An Iceland hotspot saga

Is Iceland a hotspot, with ridge-centered plume? In Iceland vigorous volcanism has built up a plateau 3.0 km higher than at a normal mid-ocean ridge with 3 to 4 times thicker crust than average oceanic crust. This volcanism can be associated with anomalous volcanism for 56–61 Ma in the form of aseismic ridges that stretch across the North Atlantic Ocean through Iceland, i.e. the Greenland-Iceland-Ridge (GIR) and the Faeroe-Iceland-Ridge (FIR). Iceland is a “meltspot” and an hotspot and the GIR and FIR may be hotspot trails. The trends or age progressions of the GIR and FIR are too uncertain to conclude if the Iceland hotspot can be a fixed reference point. There is a large seismic low-velocity anomaly (LVA) in the mantle under Iceland at least down to 400–450 km depth and with globally low velocities down to 200 km depth. The center of the LVA is at 64◦40’N and 18◦10’W between the glaciers Hofsjökull and Vatnajökull. The shape of the LVA is approximately that of a cylinder in the depth range 100–450 km, but at certain depths elongated in the northsouth direction. The LVA extends at least up to 30–40 km depth beneath Central Iceland and the rift zones. The shallower part of the LVA (i.e. above 150 km depth) extends at least 700 km outside of Iceland to the southwest, along the Reykjanes Ridge. The LVA has been numerically modelled with geodynamic methods by several authors as a ridge-centered convecting plume. They try to fit crustal thickness of the Iceland hotspot and neighbouring ridge, and the magnitude and shape of the LVA. The latest of these models find a best fit: A plume 135–150◦C hotter than background mantle, retaining in general 1% partial melt in a maximum 90 km thick melting zone, but reaching up to 2–3% partial melt in the shallowest mantle. The rest of produced melt goes into forming the crust. Considerable work has been carried out on various plume models to explain these and other observations in Iceland, but the models are still some way from reaching a mature state. As long as important observations are lacking and some key questions remain unanswered, alternatives to the plume model or more realistic variants of it in a larger tectonic framework, including heterogeneous mantle, should not be discouraged.Er Ísland heitur reitur (hotspot) sem dregur orku sína frá möttulstróki undir úthafshrygg? Þarna hefur öflug eldvirkni myndað hásléttu sem rís 3,0 km upp yfir úthafshrygginn í Norður-Atlantshafi, og jarðskorpu sem er 3–4 sinnum þykkari heldur en venjulegur úthafshryggur hefur. Þessa umfram eldvirkni Íslands má rekja 56–61 milljón ára aftur í tímann í óvirkum hryggjum sem teygja sig eftir Norður- Atlantshafinu, þ.e. í Grænlands-Íslands hryggnum (GÍH) og Færeyja-Íslands hryggnum (FÍH). Ísland er heitur reitur, svæði með óvenju mikinn kvikubúskap (meltspot), og áðurnefnda hryggi má líta á sem spor eða eftirstöðvar heita reitsins (hotspot trails). Stefna GÍH og FÍH og aldursdreifing bergs í þeim er hins vegar of óviss til þess að slá megi því föstu að íslenski heiti reiturinn sé einn af föstum viðmiðunarpunktum jarðarinnar. Undir Íslandi er stórt svæði sem tefur jarðskálftabylgjur meira en gerist annars staðar vegna lághraðasvæðis í möttlinum (LSM). LSM nær a.m.k. niður á 400–450 km dýpi. Miðja þess er staðsett 64◦40’N og 18◦10’V, það er á milli Hofsjökuls og Vatnajökuls. Í stórum dráttum hefur LSM sívalningslögun á dýpinu 100–450 km, en það teygist úr svæðinu í N-S stefnu sums staðar á þessu dýpi. Undir miðju Íslands og undir gosbeltunum nær LSM að komast næst yfirborði jarðar eða upp að 30–40 km. Grynnri hluti LSM (þ.e. 150 km) nær a.m.k. 700 km út fyrir Ísland eftir Reykjaneshryggnum. Með aflfræðilegum reiknilíkönum hefur verið hermt eftir áhrifum möttulstóks. Nýjustu líkönin og þau sem herma best eftir LSM og þykkt jarðskorpunnar undir heita reitnum og nálægum úthafshryggjum reikna með flæði efnis sem er 135–150◦C heitara en bakgrunnsmöttullinn. Í 90 km þykku (lóðrétt) bræðslusvæði stróksins er að meðaltali 1% hlutbráð. Mest nær hlutbráðin 2–3% í grynnsta hluta svæðisins. Umframbráðin stígur upp og myndar jarðskorpuna. Möttulstrókslíkön hafa þróast, en hafa ekki enn náð nægilegri fullkomnun. Á meðan ekki hefur verið lagður grunnur að öllum grundvallareiginleikum möttulstróka, ætti ekki að letja hugmyndavinnu sem sækir á önnur mið til þess að útskýra uppruna heitra reita. Möttulstrókslíkön framtíðarinnar taka væntanlega mið af stærri tektónískri heild og mismunandi efnafræðilegri gerð möttulsins.Peer Reviewe

Earthquake Sequence 1973–1996 in Bárðarbunga Volcano: Seismic Activity Leading up to Eruptions in the NW-Vatnajökull Area

A day and a half after the earthquake (mb=5.3, MS=5.6, MW=5.6) in the Bárðarbunga central volcano on Sept. 29th 1996, a volcanic eruption broke out under the Vatnajökull glacier. The eruption was located approximately 20 km SSE of the earthquake epicenter, midway between the Bárðarbunga and Grímsvötn central volcanoes. Course of events suggests a connection between earthquake and eruption and therefore a connection with a sequence of earthquakes of the same characteristics in Bárðarbunga during the years 1973–1996. The earthquakes in question are of an unusually low frequency character (corner frequency), explained by exceptionally low dynamic stress drop (< 10 bars) at shallow depth ( 5.0 km). The sequence which lasted for 22 years is characterised by annual main events of magnitudes in the range of 4.5–5.7 (mb). It intensified in the 1990s, with some of the largest earthquakes of the whole episode occurring at that time. Moment tensor solutions of teleseismic signals and locally recorded waveforms reveal that the main events are thrust faulting earthquakes with a significant non-double couple component. Arguments are presented that the faulting occurred on a steeply inward dipping caldera fault, with reactivated motion on a weak fault. As a consequence of this hypothesis magma inflation in Bárðarbunga is the most probable cause of the 1973– 1996 events. However, the loading force (the magma) may or may not have resided at a similar shallow depth as the earthquakes. Cast in the frame of the inflation model, the Bárðarbunga 1973–1996 sequence implies a resurgent caldera of at least 0.2–0.7 km3 for approximately a quarter of a century, exceeding its magma storage capacity in 1996. However, these calculations are model dependent. Bárðarbunga and neighbouring area were relatively calm during the period mid-1997 to 2004. There was a renewed activity of small earthquakes during the years 2005–2009. From the beginning of continuous seismic recording in Iceland in 1925, all eruptions in Vatnajökull on record have been accompanied with earthquake(s) of magnitude 4.0, within two months of the initial eruption.Meðalstór jarðskjálfti (MW=5,6) átti upptök sín undir eldfjallinu Bárðarbunga 29. september 1996. Þótt þessi jarðskjálfti teljist aðeins vera meðalstór miðað við jarðskjálfta yfirleitt í heiminum, þá var þetta stór skjálfti, þegar tekið er tillit til stærðar Bárðarbungu. Hann gæti hafa fært 12 km langa jarðspildu til um 65 cm. Mikil skjálftavirkni fylgdi í kjölfar meginskjálftans. Einum og hálfum degi eftir meginskjálftann varð eldgos undir Vatnajökli, 20 km suðsuðaustur frá upptökum skjálftans, miðja vegu milli Bárðarbungu og Grímsvatna. Hefur eldgos þetta verið nefnt Gjálpargos. Vegna nálægðar þessara atburða í tíma og rúmi er freistandi að athuga, hvort orsakatengsl séu þarna á milli. Skjálftinn árið 1996 var síðasti skjálfti í röð meðalstórra og minni skjálfta, sem hófust í Bárðarbungu árið 1973. Þessi skjálftaröð hefur einkennst af nærri árlegum aðalskjálftum af stærðinni 4,5–5,7 mb. Lág bylgjutíðni þeirra bendir til grunnra upptaka ( 5.0 km) og óvenju lítils spennufalls (<10 bör) miðað við tektoníska skjálfta. Lágt spennufall skýrist af grunnum upptökum í efri lögum jarðskorpunnar miðað við stærð skjálftanna. Þar er styrkur misgengja í brotgjarnri skorpu minni en á meira dýpi. Brotlausnir og vægisþinur (e. moment tensor) sýna samgengishreyfingar á sveigðum misgengjum, annaðhvort vegna aukins eða minnkaðs þrýstings í eldfjallinu. Hér eru leidd rök að því, að skjálftinn hafi orðið vegna viðsnúinnar hreyfingar á öskjumisgengi, sem hallar inn (þ.e. samgengishreyfing á siggengi). Af því leiðir, að aukinn þrýstingur sé líklegasta orsökin, og að þrýstingsaukningin stafi af flæði kviku inn undir Bárðarbungu. Í ljósi þessarar tilgátu hefur Bárðarbungufjallið verið að þenjast út í u.þ.b. aldarfjórðung, og í lok september 1996 náði þenslan hámarki, og kvika braut sér leið undan fjallinu. Eldgosið í Gjálp hefur væntanlega valdið þrýstingslækkun í Bárðarbungu og nágrenni, sem sést í lítilli skjálftavirkni í norðvesturhluta Vatnajökuls frá miðju ári 1997 til 2005 (þ.e. engir skjálftar af stærðinni >3,0).Peer Reviewe

Contemporary tectonics of the Wasatch front region, Utah, from earthquake focal mechanisms

We have completed a comprehensive study of focal mechanisms of digitally recorded earthquakes (M, -_< 4.4) that occurred in the Wasatch front region in Utah during 1980 to 1986. Single-event solutions for 24 events were determined using recently revised crustal models and a computerized grid-search technique. Overall, the mechanisms show predominantly normal faulting on N-S-striking nodal planes of moderate to steep dip (>30°). Tension-axis azimuths average 96 ° _+ 12% Thus, in general, the mechanisms indicate E-W to ESE-WNW crustal extension and vertical crustal shortening. Oblique slip, when observed, is characterized by left-lateral motion on planes striking N to NE or right-lateral motion on planes striking N to NW. Most of the mechanisms with significant amounts of oblique-slip motion occur in the southern part of the study area, where compression- axis orientations range from near vertical to near horizontal. Thus, the mechanisms suggest a possible change in stress regime from north to south along the Wasatch front. Despite geologic evidence for low-angle faults in the study area, shallowly dipping nodal planes are relatively uncommon.This research was supported by the U.S. Geological Survey, Department of the Interior, under award numbers 14-08-0001-Gl163 and 14-08-0001- G1349. Partial support also came from a scholarship awarded by the Society of Exploration Geophysicists and sponsored by the Sohio Petroleum Company.Peer Reviewe

Application of the POCS inversion method to cross‐borehole imaging

Publisher's version (útgefin grein)Cross-borehole tomography suffers from a well-known problem of data incompleteness: the limited ray coverage dictated by the poor experimental geometry implies that certain features of the velocity field are not determined by the data. Construction of a tomographic image of the velocity field therefore requires the addition of prior constraints to the inversion. In the Fourier wavenumber domain (assuming straight-line rays), the process of adding prior constraints is equivalent to specifying unmeasured wavenumber coefficients. The projection onto convex sets (POCS) algorithm can impose physically plausible constraints that allow high quality tomographic images to be produced. Each constraint is viewed as defining a set (in function space) of images that satisfy that particular constraint. The POCS method finds one or more images in the intersection of the constraining sets, which is equivalent to finding an image that simultaneously satisfies a number of constraints including the observed data. The sets of images that we employ include: those that satisfy the data in the sense of having certain known wavenumber components, those that have bounded energy in certain unmeasured wavenumber components, those that have seismic velocity bounded everywhere (e.g., nonnegative), and those in which the velocity structure is confined to the region between the boreholes. An advantage of the POCS algorithm is that it allows both space-domain and wavenumber-domain constraints to be imposed simultaneously. In our implementation of the POCS algorithm, we make use of the fast Fourier transform to rapidly iterate between the space and Fourier-wavenumber domains. We test the method on synthetic data, and show that it significantly reduces the artifacts in the image, when compared to other methods. We then apply it to data from a cross-borehole experiment in Manitoba, Canada, that were previously studied by others. We achieve a tomographic image of the velocity field that is similar in many respects to the results of others, but which possesses fewer artifacts.This work was supported by the National Science Foundation and the Office for Naval Research. This is Lamont- Doherty contribution number 5042.Peer Reviewe

The effect of local wind on seismic noise near 1 Hz at the MELT site and in Iceland

The mantle electromagnetic and tomography (MELT) experiment on the east Pacific rise near 17°S was the first large teleseismic experiment on a midocean ridge. During the six-month deployment, no compressional arrivals were well recorded above 0.5 Hz. In comparison, the ICEMELT experiment in Iceland recorded compressional arrivals at 1-2 Hz from about 2 earthquakes per month. We compare noise spectra from the two experiments and show that this difference in detection is at least in part a result of noise. Near 1 Hz, seismic noise in the oceans is produced locally by wind-generated waves. At both experiment sites, 1-Hz noise levels are well correlated with local sea-surface-wind speeds derived from satellite observations. For a given wind speed, 1-Hz noise levels are about 10-20 dB lower in Iceland. At the MELT site, cross-correlations of wind speed with the logarithm of noise in a narrow-frequency band yield correlation coefficients exceeding 0.7 at frequencies between 0.4 Hz and 2 Hz. Noise levels at 1 Hz increase with wind by 1.3-1.4 dB per m/sec for wind speeds less than 10 m/sec. For the ICEMELT experiment, high correlation coefficients extend to markedly higher frequencies for coastal stations, and there is a 10-dB drop in 1-Hz noise levels 100-km inland. Noise levels increase by about 0.8 dB per m/sec. The strong correlation between wind speed and l-Hz seismic noise provides justification for using satellite wind speed data to search for locations on the global spreading system where there is a better probability of recording high-frequency arrivals. The calmest sites are found on the northern east Pacific rise, near the equator in all oceans, and near 34 ° N and 22 ° S on the mid- Atlantic ridge.This study was supported by the National Science Foundation under grant OCE-9414299.Peer Reviewe

Evolution of Stresses Over Conjugate Faults in Hjalli‐Ölfus, South Iceland

Plain Language Summary Iceland hosts a predominantly rifting plate boundary that is offset by two ∼east-west trending, horizontally sliding (transform-faulting) segments, one in the north and the other in the south. The southern segment, known as the South Iceland Seismic Zone (SISZ), is seismically productive and is flanked by diverging volcanic arms. The SISZ has hosted several moderate to large earthquakes on north-south faults that cut across it. However, the westernmost end of the SISZ, also known as Hjalli-Ölfus, differs from the rest of the SISZ as it seems to host earthquakes in an ∼east-northeast direction, similar to its western, rifting neighbor, namely, the Reykjanes Peninsula. The activity in Hjalli-Ölfus also seems to be responsive to volcanic/magmatic activity in the Hengill volcano to its north. This suggests the existence of multiple stress fields—volcanic, tectonic, or both—acting on the Hjalli-Ölfus segment. Here, we study earthquakes in Hjalli-Ölfus from January 1991 to December 1999, including a magnitude 5.1 earthquake in November 1998, to identify possible stress changes along the segment over time. Results indicate that magmatic deformation and seismic activity near the Hengill volcano directly influence the seismic productivity of Hjalli-Ölfus, and that this seismicity is similar to that of the Reykjanes Peninsula.Hjalli-Ölfus is the westernmost segment of the east-west transform South Iceland Seismic Zone (SISZ), which is the eastward extension of the ∼ENE-trending transtensional Reykjanes Peninsula (RP). Historically, the area has shown an interactive behavior with the Hengill volcanic system to the north and the central SISZ to the east. We analyzed the state of stress and faulting mechanisms in Hjalli-Ölfus between July 1991 and December 1999, in connection with the Hengill inflation episode (Feigl et al., 2000, https://doi. org/10.1029/2000JB900209) and the 13th November 1998 Mw 5.1 Hjalli-Ölfus earthquake. We find that this region predominantly hosted oblique-normal and left-lateral strike-slip events (4–10 km-depth), with most nodal planes oriented along ∼ENE or ∼WSW directions (75° ± 15° or 255° ± 15°). We identify 5 stages of stress evolution from January 1991 to December 1999 over which Hjalli-Ölfus experiences both spatial and temporal shifts in stress-states. The Hengill inflation likely loaded both the fissure zone and western Ölfus, culminating in the Mw 5.4 (Hengill) and Mw 5.1 (Hjalli-Ölfus) earthquakes. Following these events, the maximum compressive stress (SHmax) orientation near the location of the Mw 5.1 earthquake showed a ∼5°–7° counterclockwise swing, compared to SHmax before June 1998. The average SHmax (∼40° ± 1°) and minimum principal stress (sigma3 ∼ 130° ± 1°) are comparable to geological trends in the RP. We conclude that Hjalli-Ölfus shows clockwise SHmax rotation upon loading, while a stress-drop reverses the rotation. We also posit that the region, especially the western end, behaves like the RP during interseismic periods.Icelandic Research Fund (IRF) (Rannís). Grant Number:152432-053Peer Reviewe

The 1912 Iceland earthquake rupture: Growth and development of a nascent transform system

We have mapped in detail surface ruptures of the 1912 magnitude 7.0 strike-slip earthquake in south Iceland. This earthquake ruptured fresh basalt flows that had covered the pre-existing fault. The observed style of surface fracturing closely matches both theoretical predictions of the first stages of shear fracture development and microscopic-scale observations from laboratory experiments. The shear offset distributed across the zone of surface fractures produced by this earthquake is right-lateral and is in the range of 1 to 3 m. Total mapped rupture length is 9 km, but total rupture length is probably at least ∼ 20 km. This interplate earthquake had an exceptionally high ratio of slip to fault length and, by inference, stress drop. The north-south trending rupture of the 1912 earthquake is part of the “bookshelf” faulting in the east-west trending South Iceland Seismic Zone. We ascribe the “bookshelf” faulting in the South Iceland Seismic Zone to a combination of the early development stage of the transform and regional strength anisotropy of the crust.This research was supported by the National Science Foundation, the Icelandic National Power Authority (Landsvirkjun), and the Department of Geological Sciences of Columbia University. Lamont-Dohert Contribution 5036.Peer Reviewe

Tomographic image of the Mid-Atlantic Plate Boundary in southwestern Iceland

Publisher's version (útgefin grein)The 170 km South Iceland Seismic Tomography (SIST) profile extends from the west and across the Mid‐Atlantic Ridge spreading center in the Western Volcanic Zone and continues obliquely through the transform zone (the South Iceland Seismic Zone) to the western edge of the Eastern Volcanic Zone. A total of 11 shot points and 210 receiver points were used, allowing precise travel times to be determined for 1050 crustal P wave rays and 180 wide‐angle reflections. The large amplitudes of the wide‐angle reflections and an apparent refractor velocity of 7.7 km/s are interpreted to be from a relatively sharp Moho at a depth of 20–24 km. This interpretation differs from the earlier models (based on data gathered in the 1960s and 1970s), of a 10–15 km thick crust underlain by a upper mantle with very slow velocity of 7.0–7.4 km/s. Nevertheless, these older data do not contradict our new interpretation. Implication of the new interpretation is that the lower crust and the crust‐mantle boundary are colder than previously assumed. A two‐dimensional tomographic inversion of the compressional travel times reveals the following structures in the crust: (1) a sharp increase in thickness of the upper crust (“layer 2A”) from northwest to southeast and (2) broad updoming of high velocity in the lower crust in the Western Volcanic Zone, (3) depth to the lower crust (“layer 3”) increases gradually from 3 km at the northwestern end of the profile to 7 km at the southeastern end of the profile, (4) a low‐velocity perturbation extends throughout the upper crust and midcrust into the lower crust in the area of the transform in south Iceland (South Iceland Seismic Zone), and (5) an upper crustal high‐velocity anomaly is associated with extinct central volcanos northwest of the Western Volcanic Zone. The travel time data do not support the existence of a large (> 0.5 km thick) crustal magma chamber in this part of the Western Volcanic Zone but do not exclude the possibility of a smaller one.This research was supported by the U.S. National Science Foundation, the Iceland National Science Foundation, the National Energy Authority of Iceland, the Incorporated Research Institutions for Seismology, IcelandAir, and the Lamont-Doherty Geological Observatory of Columbia University.Peer Reviewe

Seismicity on Conjugate Faults in Ölfus, South Iceland: Case Study of the 1998 Hjalli‐Ölfus Earthquake

Publisher's version (útgefin grein)The Ölfus seismic belt lies at the western end of the ~E‐W sinistral transform shear zone in South Iceland, called the South Iceland Seismic Zone (SISZ), where most seismicity and surface faulting show ~N‐S dextral slip. Unlike the rest of SISZ, seismicity in west Ölfus is predominantly along the ~ENE‐WSW direction. Throughout recorded history, Ölfus has shown an interactive behavior with the Hengill volcanic system that lies northwest of the zone. For instance, the 13 November 1998 Mw 5.1 earthquake in the Hjalli area (west Ölfus) and its ~ENE trending aftershock sequence were likely triggered by the 4 June 1998 Mw 5.4 Hengill earthquake sequence. These events point to an interplay between conjugate ~N‐S and ~ENE‐WSW faults in the region. Relative relocations of earthquakes in Hjalli‐Ölfus from July 1991 to December 1999 (Icelandic Meteorological Office, 2017) are chiefly limited to 4‐ to 8‐km depth along the ~ENE direction with a few distributed on smaller ~N‐S faults. The foreshocks of the November 1998 earthquake occurred on a ~N‐S fault until a day prior to the mainshock when they shifted to the ~ENE direction. The subsequent aftershocks are also mainly restricted to the ~ENE direction. We find that the Mw 5.1 (Global Centroid Moment Tensor moment = 5.43 × 10E16 N‐m) Hjalli‐Ölfus earthquake ruptured a near‐vertical ~ENE fault area of 24–40 km2 with left‐lateral average slip of 5–8 cm. Multiple relocations of the mainshock using various constraints indicate that the event likely occurred close to the junction of the conjugate ~ENE‐WSW and ~N‐S faults.This study was part of the project titled “4D seismic of the South Iceland Seismic Zone: Strong earthquake forecasting” (152432‐053) funded by the Icelandic Research Fund (IRF) (Rannís). The authors thank Felix Waldhauser for sharing a more recent version of the hypoDD routine, which was then modified for the purposes of this study, and for advice on fine‐tuning the analyses. The authors thank Haukur Jóhannesson, Kristján Sæmundsson, Árni Hjartarson, Maryam Khodayar, Leó Kristjánsson, and Ágúst Guðmundsson for insightful discussions. The authors also thank Páll Einarsson and Bryndís Brandsdóttir for scientific discussion and for sharing seismic data from 1974 to 1987. The project facilitated the procurement of seismic data recorded by the SIL seismic network from the Icelandic Meteorological Office (IMO), and the raw version of this data can only be obtained directly from the agency.Peer Reviewe

Method to find the Minimum 1D Linear Gradient Model for Seismic Tomography

The changes in the state of a geophysical medium before a strong earthquake can be found by studying of 3D seismic velocity images constructed for consecutive time windows. A preliminary step is to see changes with time in a minimum 1D model. In this paper we develop a method that finds the parameters of the minimum linear gradient model by applying a two-dimensional Taylor series of the observed data for the seismic ray and by performing least-square minimization for all seismic rays. This allows us to obtain the mean value of the discrete observed variable, close to zero value.This research was supported by the Icelandic Research Fund RANNIS ID: 152432-051