Deep structure of Fennoscandia from fundamental and higher mode dispersion of rayleigh waves

139

Tecfonoph_vslcs, 195 (1991) 139-149 Elsevier Science Publishers

B.V

. Amsterdam

european

Deep_ structure of Fennoscandia from fundamental and higher mode dispersion of Rayleigh waves Gildo Calcagnile Dlpurtrmento dt Geology

e Geojwca. Unruersrtci dr Barr, Ban, It&, and Osservatorro dl Geofurca e F~isrccr Cosmrca, Facoltd dl Screnre, Unroersctri dr Ban, Ban, IIO(~ (Rewed

version accepted

geotraverse

Apnl 20.1990)

ABSTRACT Calcagnile, G., 1991. Deep structure of Fennoscandia from fundamental and lugher mode dispersion of Rayleigh waves. In: R. Freeman. M. Huch and St. Mueller (Editors), The European Geotraverse. Part 7. Tectonophysxs, 195. 139-149. Surface wave dispersion for the Baltic Shield area has been Investigated through analysis of phase velocltles up to periods of about 250 s for the fundamental Rayleigh mode and about 80 s for the first two higher modes. Tlus has enabled investigation of the lithosphere-asthenosphere system and deeper regons down to about 400 km. The shear wave velocitydepth relationship obtained from the inversion of dispersion data for the paths COP-KEV/COP-KRK can be considered as representative of an average model for the Baltic Shield. Moreover the mvewon of dispersion data supplemented t,y prewous results from the Scandmavian area suggest the presence of signifxant lateral heterogeneity down to depths of abcut 350 km moving from the older Precambrian shield (COP-KEV/COP-KRK) to the penpheral area (NUR-COP). Models wth small. If any. differences in average shear velocity are consistent with the data m the depth range 350-450 km for the regions studled.

Introduction

preferred even if by this procedure the accuracy obtainable in phase velocity determinations is less than the accuracy proper of dispersion relationship obtained by stacking. The latter process actually leads to a less ambiguous interpretation of

Every major plate, except for the Nazca and Pacific plates, which are mostly oceanic, includes a Precambrian shield. The analysis of available dispersion data indicate that these shield areas have a deep, relatively rigid, high-velocity root extending to depths of at least 200 km. By using

complicated responses and to a better phase velocity determination for modes that are not dominant in amplitude

stacking techniques, Nolet (1976, 1977) has obtained dispersion relationships for the first seven

In previous Clymer, 1973; Calcagnile and

Rayleigh modes in western Europe, inferring structural information down to about 800 km. In a re-analysis of these data, Cara et al. (1980) divided the whole area in a southern

and a northern

However, since western Europe heterogeneous area (e.g. Panza

part.

is a tectonically et al., 1980a,b),

0 1991 - Elsewer

Science Publishers

and Panza,

1976; Nolet,

upper

investigations (Noponen, 1966; Neunhbfer and Guth, 1975: Panza, 1978; Stuart 1978) the

crust

and

mantle

structure

paths were

of the Fennoscandian studied by analysing

under

several

area (Baltic fundamental

Shield) mode

surface wave dispersion with the two-station method. The highest measured period in those studies was around 150 s, allowing the derivation of elastic properties to depths of about 200 km. In view of the importance of providing useful constraints on hypotheses of geodynamic processes such as glacial rebound and plate dynamics, we

with heterogeneities extending down to depths of at least several hundred kilometres (e.g. Calcagnile and Scarpa, 1985). the previous results should be considered as gross regional averages. Thus the use of the standard two-station method, which allows the sampling of more limited areas, may be 0040-1951/91/$03.50

(e.g. Nolet

1977).

B.V

have tried velocity

to extend

to higher

measurements

modes of Rayleigh

mental

of fundamental

the phase and higher

waves. The results of the inver-

sion of phase velocity longer periods

penods

measurements

than obtained

carried

out at

up to now for funda-

and higher modes should allow the acquisi-

tion of more information

on the deeper

parts

of

the mantle. Data In this paper

we report

the results

obtained

with the two-station method (Bnme and Dorman, 1963) applied to a set of events recorded at WWSSN

Fennoscandian

stations

coordinates and earthquake ters are given in Table 1.

(Fig. 1). Station

hypucentral

parame-

For the measurement of higher Rayleigh modes (Panza and Scalera, 1978) the records were digitized at 2 s intervals and processed with a multiple narrow-band

filter system

(e.g. Levshin

et al.,

1972) to obtain time-energy diagrams. However, knowledge of the group velocity tained

in such a manner

fication

is not adequate

of higher modes.

Possible

100

200

Fig. 1. Long-period seismograph network and station lines used to determine phase velocity profiles of Rayleipfi waves.

ob-

for identi-

erroneous

iden-

tification of the modes can be solved by comparing phases that have been determined using frequency filtering and time windowing against

TABLE

I i

theoretical

diagrams

of the phase dispersion

of the

first modes. The derived phase velocity relationships are shown in Fig. 2. Dispersion curves for the same

1

Coordinates

of earthquakes

and recording stations

Location

h (km)

Origin

Time

Olh 26m 36.8 s

Earthquake hypocentral data Kuriles

43.2 N,

146.2 E

76

23 June, 1964

Honshu

40.7 N,

142.9 E

41

29 March, 1965

10h 47m 38.4 s

Hokkaido

44.2 N,

145.5 E

159

25 Oct., 1965

22h 34m 22.4 s

Taiwan

24.9 N,

122.6 E

102

1 July, 1966

05h 5Om 38

Shikoku

33.3 N,

132.3 E

48

5 Aug., 1%8

16h 17m 05.5 s

Hokkaido

44.9 N,

143.2 E

238

19 Jan., 1969

07h 02m 07.9 s

Burma

23.1 N,

94.7 E

124

17 Oct., 1969

Olh 25m 11.5 s

Seismic sratrons Copenhagen

(COP)

55 41’OO”N,

12 26’OO”E

Kevo

(KEV)

69 45’19”N,

27 00’24”E

Kirkenes

(KRK)

69 43’27”N,

30 03’45”E

Kongsberg

(KON)

59 38’57”N.

09 35’54”E

Nurmijarvi

(NUR)

60 30’32”N.

24 39’05”E

Umea

(UMW

63 48’54”N,

20 14’12”E

s

DEEP

STRUDURE

OF FENNOSCANDIA

FROM

DISPERSION

OF RAYLEIGH

WAVES

$ ‘5 8

s

DEEP

STRUCTURE

OF FENNOSCANDIA

FROM

DISPERSION

OF RAYLEIGH

k

WAVES

.I

0..

.n

143

path

obtained

agreement,

from

while

different

events

dispersion

paths are in general

curves

systematically

are in good for different

different.

ever, we must note that. taking

KRK, which both cross longitudinally almost the same area of the shield and are almost equal in

How-

into account

length.

their

In the inversion

dispersion

values

step the average

will therefore

be used.

of their The re-

error bounds the curves are not completely resolved from one another: single “dispersion areas”

sults for the path COP-UME are accounted for if we observe that this path is located in the south-

partly

ern part between

overlap,

particularly

for

the

first

higher

For the fundamental ues in the period KON-KEV the shield.

the previous

As far as higher

mode.

path That

mode,

relatively

range 60-130 bordering might

s are found the western

be explained

no comparable

low val-

modes

data

paths.

values

are available

are concerned, for the area.

for the

Such modes might be compared

side of

by Nolet (1976, 1977) and Cara et al. (1980) with different

by a mixed

into

path of 50-80s shield structure and 50-20% younger Caledonian structure, as suggested by

techniques.

account

that

However, the modes

with those found it should

be taken

obtained

by these

authors are averaged over an area including the Baltic Shield and north-central Europe and this area is very heterogeneous (Panza et al., 1980a,b;

Clymer (1973) for a similar path. The lowest values are found for paths COP-NUR and KONNUR, which is to be expected since these paths lie along the southern edge of the shield. These values are, comparable to those obtained for north-central USA by Biswas and Knopoff (1974). Higher val-

Calcagnile and Scarpa, 1985). Higher mode dispersion relationships seem to be more different for the second than for the first higher mode; the second higher mode values for

ues are found

NUR-COP

for the paths COP-KEV

and COP-

at long periods

are lower than

TABLE 2 Phase velocities (H.M. = higher mode) Period (s)

Error (km/s)

Phase velocity (km/s) Fundamental

I H.M.

II H.M.

CF

(I

CII

(a) COP- -NUR 277.7

4.90

0.12

250.0

4.72

0.12

166.7

4.37

0.06

100.0

4.20

83.3

4.15

62.5 50.0

4.08

41.6

4.04

0.06 5.60

0.06

5.25

5.91

5.03

5.60

0.06

0.12 0.10

0.12

0.10

0.12

0.06

31.3

3.90

4.72

5.13

0.07

0.10

0.10

25.0

3.79

4.63

5.09

0.11

0.12

0.12

0.12

(6) COP-KEV

and COP-KRK

250.0

4.74

0.12

208.3

4.62

0.10

125.0

4.34

0.06

100.0

4.28

71.4

4.23

5.41

6.37

0.06

0.12

50.0

4.10

5.03

5.56

0.06

0.10

0.10

41.6

4.05

4.90

5.35

0.06

0.10

0.10

31.3

3.90

4.73

5.07

0.10

0.10

0.10

25.0

3.79

4.68

4.93

0.12

0.12

0.12

0.06

those

DEEP

STRCICTURF

OF FENNOSCANDIA

for COP-KEV/COP-KRK, previously,

taking

An example ments

of the quality

3 for relevant are

plotted

COP-KRK accuracy

the

and COP-NUR; are

obtained

from

of the data is given Single

paths

the estimates the

spread

for each single period,

the

error

given in Table

of the

with the rms error

of

we assumed measurement,

which was larger than the scatter among together

of the

small number

of the single

Average

phase velocity

the single

for the curve

2.

values of the whole data

set (fundamental

and higher

COP-NUR

COP-KEV/COP-KRK

and

modes)

for the paths which

are used in the inversion step are given in Table 2 together with the associated errors estimated on the basis of the spread

COP-KEV/

measurements

values.

measure-

145

WAVES

if as we said

resolved.

phase data. Due to the sometimes estimated

OF RAYLEIGH

their error bounds

profiles.

for

DISPERSION

even

into account

the curves are not completely in Fig.

FROM

that,

in the data. We stress again

also in this case,

considered

together

phase

with

give two “dispersion

their

areas”

Obviously

it cannot

differences

in the final models

by

different

However. “absolute”

velocity

ing cross sections

in Table

2.

overlap.

ruled

out

that

can be determined

assumed

as we have already

if

partly

be generally

arbitrarily inversion

error

that

values.

cross

sections.

said, ours is not an

but an inversion

are constrained

whose start-

by ea-lier results

Inversion

(mainly Calcagnile 1982; Guggisberg.

The inversion was carried out using the hedgehog procedure (e.g. Biswas and Knopoff, 1974).

Berthelsen, 1987). We therefore believe that the results of the inversion are of some significance. The S- and P-wave velocity and density m the

We are aware of the heterogeneity present uppermost 200 km of the investigated

in the area

(Calcagnile, 1982; Guggisberg, 1986). At this stage, however, we wish to focus our attention on the deeper parts of the upper mantle; hence we inverted phase velocity data for two selected paths. These correspond to two regions, each of which, according to previous studies, may be considered as being relatively homogeneous (Calcagnile, 1982). Actually, looking at the lithosphereasthenosphere system map (or at the lid map) given by Calcagnile (1982) one can see that COPKEV/COP-KRK is a mixed path of more than 80% shield structure with a lithosphere thicker than 150-170 km (and low, if any, contrast between the lid and the low-velocity layer). In contrast, COP-NUR with lithosphere

is more than 90% in a region 120-130 km thick and there is a

clear low-velocity layer lid contrast. This is also supported by Guggisberg and Berthelsen’s (1987) results. All the other paths are more complex. Thus we believe that the path COP-KEV/COP-KRK can be considered at this stage as representative of the average deep model of the Baltic Shield. A detailed analysis of the deep lateral heterogeneity in the area will be the subject of a forthcoming paper.

different

mantle

and Panza, 1978; Calcagnile, 1986; and Guggisberg and

layers

have been

fixed at com-

monly used values and by taking into account the FENNOLORA results (Guggisberg, 1986). Small differences from the quoted values respect have a significant influence

do not in this on the results

of the inversion (e.g. Knopoff and Panza, 1977). The cross sections used in the inversion are given in Table 3. The average crustal thickness in the area is Moho candia, 45 km

about 40 km: the greatest depths to the are found under northeastern Fennoswhere a fairly uniform thickness of about with a maximum in the Bothnian area of

about 50 km is observed. In contr.lst. crustal thicknesses of around 30 km have been found in the coastal area of southern Fennoscandia (Bungum

et al., 1980). Accordingly

we have fixed

at about 42 km the crustal thickness for the path COP-KEV/COP-KRK while for COP-NUR it has been fixed at 35 km (small fluctuations in crustal parameters do not significantly affect the inversion results for the periods that we are dealing with). The fine structure of the subcrustal layers down to about 90-100 km. based on Lund (1979). Cassel and Fuchs (1979) and Guggisberg (1986), was taken into account both for the path COPKEV/COP-KRK, which is almost coincident with the Fennolora profile, and for COP-NUR, which occurs on the edge of the shield.

TABLE

3a

Hedgehog

model for upper

mantle:

COP-NUR Layer thickness

Depth (km)

(km)

Y (km/s)

P Wcrn’b

r’r (km/s)

_-_

.._-_

__

0

crust plus fine subcrustal

layers

90 hd

P4

P2

8.5

3.35

P5

P3

8.4

3.50

P6

Pl

8.8

3.65

490-P4-P5-P6

Pl

9.8

4.00

90

5.2

10.0

4.10

100

6.1

11.15

4.40

300

6.35

11.7

4.60

CQ

6.50

12.0

4.70

90 + P4 channel 90 + P4 + P5 subchannel 90 + P4 + P5 + P6 “spmel”

layer

580 670 770 1070

Confidence Parameter

limits 0 = 0.065 km/s;

single point rejection

if I6C 1 > c (Table 2)

Range

Pl (km/s)

4.65

(0.10)

5.25

P2 (km/s)

4.35

(0.15)

4.80

P3 (km/s)

4.30

(0.10)

4.80

P4 (km)

15

(30)

75

P5 (km)

35

(70)

245

P6 (km)

15

(60)

195

P7 (km/s)

4.70

If in the inversion the difference between computed and experimental phase velocity is larger than _tt (Table 2) at each individual period, the model is rejected. For models that pass this test we check whether the standard deviation (rms) is less than a fixed value (in our case 0.065 km/s). The inversion result is summarized in a simplified way in Fig. 4. The models obtained for the path COP-NUR agree in the uppermost 200 km with those already published (Calcagnile, 1982). Moreover, the bottom of the low-velocity channel is found around 350 km; beneath, the subchannel, which has a shear velocity V, of 4.80 + 0.15 km/s, reaches an average depth of about 500 km and overlies a

(0.20)

5.10

layer with V, = 5.0 + 0.1 km/s corresponding to the “spinel” layer. For the region corresponding to COP-REV and COP-KRK, the layer that we have named “channel” [in the cross section (Table 3) it is formed by two layers separated by the lid] represents an average structure that is similar to that already given by Calcagnile (1982). The average shear velocity in the “channel” is 4.6 & 0.1 km/s down to about 210 km where it decreases slightly for the next 20-80 km. This supports Sacks et al’s. (1979) finding on the depth of the lithosphereasthenosphere in the Gulf of Bothnia area based on body wave studies. Below this layer the shear velocity increases to 4.7-4.8 km/s down to about

DEtP

STRUCTUKE

TABLE

OF FENNOSCANDIA

FROM

DISPERSION

OF RAYLEIGH

WAVES

(km)

V, (km/s)

147

3b

Hedgehog

model for upper

mantle:

COP-KEV/COP-KRK Layer thickness

Depth (km)

P (g/cm3)

VP (km/s)

0 crust

fme aubcrustal

plus

105 channel

layers

1

P5

P2

8.2

3.40

P5

P3

87

3 50

P6

P4

8.5

3.50

P7

Pl

8.8

3 65

475-2P5-P&P7

5.0

9.x

4.00

90

5.2

10.0

4 10

100

6.1

11 15

4.40

300

6.35

11.7

4.60

M

6.50

12.0

4.70

105 + P5 hd 105 + 2P5 channel

2

105 + 2P5 + P6 subchannel 105 + 2P5 + P6 + P7 ” spIneI” layer 580 670 770 1070

Confidence

hmlts u = 0.065 km/s;

smgle point rejection

If I6C 1 > c (Table

II). Different

values (4.9 and 5.1 km/a)

for the “spmel”

layer were also tested Parameter Pl (km/s)

4.65

(0.10)

5 25

P2 (km/s)

4.3

(0.10)

4.80

P3 (km/s)

4.5

(0.15)

4 80

P4 (km/a)

44

(0 10)

4 70

P5 (km)

30

(10)

P6 (km)

55

(20)

70 95

P7 (km)

15

(60)

195

450 km. In the deeper regions the average shear velocity is similar to that for COP-NUR, and models with small, if any, differences are con-

also made it possible for the first time to measure higher mode phase velocity at periods useful for investigating deep mantle structure.

sistent

Analysis of the fundamental and the first two higher Rayleigh modes has allowed information to be gathered on the upper mantle structure down to depths of about 400 km. A possible average mode1 has been obtained for the deep structure of the Baltic Shield. Moreover, the inversion of phase velocity data together with earlier results indicates

with both data sets.

Conclusions The analysis of Rayleigh wave dispersion has allowed phase velocities in the Fennoscandian area to be measured up to about 250 s for the fundamental mode for several new paths and to be extended to much longer periods than was previously possible.

Moreover,

this type of analysis

has

that Fennoscandia is characterized by a significant lateral heterogeneity not only in the uppermost 200 km as already found in previous studies, but

148

StiEAR VELOCITY IKMIS)

COP-NUR

COP-KEV-KRK

Fig. 4. Shear wave velocity distributions in the upper mantle. The heavy line represents a possible average velocity value. Da&xl lines give the depth uncertainty in the velocity distribution.

also down to depths of about 350 km, as pointed out also by Fuchs et al. (1983) and as is evident from the FENNOLORA experiment. Even though more deta2ed analysis is required to delineate the pattern of the heterogeneity, in-

version-of selected paths has opened up the possibility of finding homogeneous layering below about 350 km. fn the central part of the Baltic Shield this seems to be the depth to w&h the bottom of the shield might extend.

Dt.EP

STRUCTURE

OF FENNOSCANDtA

FROM

DISPERSION

OF RAYLEIGH

Acknowledgements

WAVES

149

Guggisberg,

B. and

velocity

I gratefully

acknowledge the financial support of the Italian Ministry of Education (MPI 40%).

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