O b s e r v a t i o n s o f Lg and R g w a v e s and r e m a r k s 

a b o u t t h e n a t u r e o f Lg! 

S A V A R E N S K Y E . T H . a n d V A L D N E R N . G . ( * ) 

Ricevufco il 7 dicembre 1960 

The result of the seismic sounding in the Black Sea (l) showed that 
to the South of the Crimean peninsula the granitic layer is interrupted 
in the deep part of the sea. 

The preliminary data about the weakening of the Lg and Rg waves 
when they piopagate through the central part of the Black Sea was 
exposed in our previous work (**) and by M. Bath (2). 

The further investigation consists in finding the clear waves Lg 
on the continental paths. We succeeded in this comparing the records 
of waves at stations Tiksi from the earthquakes of two groups. The 
lirst group of earthquakes: Mongolia, Pamir, Southern China. The se-
cond group: Aleutian Islands, Kamchatka, Kuril Islands, Japan. 

Por the Mongolian earthquakes very sharp beginnings of waves, 
especially Lg1( having great amplitudes with the period of 2 to 1 sec 
are observed. On the records of the station Tiksi clear waves Lg and 
Rg are noticed (fig. 1). For the earthquakes of Southern China these 
waves are clear, however the amplitudes are smaller and the periods 
of the waves are greater than previous. On the whole the first group 
of the paths corresponds to the continental structure of the earth's crust. 
The mean values of the velocities of these waves are: Lgx — 3.53 km/sec; 
Lg2 - 3.31 km/sec; Rg - 3.05 km/sec. 

For the epicentres of the Aleutian Islands no clear beginnings of 
the waves Lg and Rg are distinguished. The period of 18 to 21 sec with 
the superposition of rather weak waves of short period is predominant 
on the records of surface waves (fig. 2). The direction of the paths in 

(*) Paper read at the Helsinky Assembly of the I.U.G.G., 1960. 
(**) « Annali di geofisica», X I I I , n. 2, 1960. 



9 6 S A V A R E N S K Y E . TIL. - V A L D N E R N . G. 

this case passes through the deep part of the Bering Sea (H ^ 3.5 km) 
where the granitic layer seems to be interrupted or absent. For this 
region seif-mograms of 12 earthquakes are interpreted. 

The group of the epicentres of Kamchatka, the Kuril Islands and 
J a p a n does not present a clear pattern of the interruption of the granitic 

jr-s 

Fig. 1. - Copy of a seismogTam of the seismic station Tiksi with clear Lgx wave. 

layer or the continental structure of the earth's crust. I n this case se-
ismograms with the Lg and E g waves and without them are available. 
This can be explained by the presence of a deep part in the Okhotsk 
Sea (H = 3 km) on the path of the propagation as well as the Kuril 
trench which has some effect creating the breakings of the granitic 





9 8 S A V A R E N S K Y E . TIL. - V A L D N E R N. G. 

layer. This is the cause of the doubtful cases and the disappearance of 
Lg and Rg on records. 26 earthquakes of this group are investgated. 
For some of them the mean values of the velocities are the following: 
Lg! — 3.50 km/sec; Lg2 - 3.29 km/sec; Rg - 3.06 km/sec. 

Fig. 3 shows the results of the calculations of the dispersion curves 
of the group velocities for Love waves in the two layered crust (3). The 
ordinate axis represents the ratios of the group velocity to the velocity 
of transversal waves 6, in the upper layer. The abscissa axis represents 
the value reverslv proportional to kH; (H = lil + Ii,) expressed in terms 

t.C 

||[. Curve k,/H 

°-9' t r - t f r / H 
Fig. 3. - Plot of dispersion curves of 

Love waves in the two-layered crust. 

b T 
of the period T velocity and the thickness of the crust H, namely ' - . 

H 
The parameter of the family of curves is the ratio of the upper layer's 
thickness to the total thickness of the earth's crust H. The group 
of the curves in fig. 3 corresponds to the first mode. 

The famdv of curves for the first mode shows the presence of the 
points of maximum. Por them the amplitude in the conditions of dis-
persion is great and the appearance of such waves on a seismogram 
will form as if onsets of waves. The maximum will be seen on the back-
ground of the preceding waves. 

We suppose that such is the nature of waves Lgt. This wave cor-
responds to the maxima on the dispersion curves of the first as well 
as higher modes of the Love wave. 

For the final solution of the problem about the correctness of our 
explanation of nature of the Lg! wave the observed data of the velo-
cities and periods of Lg! wave must be compared with the theoretical 
curves in fig. 4. The comparison of empirical results with the curves was 
made on different assumptions about the thickness of the earth's crust. 



O B S E R V A T I O N S OF LG A N D RG W A V E S A N D R E M A R K S A B O U T , E T C . 9 9 

Families of curves are plotted in fig. 4. The abscissa axis represents 
the period in sec., the ordinate axis — the value of the velocities based 
on the assumption that bl = 3 . 3 km/sec. Each straight line corresponds 
to the points of the maximum for the dispersion curves of the first mode 
of the Love wave at different values of the total thickness of the earth's 

bath f i ) 
• autkox* 
+ Payo SuMrtco (5) 

•—•> Pt£M , glAfilUJ,. (4) 
Olufe/t, £u/i>n^: Pxm (6) 

Fig. 4. - Plot of lines of maxima (group velocity 1st mode) and tlie velocity 
and period of t h e Lgt wave by different authors. 

crust from 20 to 60 km. The horizontal straight lines on this chart cor-
respond to different values of the ratios of the thickness of the upper 
granitic layer h1 to the total thickness of the crust H. Points designate 
the values of velocities and the periods of their onsets obtained from 
observations by different authors: for the propagation of waves from 
the Black Sea earthquakes to Swedish stations by Bath (2), for the 
waves from the Black Sea earthquakes to the Moscow and Simferopol 
stations by the authors, for the waves propagating through the USA 



1 0 0 S A V A R E N S K Y E . TIL. - V A L D N E R N . G. 

continent by Press (4). The data of the Lg! wave from various earth-
quakes obtained at the Toledo seismic station by Payo Subiza (5) and 
finaly the data obtained by Obver, Ewing and Press for the Arctic (8). 
On the whole the points corresponding to the observation data on the 
chart for the thickness of the crust from 20 to 60 km. However their 
scattering considerably exceeds the 60 km thickness of the crust. The 
relative thickness of the upper layer of the crust ranges from 0.25 to 
0.45. Thus to a first approxim- ation the agreement with our assump-
tion t h a t Lgi is as a rule the first mode of the Love wave can be 
considered satisfactory. 

Fig. 5. - Plot of lines of ma- xima group velo-
city (2<i mode) and the velocities and periods 
of t h e Lg! wave of t h e Mongolian ear t h quakes 

More detaded comparisons show that the Lgx wave can correspond 
also to the second mode of the Love wave. This is suggested by the 
fact t h a t on the path of the propagation of waves from Mongoba to the 
station Tiksi the thickness of the earth's crust is sufficiently great. If 
to use the formula permitting to estimate the crustal thickness on the 
basis of rebef (7) (or t h e map given in the same paper) the crustal thick-
ness on this path must be of the order of 45 km approximately, the same 
thickness would be obtained by means of the method of A. A. Treskov (a). 
If to extrapolate the data obtained in the process of the deep seismic 
sounding of the Northern Tien Shan (9) to the path from Mongoba to 
North then the crustal thickness must also be estimated equal to 40-45 



O B S E R V A T I O N S OF LG A N D RG W A V E S A N D R E M A R K S , E T C . 1 0 1 

km and the relative thickness of the granitic layer can be considered 
equal to 0.3-0.5. Fig. 5 is a chart of the straight lines for the points 
of the maximum on the group velocity curves of the second mode of 
the Love wave with different crustal thickness. The points representing 
the velocities and the periods of the Lg! at the station Tiksi from Mon-
golian and Chinese earthquakes when the beginnings of Lg! were the 
most clear are also plotted on this chart. These points agree with the 
presented data about the crustal thickness expected on the investigated 
path. 

SUMMARY 

This investigation consists in finding the ivaves Lg and Rg on the 
paths in the far east. On the bases of theory and observations some con-
clusions are made concerning the nature of Lgi wave as a mode of Love 
waves in two layered crust. 

RIASSUNTO 

Questa indagine consiste nella ricerca delle onde Lg ed Rg lungo i 
tragitti nelVEstremo Oriente. Sulla base della teoria e delle osservazioni 
vengono tratte alcune conclusioni che riguardano la natura dell'onda Lgl 
considerata come onda di Love in due croste stratificate. 

R E F E R E N C E S 

( ! ) N E P R O T C I I N O V Y U . P . , Deep Crustal Structure under the Black Sea to 
South-West of the Crimea from Seismic Data. « Dokladi Acad. Nauk. », 
125, 5, 1119, (1959). 

( 2 ) B A T H M., The Elastic Waves Lg and Rg along Eurasiatic Paths. « Arkiv 
for geofisic» (Stockholm), 2, 13, 295, (1954). 

( 3 ) D O R M A N N J . , Numerical Solution for Love Wave Dispersion on a half 
Space with Double Surface Layer. ((Geophysics)), 24, 1, 12, (1959). 

( 4 ) P R E S S F . , E W I N G M . , TWO Slow Surface Waves across North America. 
«Bull. Seism. Soc. Am. », 43, 3, 219, (1952). 

( 5 ) P A Y O S U B I Z A , Report During V European Seismological Comission, Spain, 
1959. 



1 0 2 S A V A R E N S K Y E . TIL. - V A L D N E R N . G. 

( 6 ) O L I V E R I . , E W I N G M . , P R E S S P . , Crustal Structure of the Arctic Regions 
from the Lg Phase. « Bull. geol. Soc. Am. >, 66, 9, (1955). 

( ' ) D E M E N I T Z K A Y A R . M . , Techniques of the Study of the Structure of the 
Earth's Mantle Crystal Part. « Soviet Geology », 92, (1959). 

( 8 ) T R E S K O V A. A., Results of the Determination of the Crustal Thickness 
from Observations of Distant Earthquakes. « Bulletin of t h e Seismolo-
gical Council of t h e USSR », Acad. Sci. USSR, 6, 76, (1957). 

( 9 ) K O S M I N S K A Y A I . P . , Crustal Structure from Seismic Data, Bulletin of 
the Moscow Society of the Nature Investigators. « Section of Geology », 
33, 4, 25, (1958).