Seismological evidence for lateral magma intrusion during the July 1978 deflation of the Krafla volcano in NE-Iceland

The July 1978 deflation of Krafla volcano in the volcanic rift zone of NE-Iceland was in most respects typical of the many deflation events that have occurred at Krafla since December 1975. Separated by periods of slow inflation, the deflation events are characterized both by rapid subsidence and volcanic tremor in the caldera region, as well as extensive rifting in the fault swarm that transects the volcano. Earthquakes increase in the caldera region shortly after deflation starts and propagate along the fault swarm away from the central part of the volcano, sometimes as far as 65 km. The deflation events are interpreted as the result of subsurface magmatic movements, when magma from the Krafla reservoir is injected laterally into the fault swarm to form a dyke. In the July 1978 event, magma was injected a total distance of 30 km into the northern fault swarm. The dyke tip propagated with a velocity of 0.4-0.5 m/s during the first 9 h, but the velocity decreased as the length of the dyke increased. Combined with surface deformation data, these data can be used to estimate the cross-sectional area of the dyke and the driving pressure of the magma. The cross-sectional area is variable along the dyke and is largest in the regions of maximum seismic energy release. The average value is about 1,200 m2. The pressure difference between the magma reservoir and the dyke tip was of the order of 10-40 bars and did not change much during the injection.           ARK: https://n2t.net/ark:/88439/y057911 Permalink: https://geophysicsjournal.com/article/134 &nbsp

AVO Analysis of Multibeam Backscatter, an Example from Little Bay, NH and Skjalfandi Bay, Iceland

In the seismic reflection method, it is well known that seismic amplitude varies with the offset between the seismic source and detector, and that this variation is the key to the direct determination of lithology and pore fluid content of subsurface strata. Based on this fundamental property, amplitude-versus-offset (AVO) analysis has been used successfully in the oil industry for the exploration and characterization of subsurface reservoirs. Multibeam sonars acquire acoustic backscatter over wide range of incidence angles, and the variation of the backscatter with the angle of incidence is an intrinsic property of the seafloor. With the necessary changes being made, a similar approach to seismic AVO analysis can applied to the acoustic backscatter. To illustrate this approach, AVO analysis was applied to a Simrad EM3000 (300kHz) multibeam sonar dataset from Little Bay, NH, and to a Simrad EM300 (30kHz) multibeam sonar dataset from Skjalfandi Bay, Iceland. The analysis starts with the backscatter time series stored in raw Simrad datagrams, which are then corrected for seafloor slope, insonification area, time varying and angle varying gains. Then, a series of AVO attributes (near, far, slope, gradient, fluid factor, product etc) are calculated from the stacking of a number of consecutive time series. Based on the calculated AVO attributes and the inversion of a modified Jackson et al (1986) acoustic backscatter model we estimate the acoustic impedance, the roughness, and consequently the grain size of the insonified area on the seafloor. In Little Bay, the estimated impedance and the grain size were compared to in-situ measurements of sound speed and to the direct analysis of grain size in grab samples, showing a very good correlation. In Skjalfandi Bay, the AVO attribute of fluid factor was calculated, which presented an estimate of the gas/fluid content in the sediment structure. The areas with high fluid factor anomalies correlated to regions that showed evidence of gas in seismic profiles

Lost in Iceland? Fracture Zone Complications Along the mid-Atlantic Plate Boundary

The mid-Atlantic plate boundary breaks up into a series of segments across Iceland. Two transform zones, the South Iceland Seismic Zone (SISZ) and the Tj\ {o}rnes Fracture Zone (TFZ) separate the on land rift zones from the Reykjanes Ridge (RR), and the Kolbeinsey Ridge (KR), offshore N-Iceland. Both are markedly different from fracture zones elsewhere along the plate boundary. The 80 km E-W and 10--15 km N-S SISZ is made up of more than 20 N-S aligned, right-lateral, strike-slip faults whereas the TFZ consists of a broad zone of deformation, roughly 150 km E-W and 75 km N-S. The over-all left-lateral transform motion within the SISZ is accommodated by bookshelf faulting whereas the right-lateral transform motion within the TFZ is incorporated within two WNW-trending seismic zones, spaced $\sim$40 km apart, the Gr\\u27{\i}msey Seismic Zone (GSZ) and the H\\u27{u}sav\\u27{\i}k-Flatey fault (HFF). Recently collected EM300 and RESON8101 multibeam bathymetric data along with CHIRP subbottom data has unveiled some tectonic details within the TFZ. The GSZ runs along the offshore extension of the Northern Volcanic Rift Zone (NVRZ) and is made up of four left-stepping, en-echelon, NS-striking rift segments akin to those on land. Large GSZ earthquakes seem to be associated with lateral strike-slip faulting along ESE-striking fault planes. Fissure swarms transecting the offshore volcanic systems have also been subjected to right-lateral transformation along the spreading direction. As the Reykjanes Peninsula, the on land extension of the RR, the GSZ bears the characteristics of an oblique rift zone. The plate boundary segments connecting to the RR and KR are thus symmetrical with respect to the plate separation vector (105$\deg$) and orientation of individual volcanic systems. The HFF has an overall strike of N65$\deg$W and can be traced continuously along its 75--80 km length, between the Theistareykir volcanic system within the NVRZ, across the central TFZ-graben, the Skj\\u27{a}lfandi bay, and into the largest and westernmost graben, Eyjafjardar\\u27{a}ll (EG). Four pull-apart basins occur along the fault, the largest at the intersection with the EG, the southward magma-starved, continuation of the KR. Dikes, parallel to the HFF bear witness to it being a leaky transtensional feature. RESON8101 maps expose the tectonic fabric along the tidally swept shoreline adjacent to the main fault. The southwesternmost margin of the fault is characterised by NE-striking lavas which, along the coast, dip steeply (30--50$\deg$) westwards, towards the EG. The lavas are dissected by en echelon arrays of minor strike-slip faults intersecting the main fault at angles of N20$\deg$--30$\deg$W and N20$\deg$E. Some can be traced onto land where they exhibit complicated flower patterns. Destructive earthquakes occurred on the HFF in 1755, 1867 during an eruption offshore Tj\ {o}rnes, i.e. north of the fault, and in 1872. The 1867 earthquakes where most likely associated with rift-transform interaction on the easternmost section of the fault. An intense earthquake sequence on April 17, 1872 culminated with two $\sim$M6.5 earthquakes at 10 a.m. and 11 a.m. the next day. Based on intensity and damage reports, the $\sim$M6.5 earthquakes originated at different segments of the HFF, near H\\u27{u}sav\\u27{\i}k and Flatey

The Tectonic Evolution of the Tjornes Fracture Zone, Offshore Northern Iceland-ridge Jumps and Rift Propagation

The Tjornes Fracture Zone (TFZ) links the rift zones in northern Iceland with the Kolbeinsey Ridge north of Iceland. The TFZ was initiated during the Miocene (7-9 Ma), following an eastward jump of the spreading axis in northern Iceland. A roughly 150 km long (EW) and 50 km wide (NS) deformation zone has since developed which includes both right-lateral, strike-slip faults and three N-S trending extensional grabens (from west to east the Eyjafj\ {o}rdur, Skj\\u27{a}lfandi and \ {O}xarfj {o}rdur basins) which are filled with a 0.5-4 km thick sedimentary sequence. There are two WNW-striking bands of seismicity in the TFZ, a northern band known as the Gr\\u27{\i}msey lineament and a southern band associated with the WNW-trending H\\u27{u}sav\\u27{\i}k-Flatey fault (HFF). Over the past three field seasons we have mapped a large portion of TFZ utilizing multibeam echo sounders (both EM300 and a Reson 8101 shallow water system), collected high-resolution multichannel seismics and Chirp sonar, and obtained bottom photographs. The HFF can be traced offshore from H\\u27{u}sav\\u27{\i}k village across Skj\\u27{a}lfandi Bay as two WNW-trending, south-facing fault scarps and northwest of Flatey Island into the southern Eyjafj\ {o}rdur basin as a WNW-trending, north-facing scarp. In Skj\\u27{a}lfandi Bay several smaller WNW-trending faults are located sub-parallel of the main HFF. Offshore Flateyjarskagi, west of Flatey Island, a zone of intense deformation has been mapped, including clear evidence of right-lateral strike-slip faulting. The sediment-filled basins north of the HFF are bounded by numerous NS-trending faults, some of which extend to the seafloor, suggesting they are actively extending. The very subtle expression of the HFF in eastern Skj\\u27{a}lfandi Bay, and the more prominent but simple expression of recent (post-glacial) faulting along the western HFF near Flatey Island are consistent with historical and recent seismicity which is concentrated on the H\\u27{u}sav\\u27{\i}k fault system on the Tj\ {o}rnes peninsula, along the western HFF and in the southern Eyjafj\ {o}rdur, basin. A GPS geodetic station on the Tj\ {o}rnes peninsula, northeast of the HFF, maintained by the Iceland Meteorological Service shows that over the past 2 years the southern TFZ has been moving with the North American plate suggesting that little strain accommodation is currently occurring along the main HFF. These observations are consistent with a model for the tectonic evolution of the TFZ in which the continued northward propagation of the northern rift zone in Iceland has progressively shifted relative motion between the North American and Eurasian plates northward to the series of NNE-SSW trending rift zones along the Gr\\u27{\i}msey seismic lineation

Tectonic Details of the Tjornes Fracture Zone, an Onshore-Offshore Ridge-transform in N-Iceland

The Tjornes Fracture Zone (TFZ) links the northern rift zone (NVZ) in Iceland with the Kolbeinsey Ridge north of Iceland. The TFZ was initiated during the Miocene (about 7 Ma), following an eastward jump of the spreading axis in northern Iceland. A roughly 150 km long (EW) and 50 km wide (NS) deformation zone has since developed incorporating both right-lateral movement along WNW-trending strike-slip faults and oblique extension (105�) within three major N-S trending grabens (from west to east the Eyjafjardar�ll, Skj�lfandi and Oxarfjordur basins). Recently collected EM300 and RESON8101 multibeam bathymetric data, and CHIRP subbottom data combined with onshore mapping have enhanced our understanding of the rift-transform interactions within the TFZ. The transform motion is incorporated within two seismically active WNW trending zones, the Gr�msey Seismic Zone (GSZ) and the H�sav�k-Flatey fault (HFF), spaced ~40 km apart along the margins of the extensional basins. Being the propagating continuation of the NVZ offshore the GSZ has both the characteristics of an oblique rift zone and a transform whereas the HFF is more akin to oceanic transform faults. Four left-stepping, en-echelon, NS-striking rift segments (volcanic systems) exist along the GSZ. Large GSZ earthquakes, however, seem to be mainly associated with lateral strike-slip faulting along WNW-striking fault planes. Fissure swarms transecting the offshore volcanic systems also indicate right-lateral strike-slip motion parallel to the spreading direction. The HFF has an overall strike of N65�W and can be traced continuously onshore and offshore along its 75-80 km length, between the NVZ, across Skj�lfandi and into Eyjafjardar�ll. Four pull-apart basins occur along the fault, the largest at the intersection with Eyjafjardar�ll, the southward but magma-starved, continuation of the KR. Tertiary dikes, parallel to the HFF indicate it has been a leaky transtensional feature. The southwestern margin of the fault is characterized by NE-striking lavas which dip steeply (30-50�) towards Eyjafjardar�ll. The lavas are dissected by en echelon arrays of conjugate strike-slip faults intersecting the HFF fault at angles of N20�-30�W and N20�E. Some can be traced onto land where they exhibit complicated flower patterns. Destructive earthquakes occurred on the HFF in 1755, 1867 and 1872. The 1867 events were most likely associated with rift-transform interaction within the M�n�reyjar volcanic system, similar to the 1975-1989 Krafla rifting episode, when a lateral intrusion event triggered a M6.5 strike-slip earthquake at the junction of the Krafla fissure swarm and the GSZ. Although transform motion within the TFZ is currently taken up by two parallel fault systems the Tjornes microplate will merge with the North American plate as continued northward propagation of the divergent plate boundary gradually deactivates the extensional basins and HFF

Geophysical constraints on the dynamics of spreading centres from rifting episodes on land

Most of the Earth's crust is created along 60,000 km of mid-ocean ridge system. Here, tectonic plates spread apart and, in doing so, gradually build up stress. This stress is released during rifting episodes, when bursts of magmatic activity lead to the injection of vertical sheets of magma — termed dykes — into the crust. Only 2% of the global mid-ocean ridge system is above sea level, so making direct observations of the rifting process is difficult. However, geodetic and seismic observations exist from spreading centres in Afar (East Africa) and Iceland that are exposed at the land surface. Rifting episodes are rare, but the few that have been well observed at these sites have operated with remarkably similar mechanisms. Specifically, magma is supplied to the crust in an intermittent manner, and is stored at multiple positions and depths. It then laterally intrudes in dykes within the brittle upper crust. Depending on the availability of magma, multiple magma centres can interact during one rifting episode. If we are to forecast large eruptions at spreading centres, rifting-cycle models will need to fully incorporate realistic crust and mantle properties, as well as the dynamic transport of magma