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Postglacial deformation of bedrock in Finland V. Juhani Ojala, Aimo Kuivamäki and Paavo Vuorela
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Abstract
V. Juhani Ojala, Aimo Kuivamäki and Paavo Vuorela 2004. Postglacial deformation of bedrock in Finland. Geological Survey of Finland, Nuclear Waste Disposal Research. Report YST-120, 23 pages, 10 figures. Glacial isostatic adjustment controls the three-dimensional deformation in Fennoscandia. Maximum vertical uplift rates based on the GPS measurements are about 11 mm/yr and horizontal motions are up to 2 mm/yr. Tectonic component is about 10% of the land uplift (or 1 mm/yr). Horizontal motions are directed outward from area of the fastest uplift. Horizontal tectonic motions are also less than 1 mm/yr. Seismic activity in Finland is low and heterogeneously distributed and the earthquake density maximums and the areas of postglacial faults have a spatial correlation. Detailed geodetic surveys indicate that crustal deformation occurs unevenly. However, the bedrock in Finland is so fractured that the deformation is distributed over a number of structures and that deformations and displacements along individual structures are very small and difficult to resolve. Fault intersections can form a locked area where stresses large enough to trigger intraplate earthquakes can build up. In the absence of intersections, the pre-existing faults can creep at a lower stress threshold. In Fennoscandia, plate-boundary tectonic stresses drive the regional compressive stress field, but to account for the current level of seismicity the glacial isostatic adjustment has a very important role. Brittle crust is near the point of failure, and, consequently, small changes, like glacial rebound related, (0.1 Mpa) in the state of stress can nucleate earthquakes are sufficient to reactive optimally oriented pre-excising weaknesses. Stress orientations inferred from the strain measurements of the first order triangulation network and seismological stress data shows a) the dominating ridge-push/mantle drag related compression and, b) evidence on significant local variations of the surface stress field influenced by the orientation of major fracture zones. Postglacial faults are re-activated old faults and the areas of postglacial faulting are still the most seismically active areas in Fennoscandia. The association of seismicity with glacial rebound suggests that in areas experiencing diminishing rebound, the level of seismicity decreases over time. Therefore, palaeoseismicity and geological modelling have to be combined to predict the likely incidence, magnitude and frequency of future earthquake activity.
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Table of contents
Abstract...................................................................................................................................1
Table of contents ....................................................................................................................2
1 Introduction ....................................................................................................................3
2 Seismic activity ..............................................................................................................3
2.1 Earthquake focal mechanisms ................................................................................4
3 Postglacial faults.............................................................................................................9
4 Recent bedrock movements..........................................................................................12
4.1 Vertical movements..............................................................................................12
4.2 Horizontal movements..........................................................................................15
4.2.1 Detailed measurements of horizontal movements........................................15
5 Discussion and conclusions ..........................................................................................19
References ............................................................................................................................21
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1 Introduction
Postglacial and currently occurring bedrock deformation can be divided into episodic and
continuous. Episodic deformation happens as measurable seismic events, which may be
related to certain faults and is mostly brittle. Continuous deformation needs longer
observation period and is detected by accurate geodetic methods like traditional levelling and
modern GPS measurements and include elastic and plastic deformation, and brittle
deformation, which occurs in such small events that they cannot be separated with currently
used instrumentation.
Purpose of this paper is to summarise the current knowledge of the postglacial deformation of
bedrock in Finland. These are divided into recent earthquake activity, postglacial faulting,
glacial isostatic rebound and current horizontal crustal deformation. There are exhaustive and
detailed case studies on postglacial faults or other deformational features and structures,
which have resulted from bedrock deformation after glaciations and cut glacial or later
deposits in Finland and Fennoscandia (eg. Kuivamäki et al. 1998; Kuivamäki and Vuorela
2002, Olesen et al. in press). In addition, tectonics and deformation related to glaciations are
discussed in several papers in a Special issue of the Quaternary Science Reviews Volume 19,
Issues 14-15) (2000).
2 Seismic activity
Earthquakes are the most obvious manifestation of the currently occurring bedrock
deformation. In Fennoscandia, the earthquake records date back to 1375 but the early events
have been estimated from historical records and the current understanding is largely based on
20 years of microseismic data collected with modern instrumentation. In the Fennoscandia,
the seismic activity is low and is concentrated offshore near the coastline of Southern and
Middle Norway and few onshore regions (Fig 1., Table 1). Onshore activity has lower activity
and magnitudes, especially in Finland where the magnitude of all earthquakes in human
records have magnitudes less than five (Fig 2 and 3.). However, the large postglacial fault
would have had magnitudes up to eight (Kuivamäki et al. 1998). The most earthquake active
areas are Southern Sweden, Swedish coast of Gulf of Bothnia, Northern Fennoscandian
postglacial fault province and Kuusamo. Note that Kuusamo and Southern Sweden are the
only onshore areas where postglacial fault have not been recognised yet. In Finland, within
the modern observation period, it has not been possible so far unequivocally to point out
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individual faults, which have moved during earthquakes. The ongoing detailed geodetic and
GPS measurements around the some of the postglacial faults (eg. Poutanen and Ollikainen,
1995; Ahola, 2001) will reveal the magnitude and extend of deformation in future.
Several studies demonstrated that the brittle crust is near the point of failure, and that very
small changes (0.1 Mpa) in the state of stress can nucleate earthquakes (eg. Thorson 2000).
Consequently, the stress build up due to deformation of the brittle crust is released mainly
through microseismisity below the detection limit of sparse seismic network and large
earthquakes are rare. In addition, most of the earthquakes happen at 5 to 20 kilometers depth
without a surface trace. Faults may eventually break to the bedrock surface and disturb glacial
deposits or features. Disturbances of glacial deposits or recent land surface are rare and in
many cases these are related to instability of slopes, rock faces, water-saturated sediments on
land or under water (Kuivamäki et al. 1998; Olesen et al. in press). In addition, ground
shaking related to earthquakes can trigger the failure of instable slopes considerable distances
away from the epicentres.
2.1 Earthquake focal mechanisms
The earthquake focal mechanism resolves the main stress directions and mode of deformation.
An earthquake is generated through stress build-up and subsequent release. Regional stress
generating mechanisms typically dominate and focal mechanism indicates stress direction in
reasonably large area. By grouping many focal mechanisms the impact of local features can
be reduced and the regional stress imprint and the regional tectonism can be enhanced.
Unfortunately, in Finland, the low level of seismic activity, the sparse coverage of the national
seismic network and the limitations of the instrumentation have restricted the source
mechanism inversions to few optimally situated earthquakes (Uski et al. 2003). Figure 4
shows the available fault plane solutions and inferred horizontal stress directions. The stress
field in Finland is characterized by NW–SE-oriented horizontal compression caused by the
ridge-push force or mantle drag resulting from the opening of the North Atlantic Ocean. The
prevailing orientation of maximum horizontal stresses has been confirmed by in situ stress
measurements and earthquake observations (eg. Saari, 1992; Zoback, 1992; Tolppanen and
Johansson 1996).
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Summary:
• Earthquakes are the most obvious manifestation of the current bedrock deformation.
• The seismic activity and magnitudes in Finland are lowest in the Fennoscandia
• Areas of postglacial faulting are still the most seismically active areas in Fennoscandia
• Earthquake focal mechanisms gives indication of stress state in large rock mass
Table 1. Northern Europe Earthquakes 1375-2003 (Compiled from the Bulletins of earthquakes of the Institute of Seismology, the University of Helsinki). Magnitude Number of Earthquakes
Year 2002-2003 Number of Earthquakes Year 1375-2003
1 115 4944 2 153 1190 3 57 889 4 5 255 5 1 48 6 0 2 7 0 1
2500000
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3000000
3000000
3500000
3500000
4000000
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6000
000 6000000
6500
000 6500000
7000
000 7000000
7500
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8000
000 8000000
Post-glacial faults
Eqmag_d600 - 0.001
0.001 - 0.002
0.002 - 0.003
0.003 - 0.004
0.004 - 0.005
0.005 - 0.006
0.006 - 0.007
0.007 - 0.008
0.008 - 0.009
0.009 - 0.01
0.01 - 0.02
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0.06 - 0.08
0.08 - 0.12
0.12 - 0.16
0.16 - 0.2
0.2 - 0.24
No Data
Earthquake density
0 500 Kilometers
Figure 1. Earthquake density and main post-glacial faults in Fennoscandia. All earth quakes from 1375 to 2003 from the Bulletins of Earthquakes of the department of seismology, University of Helsinki. Earthquake density has been calculated using 60 km search radius. Post-glacial faults from Kuivamäki et al (1998).
KuusamoSweden
Norway
Russia
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0 500 Kilometers
Post-glacial faults
Fshield_eq_kkj.shp0.1 - 1 Magnitude#
1 - 2#
2 - 3#
3 - 4#
4 - 5#
5 - 6#
Earthquakes 2002-
Figure 2. Earthquakes 2002- 2003 from the Bulletins of Earthquakes of the department of seismology, University of Helsinki. Post-glacial faults from Kuivamäki et al. (1998).
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Earthquakes > Mag 5
Figure 3. Earthquakes over magnitude 5 in Northern Europe. All earth quakes from 1375 to 2003 from the Bulletins of Earthquakes of the department of seismology, University of Helsinki. Post-glacial faults from Kuivamäki et al. (1998).
8
Figure 4. Available fault plane solutions for the earthquakes in Finland (lower hemisphere equal area projection). The epicenters (asterisks) are shown on a seismotectonic map, filled circles denoting earthquakes occurred in 1970– 2000. Structural features are modified from Korsman et al. (1997). The compressional quadrants of the new fault plane solutions are shaded with dark grey and those of the earlier solutions with light grey. The black and white dots show the trend and plunge of the P- and T-axes, respectively. Broad arrows show the interpreted main horizontal component of stress. Note that in the Kuusamo area, where the stress field is extensional, the stress field deviates significantly from the broadly NW-SE stress field. Modified from Uski et al. (2003).
9
3 Postglacial faults
The bedrock has deformed several times during its geological history and it has resulted in the
mosaic like block structure, where fracture zones surround more or less intact blocks of
different sizes. The old block structure existing now is very stable, i.e. the sizes and
surrounding are fracture zones at optimum orientation for any tectonic stress direction to
reactive them with very small loads and to release tectonic stresses. Large-scale postglacial
faults were discovered in 1960’s in northern Finland (Kujansuu 1964, 1972) and later
elsewhere the northern Fennoscandia (Lagerbäck 1979, Olesen et al. in press). The faults are
classified as postglacial as the faults cut the Quaternary deposits overlaying the fault zone or
the bedrock surface polished by glacial ice and have displacements from few centimeters to
30 metres. They formed very soon after deglaciation and were accompanied probably the
most remarkable seismic events with magnitudes up to eight in Fennoscandia (Kuivamäki et
al. 1998; Olesen et al. in press)
Significant postglacial faults cutting quaternary deposits have been recognized only in
northern part of Fennoscandia (Figs 1-3). Only small PG-faults located in ice polished
bedrock outcrops and with displacements up to 20 cm have been found in southern Finland,
This does not however rule out the possibility of larger displacements. The main part of
southern Finland belongs to an area, which has been below the highest shoreline and the
coastal erosion and deposition may mask the faults. However, there might not be large faults.
Recent modelling suggests that these different scales of glacial load may have induced very
different crustal responses. In particular, Johnston et al. (1998) have shown that the patterns
and magnitudes of stress in a flexed lithosphere depend on the dominant wavelength of the
load (twice the ice-sheet diameter). Earth-model parameters used by Johnston et al. (1998),
horizontal stresses are amplified to their greatest extent for ice loads with a radius of ca 280
km. This could explain why the largest faults have are in Northern Fennoscandia where faults
could have formed at the stage of deglaciation when the shrinking ice sheet had reached this
size.
All large postglacial oriented NE-SW have a reverse sense of slip and they dip SE
(Kuivamäki et al 1998, Olesen et al. in press). Johnston (1987, 1989) noted an apparent
tendency for large continental ice sheets to suppress tectonic activity. Conversely, unloading
would increase seismic activity as illustrated in Figure 5 (Muir-Wood 1989). Ice load was
10
removed last from the western parts of Fennoscandia, rebounding eastern parts with NW-SE
compression favored reactivation weaknesses trending NE-SW and dipping SE.
The studies of the large Fennoscandian postglacial faults indicate that they are reactivated
faults (Kuivamäki et al. 1998, Olesen et al. in press) of which history have started well in
Precambrian. However, the small faults in Ilmantsi are very brittle and are only broadly same
orientation as foliation (Kuivamäki et al. 1998).
Summary:
• Large postglacial faults are rare and they formed soon after deglaciation
• Large postglacial faults are reactivated old fault zones
• Large postglacial fault are result of unloading stress release after deglaciation
• Bedrock stress is dominated by Mid-Atlantic ridge push/mantle drag
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Figure 5. Diagrammatic representation of the impact of glacial loading (a) and unloading (b) on the crust and the resulting stress changes in a region with a compressive stress regime. Note the considerably exaggerated vertical scale difference between the crustal thickness (~100 km) and the ice-sheet thickness (~3 km) (Modified from Stewart et al. 2000). Schematic Mohr diagrams illustrating stabilization of structures under the ice load as the vertical stress (lithostatic load plus glacial load) (σ3) increases and differential stress decreases. During unloading (σ3) decreases, differential stress increases which can lead to failure of a plane of weakness.
σ3 σ 1
()
normal stress
shearstress
failure envelope for plane of weakness
failure envelope for intact rock
σ3 σ 1
()
normal stress
shearstress
failure envelope for plane of weakness
failure envelope for intact rock
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4 Recent bedrock movements
The earths crust deforms in three dimensions but for the practical reasons the traditional
geodetic practice has been to observe these separately. Just over a decade, GPS positioning
has allowed three-dimensional observations of crustal movements (eg. Milne et al. 2001).
4.1 Vertical movements
The land uplift rate was first measured from tide gauge recording and precise levelling in
Fennoscandia (Fig. 6). This allowed also the total amount of uplift to be estimated (Fig. 7). In
addition, this data was used to model the tectonic and glacial rebound components in the
present uplift (eg. Kakkuri 1987). The principle was that the misfit between observations and
the isostatic uplift modelling was interpreted to reflect a tectonic component of the uplift (Fig.
8). Anomalies in the uplift has been studied by several workers (Kakkuri 1987;Chen 1991;
Fjeldskaar et al. 2000; Milne et al, 2001) and all of them have come to same conclusion that
the tectonic component of the uplift is order of 10% (or 1 mm/yr).
Veriö et al. (1993) studied the present vertical bedrock movements in detail in several areas
with levelling using profiles crossing fault zones. They found few areas where the uplift
deviate substantially from predictions based on uniform elastic uplift. These results together
with the results of the third precise levelling of Finland carried out by the Geodetic Institute of
Finland (Lehmuskoski 1996), suggest that the present day land uplift can at regional scale
considered plastic (deformation is distributed into large number of structures), but at local
scale there can be small block movements. These block movements are preferentially
concentrated within zones of pre-existing weakness. However, it has not been possible to
pinpoint individual structures, which would have controlled the deformation, and the
deformation distributed in a number of structures.
Summary:
• Postglacial isostatic adjustment is dominating the land uplift in Finland
• Tectonic component of the land uplift is about 10%
• Regionally uplift can be considered plastic but locally there are block movements
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500km
0
333333333 222222222000000000111111111
555555555444444444
777777777
666666666
999999999888888888
Figure 6. Recent land uplift rate mm/yr in Fennoscandia (Modified from Ekman 1996)
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200 0
Moscov
km
London
Paris
100
800
500600
700
400300
200
-150 -100
0
0
-170 -150
-100
Figure 7. Total uplift contours (in metres) of Fennoscandia. The solid line shows the limit of the last glacial maximum (modified from Mörner 1980)
Figure 8. Areas with significant (a) negative deviations (<1.0 mm/yr) and (b) positive deviations (>1 mm/yr) between the observations and the calculated glacial isostatic uplift (modified from Fjeldskaar et al. 2000).
15
4.2 Horizontal movements
First estimations of the horizontal crustal deformations in Finland were based on
measurements of the first order triangulation network. Chen (1991) analysed the horizontal
crustal deformations in Finland using the measurements of the first order triangulation
network with observation spanning from 1920 to 1985. At the large scale, compression in the
NW-SE direction is dominating (Chen 1991). Furthermore, Chen (1991) showed that the
strain pattern is heterogeneous (or has a “block like structure”) (Fig. 9). He calculated the
change rates of some lines across proposed geological boundaries to describe the relative
movement between adjacent blocks found extension rates up to 21.8±10.4 mm/yr on a line
140 km long in Pohjanmaa and compression rates up to 10.2±6.6 mm/yr along a 83 km long
line in Kajaani. Although results are consistent, these measurements have large errors
compared to the strain rates. However, the stress field correlate well with seismological data
presented above.
The GPS and the Finnish Permanent GPS Network offer possibility to get data on horizontal
bedrock movements in shorter observation periods (Chen & Kakkuri 1994). The knowledge
of the deformation of Fennoscandian shield in three dimensions is continuously improving as
more data is collected by the BIFROST GPS-network of Fennoscandia (Milne et al.2001).
The first analysis of BIFROST data confirms the glacial isostatic adjustment dominates the
ongoing three-dimensional crustal deformation in Fennoscadia (Milne et al. 2001). The
vertical uplift maximum 11.2 ± 0.2 mm/year is located near the site Umeå. Horizontal
movements are directed outward from this location on all sides (Fig.10). In further agreement
with the numerical predictions, these rates increase with distance away from the uplift centre,
and they reach about 2 mm/year at sites marking the perimeter of the BIFROST network.
Consequently, the uplift in Fennoscandia causes extensional component to the stress field.
The horizontal rates are higher in the western side the Gulf of Bothnia than eastern side
(Milne et al. 2001).
4.2.1 Detailed measurements of horizontal movements
The first horizontal movements revealed by GPS were measured in the Lake Lappajärvi area
in 1990, where sinistral horizontal movement (111±30 mm) was detected and the movement
had possibly taken place in connection with Lappajärvi earthquakes in 1976 (Veriö 1992).
16
The Geodetic institute of Finland and the National Land Survey of Finland have established
levelling and GPS networks at the Pasmajärvi and Nuottavaara areas to monitor possible
movements (Kuivamäki et al 1998). In the Pasmajärvi–Ruostejärvi area, there are a NW-SE
trending main fracture zone. Measurements have not unequivocally resolved any movements
along the Pasmajärvi PG-fault (Ahola 2001). Due to small deformation rates in these areas,
and elsewhere in Finland, more measurements and time will be needed for sound conclusions.
Also the levelling network established in the same area by the National Land Survey of
Finland may produce new results in relevellings. Many fault-lines indicate small movements
in different parts of Finland (Kuivamäki et al 1998), but most postglacial faults may have
stayed inactive after a burst of activity soon after the deglaciation.
Summary:
• Glacial isostatic adjustment controls the horizontal crustal deformation in Finland
• Horizontal velocities are up to 2 mm/yr of which tectonic component is less than 1 mm/yr
• Horizontal strain in Finland is heterogeneous and geologically controlled
17
Figure 9. Principal strain distributions and their directions in Finland. The length of the bars shows the relative magnitude. (modified from Chen 1991).
18
Figure 10. a) Horizontal velocity vectors estimated at each of the BIFROST sites. The scale of the vectors, as well as with the associated 1σ error ellipses, is given at the base of the plot (Milne et al. 2001). The location of known PG faults have been added to the map. b) Horizontal velocity vectors and uplift rate from the motion solution calculated by Johannson et al. 2002.
a)
b)
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5 Discussion and conclusions
Clearly, the glacial isostatic adjustment and related land uplift horizontal motions in addition
to earthquakes are the most significant indications of the ongoing bedrock deformation in
Fennoscandia, and these are closely connected. In Fennoscandia, plate-boundary tectonic
stresses drive the regional compressive stress field, but to account for the current level of
seismicity, other regional or local mechanisms must operate, such as glacial rebound present
day uplift (Stewart et al. 2000). Brittle crust is near the point of failure, and, consequently,
small changes (0.1 Mpa) in the state of stress can nucleate earthquakes (King et al. 1994,
Thorson 2000). These changes can be a result a nearby or a distant tectonic event, long-range
elastic adjustments. Modelling results strongly suggest that glacial rebound related stress
changes are sufficient to reactive optimally oriented pre-excising weaknesses and that both
tectonic forces and rebound stress are needed to explain the distribution and style of
contemporary earthquakes in Fennoscandia (Wu et al. 1999). Furthermore, the association of
seismicity with glacial rebound suggests that in areas experiencing diminishing rebound, the
corresponding level of seismicity decreases over time. Therefore, palaeoseismicity and
geological modelling have to be combined to predict the likely incidence, magnitude and
frequency of future earthquake activity.
Seismic activity is heterogeneously distributed and the earthquake maximums and postglacial
fault have a good spatial correlation. In addition, detailed geodetic surveys indicate that
crustal deformation occurs unevenly. However, the bedrock in Finland is so fractured that the
deformation is distributed between many structures. Therefore, displacements along
individual structures are very small and difficult to resolve. However, fault intersections can
form a locked area where stresses large enough to trigger intraplate earthquakes can build up.
In the absence of intersections, the faults can creep at a lower stress threshold; this slow creep
is probably the dominating deformation style in Finnish bedrock.
Stress orientations inferred from the strain measurements of the first order triangulation
network and seismological stress data shows a) the dominating ridge-push/mantle drag related
compression and, b) evidence on significant local variations of the surface stress field
influenced by the orientation of major fracture zones.
Modeling done by Kakkuri (1987) Chen (1991) and Fjeldskaar et al. (2000) suggest that
currently the tectonic uplift component is in order of 1 mm/a or about 10% in some areas.
20
Recent three-dimensional movement measurements provided by a network of 34 GPS stations
in Fennoscandia (Milne et al., 2001) indicate extensional component caused by glacial
unloading. The horizontal velocities are smallest in the vicinity of the uplift centre, and they
are directed outward from this area on all sites with up to two mm/yr velocities at the
perimeter of the GPS network.
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References
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