Locking depth and slip-rate of the Húsavík Flatey fault, North Iceland, derived from continuous GPS data 2006-2010

Located at the northern shore of Iceland, the Tjörnes Fracture Zone (TFZ) is a 120 km offset in the mid-Atlantic Ridge that connects the offshore Kolbeinsey Ridge to the on-land Northern Volcanic Zone. This transform zone is seismically one of the most active areas in Iceland, exposing the population to a significant risk. However, the kinematics of the mostly offshore area with its complex tectonics have not been adequately resolved and the seismic potential of the two main transform structures within the TFZ, the Grímsey Oblique Rift (GOR) and the Húsavík Flatey Fault (HFF) in particular, is not well known. In summer 2006, we expanded the number of continuous GPS (CGPS) stations in the area from 4 to 14. The resulting GPS velocities after four years of data collection show that the TFZ accommodates the full plate motion as it is predicted by the MORVEL plate motion model. In addition, ENVISAT interferograms reveal a transient uplift signal at the nearby Theistareykir central volcano with a maximum line-of-sight uplift of 3 cm between summers of 2007 and 2008. We use a combination of an interseismic backslip and a Mogi model in a homogeneous, elastic half-space to describe the kinematics within the TFZ. With a non-linear optimization approach we fit the GPS observations and estimate the key model parameters and their uncertainties, which are (among others) the locking depth, the partition of the transform motion between the two transform structures within the TFZ and the slip rate on the HFF. We find a shallow locking depth of 6.3+1.7- 1.2 km and transform motion that is accommodated 34 ± 3 per cent by the HFF and 66 ± 3 per cent by the GOR, resulting in a slip velocity of 6.6 ± 0.6 mm yr-1 for the HFF. Assuming steady accumulation since the last two large M6.5 earthquakes in 1872 the seismic potential of the fault is equivalent to a Mw6.8 ± 0.1 even

Geodynamic signals detected by geodetic methods in Iceland

The geodynamics laboratory provided by Iceland’s position on an active mid-ocean ridge has been recognized for several decades. Geodetic experiments have been designed and carried out in Iceland since 1938 to verify various global geodynamic theories, such as Wegener’s theory of continental drift, the sea floor spreading hypothesis, plate tectonics, mantle plumes etc. State-of-the-art techniques have been used to obtain data on crustal displacements with ever increasing accuracy to constrain the theories. Triangulation and optical levelling were used in the beginning, later EDM-trilateration. Network GPS surveying began in 1986 and has been used extensively since then to study crustal movements. With the addition of InSAR and continuous GPS in the last decade we have made a significant stride towards the goal of giving a continuous representation of the displacement field in time and space. The largest and most persistent signal is that of the plate movements. Geodetic points in East and West Iceland move with the Eurasia and North America Plates, respectively, and the vectors are consistent with global models of plate movements. The plate boundary zones are a few tens of kilometers wide, within which strain accumulates. This strain is released in rifting events or earthquakes that have a characteristic displacement field associated with them. In the Krafla rifting episode in 1975-1984 a 100 km long section of the plate boundary in North Iceland was affected and divergent movement as large as 8-9 m was measured. The June 2000 earthquakes in the South Iceland Seismic Zone were the most significant seismic events in the last decades. Two magnitude 6.5 earthquakes and several magnitude 5 events were associated with strike-slip faulting on several parallel faults along the transform-type plate boundary. Slow post-rifting and post-seismic displacements were detected in the months and years following these events, caused by coupling of the elastic part of the crust with the visco-elastic substratum. Viscosities in the range 0.3-30 x 1018 Pa s have been estimated from the time-decay of these fields. Similar values are obtained from crustal uplift measured around the Vatnajökull glacier due to the reduced load of the glacier in the last century. Magma movements in the roots of volcanoes are reflected by deformation fields measureable around them. The volcanoes inflate or deflate in response to pressure increase or decrease in magma chambers, and intrusive bodies are revealed by bulging of the crust above them. The most active volcanoes in Iceland, Katla, Hekla, and Grímsvötn, appear to be inflating at the present time, whereas Krafla and Askja are slowly deflating. An intrusion episode was documented near the Hengill volcano in 1994-1998 and two intrusion events occurred in the Eyjafjallajökull volcano in 1994 and 1999, all of which were accompanied by characteristic deformation fields

Temporal Seismic Velocity Changes During the 2020 Rapid Inflation at Mt. Þorbjörn-Svartsengi, Iceland, Using Seismic Ambient Noise

Publisher Copyright: © 2021. The Authors.Repeated periods of inflation-deflation in the vicinity of Mt. Þorbjörn-Svartsengi, SW-Iceland, were detected in January–July, 2020. We used seismic ambient noise and interferometry to characterize temporal variations of seismic velocities (dv/v, %). This is the first time in Iceland that dv/v variations are monitored in near real-time during volcanic unrest. The seismic station closest to the inflation source center (∼1 km) showed the largest velocity drop (∼1%). Different frequency range measurements, from 0.1 to 2 Hz, show dv/v variations, which we interpret in terms of varying depth sensitivity. The dv/v correlates with deformation measurements (GPS, InSAR), over the unrest period, indicating sensitivity to similar crustal processes. We interpret the velocity drop to be caused by crack opening triggered by intrusive magmatic activity. We conclude that single-station cross-component analyses provide the most robust solutions for early detection of magmatic activity.Peer reviewe

Locking depth and slip-rate of the Husavik Flatey fault, North Iceland, derived from continuous GPS data 2006-2010

ISSN:0956-540XISSN:1365-246

GNSS coordinate time series at the four stations in the Krafla and Þeistareykir area

GNSS coordinate time series at station BJAC, MYVA and KRAC at Krafla and station THRC at Þeistareykir. Different sheets correspond to different stations. The first rows of the sheets indicate the content of each column

Icelandic rhythmics: annual modulation of land elevation and plate spreading by snow load. Geophys

[1] We find strong correlation between seasonal variation in CGPS time series and predicted response to annual snow load in Iceland. The load is modeled using Green's functions for an elastic halfspace and a simple sinusoidal load history on Iceland's four largest ice caps. We derive E = 40 ± 15 GPa as a minimum value for the effective Young's modulus in Iceland, increasing with distance from the Eastern Volcanic Zone. We calculate the elastic response over all of Iceland to maximum snow load at the ice caps using E = 40 GPa. Predicted annual vertical displacements are largest under the Vatnajökull ice cap with a peak-to-peak seasonal displacement of $37 mm. CGPS stations closest to the ice cap experience a peak-to-peak seasonal displacement of$16 mm, consistent with our model. East and north of Vatnajökull we find the maximum of annual horizontal displacements of $6 mm resulting in apparent modulation of plate spreading rates in this area. Citation: Grapenthin, R.

Conduit formation and crustal microxenolith entrainment in a basaltic fissure eruption: Observations from Thríhnúkagígur Volcano, Iceland

Thríhnúkagígur Volcano, Iceland, is a composite spatter cone and lava field characteristic of basaltic fissure eruptions. Lava drainback at the end of the eruption left ~60 m of evacuated conduit, and a 4 × 104 m3 cave formed by the erosion of unconsolidated tephra by the feeder dike. Field relationships within the shallow plumbing system provide three-dimensional insight into conduit formation in fissure systems. Petrographic estimates and the relative volumes of the cave and erupted lavas both indicate xenolithic tephra comprises 5–10 % of the erupted volume, which cannot be reproduced by geochemical mixing models. Although crustal xenolith entrainment is not geochemically significant, we posit that this process may be common in the Icelandic crust. The Thríhnúkagígur eruption illustrates how pervasive, poorly consolidated tephra or hyaloclastite can act as a mechanically weak pre-existing structure that provides a preferential pathway for magma ascent and may influence vent location

Ground deformation after a caldera collapse: Contributions of magma inflow and viscoelastic response to the 2015-2018 deformation field around Bárðarbunga, Iceland

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