Postglacial deformation of bedrock in Finland V. Juhani Ojala, Aimo Kuivamäki and Paavo Vuorela

1

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

3

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

4

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).

5

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

2500000

3000000

3000000

3500000

3500000

4000000

4000000

6000

000 6000000

6500

000 6500000

7000

000 7000000

7500

000 7500000

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

0.02 - 0.03

0.03 - 0.04

0.04 - 0.06

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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Post-glacial faults

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

11

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

12

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

13

500km

0

333333333 222222222000000000111111111

555555555444444444

777777777

666666666

999999999888888888

Figure 6. Recent land uplift rate mm/yr in Fennoscandia (Modified from Ekman 1996)

14

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)

19

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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