorleifur -- Feigl, Kurt L -- England -- Nature. 2010 Nov 18;468(7322):426-30. doi: 10.1038/nature09558.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Askja, Sturlugata 7, Reykjavik IS-101, Iceland. fs@hi.is〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/21085177" target="_blank"〉PubMed〈/a〉

Description: Crust at many divergent plate boundaries forms primarily by the injection of vertical sheet-like dykes, some tens of kilometres long. Previous models of rifting events indicate either lateral dyke growth away from a feeding source, with propagation rates decreasing as the dyke lengthens, or magma flowing vertically into dykes from an underlying source, with the role of topography on the evolution of lateral dykes not clear. Here we show how a recent segmented dyke intrusion in the Baretharbunga volcanic system grew laterally for more than 45 kilometres at a variable rate, with topography influencing the direction of propagation. Barriers at the ends of each segment were overcome by the build-up of pressure in the dyke end; then a new segment formed and dyke lengthening temporarily peaked. The dyke evolution, which occurred primarily over 14 days, was revealed by propagating seismicity, ground deformation mapped by Global Positioning System (GPS), interferometric analysis of satellite radar images (InSAR), and graben formation. The strike of the dyke segments varies from an initially radial direction away from the Baretharbunga caldera, towards alignment with that expected from regional stress at the distal end. A model minimizing the combined strain and gravitational potential energy explains the propagation path. Dyke opening and seismicity focused at the most distal segment at any given time, and were simultaneous with magma source deflation and slow collapse at the Baretharbunga caldera, accompanied by a series of magnitude M 〉 5 earthquakes. Dyke growth was slowed down by an effusive fissure eruption near the end of the dyke. Lateral dyke growth with segment barrier breaking by pressure build-up in the dyke distal end explains how focused upwelling of magma under central volcanoes is effectively redistributed over long distances to create new upper crust at divergent plate boundaries.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Sigmundsson, Freysteinn -- Hooper, Andrew -- Hreinsdottir, Sigrun -- Vogfjord, Kristin S -- Ofeigsson, Benedikt G -- Heimisson, Elias Rafn -- Dumont, Stephanie -- Parks, Michelle -- Spaans, Karsten -- Gudmundsson, Gunnar B -- Drouin, Vincent -- Arnadottir, Thora -- Jonsdottir, Kristin -- Gudmundsson, Magnus T -- Hognadottir, Thordis -- Fridriksdottir, Hildur Maria -- Hensch, Martin -- Einarsson, Pall -- Magnusson, Eyjolfur -- Samsonov, Sergey -- Brandsdottir, Bryndis -- White, Robert S -- Agustsdottir, Thorbjorg -- Greenfield, Tim -- Green, Robert G -- Hjartardottir, Asta Rut -- Pedersen, Rikke -- Bennett, Richard A -- Geirsson, Halldor -- La Femina, Peter C -- Bjornsson, Helgi -- Palsson, Finnur -- Sturkell, Erik -- Bean, Christopher J -- Mollhoff, Martin -- Braiden, Aoife K -- Eibl, Eva P S -- England -- Nature. 2015 Jan 8;517(7533):191-5. doi: 10.1038/nature14111. Epub 2014 Dec 15.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, IS-101 Reykjavik, Iceland. ; Centre for the Observation and Modelling of Earthquakes and Tectonics (COMET), School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK. ; GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand. ; Icelandic Meteorological Office, IS-150 Reykjavik, Iceland. ; 1] Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, IS-101 Reykjavik, Iceland [2] Icelandic Meteorological Office, IS-150 Reykjavik, Iceland. ; Canada Centre for Mapping and Earth Observation, Natural Resources Canada, 560 Rochester Street, Ottawa, Ontario K1A 0E4, Canada. ; Department of Earth Sciences, University of Cambridge, Madingley Road, Cambridge CB3 0EZ, UK. ; Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. ; Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. ; Department of Earth Sciences, University of Gothenburg, SE-405 30 Gothenburg, Sweden. ; Seismology Laboratory, School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/25517098" target="_blank"〉PubMed〈/a〉

Description: Two types of signals are clearly visible in continuous GPS (cGPS) time-series in Iceland, in particular in the vertical component. The first one is a yearly seasonal cycle, usually sinusoid-like with a minimum in the spring and a maximum in the fall. The second one is a trend of uplift, with higher values the closer the cGPS stations are to the centre of Iceland and ice caps. Here, we study the seasonal cycle signal by deriving its average at 71 GPS sites in Iceland. We estimate the annual and semi-annual components of the cycle in their horizontal and vertical components using a least-squares adjustment. The peak-to-peak amplitude of the cycle of the vertical component at the studied sites ranges from 4 mm near the coastline up to 27 mm at the centre of the Vatnajökull, the largest ice cap in Iceland. The minimum of the seasonal cycle occurs earlier in low lying areas than in the central part of Iceland, consistent with snow load having a large influence on seasonal deformation. Modelling shows that the seasonal cycle is well explained by accounting for elastically induced surface displacements due to snow, atmosphere, reservoir lake and ocean variations. Model displacement fields are derived considering surface loads on a multilayered isotropic spherical Earth. Through forward and inverse modelling, we were able to reproduce a priori information on the average seasonal cycle of known loads (atmosphere, snow in non-glaciated areas and lake reservoir) and get an estimation of other loads (glacier mass balance and ocean). The seasonal glacier mass balance cycle in glaciated areas and snow load in non-glaciated areas are the main contributions to the seasonal deformation. For these loads, induced seasonal vertical displacements range from a few millimetres far from the loads in Iceland, to more than 20 mm at their centres. Lake reservoir load also has to be taken into account on local scale as it can generate up to 20 mm of vertical deformation. Atmosphere load and ocean load are observable and generate vertical displacements in the order of a few millimetres. Inversion results also shows that the Iceland crust is less rigid than the world average. Interannual deviation from the GPS seasonal cycle can occur and are caused by unusual weather conditions over extended period of time.