Sigrún Hreinsdóttir

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 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 ∼0.05 km3 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.

Complex multifault rupture during the 2016 Mw 7.8 Kaikōura earthquake, New Zealand

An earthquake with a dozen faults The 2016 moment magnitude (Mw) 7.8 Kaikōura earthquake was one of the largest ever to hit New Zealand. Hamling et al. show with a new slip model that it was an incredibly complex event. Unlike most earthquakes, multiple faults ruptured to generate the ground shaking. A remarkable 12 faults ruptured overall, with the rupture jumping between faults located up to 15 km away from each other. The earthquake should motivate rethinking of certain seismic hazard models, which do not presently allow for this unusual complex rupture pattern. Science, this issue p. eaam7194 At least 12 faults spaced up to 15 kilometers apart ruptured during the magnitude 7.8 Kaikōura earthquake. INTRODUCTION On 14 November 2016 (local time), northeastern South Island of New Zealand was struck by a major moment magnitude (Mw) 7.8 earthquake. The Kaikōura earthquake was the most powerful experienced in the region in more than 150 years. The whole of New Zealand reported shaking, with widespread damage across much of northern South Island and in the capital city, Wellington. The earthquake straddled two distinct seismotectonic domains, breaking multiple faults in the contractional North Canterbury fault zone and the dominantly strike-slip Marlborough fault system. RATIONALE Earthquakes are conceptually thought to occur along a single fault. Although this is often the case, the need to account for multiple segment ruptures challenges seismic hazard assessments and potential maximum earthquake magnitudes. Field observations from many past earthquakes and numerical models suggest that a rupture will halt if it has to step over a distance as small as 5 km to continue on a different fault. The Kaikōura earthquake’s complexity defies many conventional assumptions about the degree to which earthquake ruptures are controlled by fault segmentation and provides additional motivation to rethink these issues in seismic hazard models. RESULTS Field observations, in conjunction with interferometric synthetic aperture radar (InSAR), Global Positioning System (GPS), and seismology data, reveal the Kaikōura earthquake to be one of the most complex earthquakes ever recorded with modern instrumental techniques. The rupture propagated northward for more than 170 km along both mapped and unmapped faults before continuing offshore at the island’s northeastern extent. A tsunami of up to 3 m in height was detected at Kaikōura and at three other tide gauges along the east coast of both the North and South Islands. Geodetic and geological field observations reveal surface ruptures along at least 12 major crustal faults and extensive uplift along much of the coastline. Surface displacements measured by GPS and satellite radar data show horizontal offsets of ~6 m. In addition, a fault-bounded block (the Papatea block) was uplifted by up to 8 m and translated south by 4 to 5 m. Modeling suggests that some of the faults slipped by more than 20 m, at depths of 10 to 15 km, with surface slip of ~10 m consistent with field observations of offset roads and fences. Although we can explain most of the deformation by crustal faulting alone, global moment tensors show a larger thrust component, indicating that the earthquake also involved some slip along the southern end of the Hikurangi subduction interface, which lies ~20 km beneath Kaikōura. Including this as a fault source in the inversion suggests that up to 4 m of predominantly reverse slip may have occurred on the subduction zone beneath the crustal faults, contributing ~10 to 30% of the total moment. CONCLUSION Although the unusual multifault rupture observed in the Kaikōura earthquake may be partly related to the geometrically complex nature of the faults in this region, this event emphasizes the importance of reevaluating how rupture scenarios are defined for seismic hazard models in plate boundary zones worldwide. Observed ground deformation from the 2016 Kaikōura, New Zealand, earthquake. (A and B) Photos showing the coastal uplift of 2 to 3 m associated with the Papatea block [labeled in (C)]. The inset in (A) shows an aerial view of New Zealand. Red lines denote the location of known active faults. The black box indicates the Marlborough fault system

Eocene to present subduction of southern Adria mantle lithosphere beneath the Dinarides

We modeled global positioning system measurements of crustal velocity along a N13°E profile across the southern Adria microplate and south-central Dinarides mountain belt using a one-dimensional elastic dislocation model. We assumed a N77°W fault strike orthogonal to the average azimuth of the measured velocities, but we used a constrained random search algorithm minimizing misfit to the velocities to determine all other parameters of the model. The model fault plane reaches the surface seaward of mapped SW-verging thrusts of Eocene and perhaps Neogene age along the coastal areas of southern Dalmatia, consistent with SW-migrating deformation in an active fold-and-thrust belt. P-wave tomography shows a NE-dipping high-velocity slab to ∼160 km depth, which reaches the surface as Adria, dips gently beneath the foreland, and becomes steep beneath the Dinarides topographic high. The thrust plane is located directly above the shallowly dipping part of the slab. The pattern of precisely located seismicity is broadly consistent with both the tomography and geodesy; deeper earthquakes (down to ∼70 km) correlate spatially with the slab, and shallower earthquakes are broadly clustered around the geodetically inferred thrust plane. The model fault geometry and loading rate, ages of subaerially exposed thrusts in the fold-and-thrust belt, and the length of subducted slab are all consistent with Adria-Eurasia collision involving uninterrupted subduction of southern Adria mantle lithosphere beneath Eurasia since Eocene time.

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

Active aseismic creep on the Alto Tiberina low-angle normal fault, Italy

The existence of active low-angle normal faults has been questioned because the standard theory of fault mechanics precludes normal faults from slipping at low (

Coseismic slip distribution of the 2002 MW7.9 Denali fault earthquake, Alaska, determined from GPS measurements

[1] On 3 November 2002 an Mw7.9 earthquake occurred in central Alaska. The earthquake ruptured portions of the Susitna Glacier, Denali, and Totschunda faults. Inversion of the GPS-measured displacement field indicates that the event was dominated by a complex, right-lateral strike-slip rupture along the Denali fault. GPS sites closest to the epicenter show the effect of thrust motion on the Susitna Glacier fault. The preferred coseismic slip model, with M w 7.8, indicates relatively low slip on the western part of the rupture and high slip from about 60 km east of the hypocenter extending to the junction of the Denali and Totschunda faults. We find mostly shallow slip from the surface to 15 km depth, but the inversion suggests one large deep slip patch about 110 km east of the hypocenter. Our model predicts surface slip in good agreement with surface geological observations, where model resolution is good.

Crustal deformation at the oblique spreading Reykjanes Peninsula, SW Iceland: GPS measurements from 1993 to 1998

In 1993 and 1998 a 38-point GPS network was surveyed on the Reykjanes Peninsula, SW Iceland. According to the NUVEL-1A plate motion model the spreading rate of the North American and Eurasian plates in SW Iceland is 18.9±0.5 mm/yr toward N102.7°±1.1°E, highly oblique to the plate boundary. Instead of oblique spreading, the measurements indicate left-lateral shear strain accumulation parallel to the Reykjanes Peninsula seismic zone (∼N76°E) at the rate of about eyx ≈ −0.2 μstrain/yr (tensor shear strain). Subsidence is generally observed toward the seismic zone. A local maximum subsidence of 60 mm was measured in the Svartsengi geothermal area. Subsidence in this area has previously been detected with geodetic measurements and is considered a result of geothermal usage. Expansion was observed in the area of the Hengill triple junction. This is probably a result of magma accumulation beneath! mount Hromundartindur, as previous seismic and geodetic measurements have indicated. Using a simple screw dislocation model, we fit the majority of the data. Assuming a left-lateral shear zone at depth along the seismic zone, we estimate locking depth of ∼6.5 km and deep slip rate of ∼16.5 mm/yr. The maximum left-lateral displacement predicted by the screw dislocation model, 11.85±0.06 mm/yr, is consistent with the observed value of 11.9±0.5 mm/yr. If the Hengill area is excluded, little extension is observed across the peninsula. The discrepancy between the NUVEL-1A oblique spreading and the observed transcurrent motion is thought to be caused by lack of magma intrusion into the crust during this time period.

Glacio‐isostatic deformation around the Vatnajökull ice cap, Iceland, induced by recent climate warming: GPS observations and finite element modeling

[1] Glaciers in Iceland began retreating around 1890, and since then the Vatnajokull ice cap has lost over 400 km 3 of ice. The associated unloading of the crust induces a glacio-isostatic respo ...

Coseismic deformation of the 2002 Denali Fault earthquake: Insights from GPS measurements

[1] We estimate coseismic displacements from the 2002 M w 7.9 Denali Fault earthquake at 232 GPS sites in Alaska and Canada. Displacements along a N-S profile crossing the fault indicate right-lateral slip on a near-vertical fault with a significant component of vertical motion, north-side up. We invert both GPS displacements and geologic surface offsets for slip on a three-dimensional (3-D) fault model in an elastic half-space. We restrict the motion to right-lateral slip and north-side-up dip slip. Allowing for oblique slip along the Denali and Totschunda faults improves the model fit to the GPS data by about 30%. We see mostly right-lateral strike-slip motion on the Denali and Totschunda faults, but in a few areas we see a significant component of dip slip. The slip model shows increasing slip from west to east along the Denali Fault, with four localized higher-slip patches, three near the Trans-Alaska pipeline crossing and a large slip patch corresponding to a M w 7.5 subevent about 40 Ion west of the Denali-Totschunda junction. Slip of 1-3 m was estimated along the Totschunda Fault with the majority of slip being at shallower than 9 km depth. We have limited resolution on the Susitna Glacier Fault, but the estimated slip along the fault is consistent with a M w 7.2 thrust subevent. Total estimated moment in the Denali Fault earthquake is equivalent to M w 7.89. The estimated slip distribution along the surface is in very good agreement with geological surface offsets, but we find that surface offsets measured on glaciers are biased toward lower values.

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.