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Locking depth and slip-rate of the Húsavík Flatey fault, North Iceland, derived from continuous GPS data 2006–2010
Sabrina Metzger 1,
Sigurjón Jónsson 2,
Halldór Geirsson 3,4
Article first published online: 23 SEP 2011
Geophysical Journal International
Volume 187, Issue 2, pages 564–576, November 2011
1 Institute of Geophysics, ETH Zürich, Sonneggstrasse 5, 8092 Zürich, Switzerland. E-mail: email@example.com
2 King Abdullah University of Science and Technology (KAUST), Thuwal, Saudi Arabia
3 The Pennsylvania State University, University Park, PA, USA
4 Icelandic Meteorological Office (IMO), Reykjavík, Iceland
Issue published online: 14 OCT 2011
Article first published online: 23 SEP 2011
Accepted 2011 July 29. Received 2011 June 29; in original form 2011 March 11
Time series analysis;
Oceanic transform and fracture zone processes;
Kinematics of crustal and mantle deformation
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 event.
The magnitude estimation of the accumulated moment along
the HFF of Mw = 6.8 ± 0.1 is based on four assumptions: (1)
Complete stress relaxation by the 1872 Mw = 6.5 earthquakes and
steady stress accumulation since then, (2) uniform slip rate and a
constant locking depth, (3) a rupture along the whole total fault
plane with a dimension constrained by the locking depth and (4) the
fault model length, which is the sum of the segments B and C in
Fig. 7. In fact, the onshore segment C ends within the Theistareykir
fissure swarm and is ∼18 km shorter than the model segment.
Using eqs (4) and (5) with the adapted length reduces the magnitude
estimation only within the rounding precision (Mw ± 0.05). Also,
the stress accumulation on HFF might have been influenced by
the Krafla rifting episode 1975–1984 that appears to have reduced
the seismicity on the eastern end of the fault (Rögnvaldsson et al.
1998). Another fact that might be taken into account to estimate the
potential devastating energy would be the direction of rupture. If this
potential event would initiate at the northwestern end of the fault,
the rupture would propagate ‘towards’ Húsavík and the surrounding
farms, which causes a superposition and thus enhancement of the
The CGPS time-series presented in this paper covers the whole
TFZ (150 km by 100 km) in North Iceland expanding the existing
network from 4 to 14 stations. The resulting GPS velocities from
4 yr of data show clearly the transformmotion in the TFZ and the full
plate spreading between the North American and the Eurasian Plate.
The transform motion is accommodated by the HFF and the GOR
in a ratio of 34 per cent/66 per cent with an uncertainty of ±3 per
cent. In addition, the GPS velocities show influence from uplift
at Theistareykir central volcano, which likely is caused by magma
accumulation at ∼8.5 km depth. We used a combined backslip and
Mogi source model to describe the surface deformation as seen
with the CGPS data, and for the first time key parameters of the
kinematics of the TFZ were estimated with uncertainties.
Public ID 2012p812341
Universal Time October 27 2012 at 13:35:44
NZ Daylight Time Sunday, October 28 2012 at 2:35:44 am
Latitude, Longitude -40.14, 174.87
Intensity ? moderate
Focal Depth 34 km
Location 30 km south-west of Wanganui
7.1 128km S of Masset, Canada 2012-10-28 03:04:11 52.863°N 131.942°W 19.0
Originally posted by kennvideo
Revised (7.1 -> 7.7): 7.7 earthquake, 139km S of Masset, Canada. Oct 27 20:04 at epicenter (10m ago, depth 9.9km)
Originally posted by Nyiah
About how far was this one from Seattle, do you think? I have a friend there that I cannot get a hold of, and am curious if they felt in some form or another. Would they have been able to feel it in that area of WA? (probably, I assume)