| 1. |
- Árnadóttir, Thóra, et al.
(författare)
-
Glacial rebound and plate spreading : results from the first countrywide GPS observations in Iceland
- 2009
-
Ingår i: Geophysical Journal International. - 0956-540X .- 1365-246X. ; 177:2, s. 691-716
-
Forskningsöversikt (refereegranskat)abstract
- Iceland is one of the few places on Earth where a divergent plate boundary can be observed on land. Direct observations of crustal deformation for the whole country are available for the first time from nationwide Global Positioning System (GPS) campaigns in 1993 and 2004. The plate spreading across the island is imaged by the horizontal velocity field and high uplift rates (>= 10 mm yr(-1)) are observed over a large part of central and southeastern Iceland. Several earthquakes, volcanic intrusions and eruptions occurred during the time spanned by the measurements, causing local disturbances of the deformation field. After correcting for the largest earthquakes during the observation period, we calculate the strain rate field and find that the main feature of the field is the extension across the rift zones, subparallel to the direction of plate motion. Kinematic models of the horizontal plate spreading signal indicate a slightly elevated rate of spreading in the Northern Volcanic Zone (NVZ) (23 +/- 2 mm yr(-1)), while the rates at the other plate boundary segments agree fairly well with the predicted rate of plate spreading (similar to 20 mm yr(-1)) across Iceland. The horizontal ISNET velocities across north Iceland therefore indicate that the excessive spreading rate (>30 mm yr(-1)) observed by GPS in 1987-1992 following the 1975-1984 Krafla rifting episode was significantly slower during 1993-2004. We model the vertical velocities using glacial isostatic adjustment (GIA) due to the recent thinning of the largest glaciers in Iceland. A layered earth model with a 10-km thick elastic layer, underlain by a 30-km thick viscoelastic layer with viscosity 1 x 10(20) Pa s, over a half-space with viscosity similar to 1 x 10(19) Pa s can explain the broad area of uplift in central and southeastern Iceland. A wide area of significant residual uplift ( up to 8 mm yr(-1)) is evident in north Iceland after we subtract the rebound signal from the observed rates, whereas the Reykjanes Peninsula and the Western Volcanic Zone (WVZ) appear to be subsiding at a rate of 4-8 mm yr(-1). We observe a coherent pattern of small but significant residual horizontal motion (up to 3 mm yr(-1)) away from Vatnajokull and the smaller glaciers that is most likely caused by glacial rebound. Our study demonstrates that the velocity field over a large part of Iceland is affected by deglaciation and that this effect needs to be considered when interpreting deformation data to monitor subglacial volcanoes in Iceland.
|
|
| 2. |
- Hensch, Martin, et al.
(författare)
-
Temporal stress changes associated with the 2008 May 29 Mw 6 earthquake doublet in the western South Iceland Seismic Zone
- 2016
-
Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 204:1, s. 544-554
-
Tidskriftsartikel (refereegranskat)abstract
- On 2008 May 29, two magnitude Mw ~ 6 earthquakes occurred on two adjacent N-S faults in the western South Iceland Seismic Zone. The first main shock was followed approximately 3 s later by the rupture on a parallel fault, about 5 km to the west. An intense aftershock sequence was mostly confined to the western fault and an E-W aligned zone, extending west of the main shock region into the Reykjanes oblique rift. In this study, a total of 325 well-constrained focal mechanisms were obtained using data from the permanent Icelandic SIL seismic network and a temporary network promptly installed in the source region following the main shocks, which allowed a high-resolution stress inversion in short time intervals during the aftershock period. More than 800 additional focal mechanisms for the time period 2001-2009, obtained from the permanent SIL network, were analysed to study stress changes associated with the main shocks. Results reveal a coseismic counter-clockwise rotation of the maximum horizontal stress of 11 +/- 10 degrees ( 95 per cent confidence level) in the main rupture region. From previous fault models obtained by inversion of geodetic data, we estimate a stress drop of about half of the background shear stress on the western fault. With a stress drop of 8-10 MPa, the pre-event shear stress is estimated to 16-20 MPa. The apparent weakness of the western fault may be caused by fault properties, pore fluid pressure and the vicinity of the fault to the western rift zone, but may also be due to the dynamic stress increase on the western fault by the rupture on the eastern fault. Further, a coseismic change of the stress regime-from normal faulting to strike-slip faulting-was observed at the northern end of the western fault. This change could be caused by stress heterogeneities, but may also be due to a southward shift in the location of the aftershocks as compared to prior events.
|
|
| 3. |
|
|
