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The velocity section of the upper mantle in the transition zone from Asia to the pacific ocean
Tectonophysics - Elsevier Publishing Company, Amsterdam Printed in The Netherlands
THE VELOCITY SECTION OF THE UPPER MANTLE IN THE TRANSITION ZONE FROM ASIA TO THE PACIFIC OCEAN
R. Z . TARAKANOV
Sakhalin Complex Scientific Research Institute, Academy of Sciences of the U.S.S.R., Novoaleksandrovsk, Sakhalin (U.S.S.R.) (Received
May 29, 1964)
SUMMARY Velocities of P waves are measured in the upper mantle of the earth to a depth of 300 km along the profile of the middle Kuriles-central Japan. Three layers - at the depths of 30-80 km, 80-125 km and more than 125 km - are distinguished within the mantle. These layers are characterized by different average velocity graaients. The presence of a wave guide with a sharp drop of 0.2-0.3 km/set in the velocity of the longitudinal waves is quite possible at the depth of more than 80 km and more than 125 km in this profile.
INTRODUCTION
During the treatment of data concerning earthquakes in the Kuriles and northern Japan, the unusually short travel times of longitudinal waves were marked in comparison with the travel-time curve of Wadati (Wadati and Oki, 1933). The average empirical travel-time curve constructed by the author, and that of Wadati for focal depths of 30 and 80 km are compared in Fig. 1. The differences in travel times increase with the epicentral distances, and reach 4-6 set for the longitudinal waves, and more than 10 set for the fictitious S-P waves, at A = 1,500-2,000 km. These differences cannot be explained only by the differences in the structure of the earth’s crust. It is naturally supposed, therefore, that the upper mantle under the Kuriles and northern Japan is characterized by higher velocities of longitudinal and transverse waves than the velocities obtained by Wadati. In this paper the dependence of velocity on depth, only for the longitudinal waves, is considered.
NEW AVERAGE
EMPIRICAL
TRAVEL-TIME
CURVES
The upper mantle velocity section was constructed on the basis of average empirical travel-time curves for focal depths of 0, 30, 60, 80 and 120 km. For the construction of average travel-time curves the most qualitative observations of 69 earthquakes for the period of 1953-1961 representing about 2,000 experimental means of travel time, were used. Tectonophysics,
2 (2/3) (1965) 227-237
228
Tectonophysics,
2 (2/3) (1965) 227-237
I
200
400
600
800
1000
I3 4.
2
@
----
no0
-A&m)
S-P
Fig.1. Comparison between empiric travel-time curves and that of Wadati for the focal depths: A. 30 km. B. 80 km. (1) The empiric travel-time curves of the P and S-P waves; (2) the travel-time curve by Wadati; (3,4) travel-time curve points based on the station data: (3) the stations of general type, (4) the stations of regional type.
0 B
H=80 km
P
230
i
Fig,2 (Legend see p.2 Tectonophysics, 2 (Z/3) (1965) 227437
UPPER
MANTLE
VELOCITIES
IN ASIA-PACIFIC
OCEAN
ZONE
231
The epicenters of the earthquakes, and the seismological stations, whose observations were used for the construction of travel-time curves, are located approximately on the south Kamchatka-central Japan profile (Fig.2). The main earthquake parameters were determined by methods independent of the adopted travel times. The mean errors in the determination of individual parameters are: for the epicenter ca. lo-20 km, for the focal depth ca. 5-10 km, and for the time in foci ca. 0.5-l sec. The use of observational data from the regional seismological stations on the south Kuriles increased the accuracy of the earthquake parameter determinations. According to their data, the arrival times of waves are determined with an accuracv of ca. O-O.2 sec. (Fedotov et al., 1561.) The location of the stations and epicenters provides for the nearly uniform distribution of observations up to distances of l,OOO-1,200 km. The standard error of experimental travel-time curves is ca. 1.5 sec.
METHODS
OF VELOCITIES
CALCULATION
The construction of the depth-velocity distribution, as it is known, is reduced to the differentiation of the empirical travel-time curve, and the estimation of Vand h corresponding to the top of the ray. The results of the determination of the derivative of the travel-time curve by two methods are shown in Fig.SA, B. In the first case the traveltime curve is represented by overlapping linear sections, and in the second by a system of overlapping parabolas (&gayevsky, 195’7). The accuracy of the determination is shown by vertical segments in Fig.SB. In both cases the derivatives are determined by the least square method. It can be seen from both figures that a general decrease of the derivatives is accompanied by their relative increase within distances of 800-950 and 1,200-1,400 km. These anomalous changes in the behaviour of the derivative are probably ‘connected with the peculiarities of the upper mantle structure in the considered region. For the determination of the dependence of velocity on depth, based on travel-time derivatives, the Gerglotz-Wichert method is generally used. (Savarensky and Kirnos, 1955.) However, it cannot be used for those parts of the curve: dt,‘dh
=-f(A)
(I)
where
the derivative increases with distance. To determine the dependence method, we were forced to change the complex empirical curve by monotonously decreasing the curve (Fig.SA). Consequently, it is imposV(h) by this
Fig.2. The earthquake epicenters and seismological stations, data of which were used for the velocity-section construction. (l-3) Classifications of the earthquakes according to magnitude: (1) 6f
2
(2/3)
(1965)
227-237
232
N
6
Tectonophysics,
2 (2/3) ( 965) 227-237
UPPER MANTLE VELOCITIES
IN ASIA-PACIFIC
-.__I_ ____
-
-,
-.-
I_ ~.-
-.____ -.-.-
Tectonophysies, 2 (Z/3) (1965) 227-237
OCEAN ZONE
233
234
h (km) 30
50
IP
IS
I
ot
I
I
1
I
2OS
-i-
I
---.-b-
2so
-1 -2 -.-.
3
.. . . .. . . , soo-
--_
3
t: @
8
0
0
1 ac
Tectonophysics,
2 (2/3) (1965) 227-237
9.0
a!5
Fig.4
(Legend
see p.235).
UPPER
MANTLE
VELOCITIES
IN ASIA-PACIFIC
OCEAN
ZONE
235
sible to find all the peculiarities of the velocity section by the Gerglotz-Wichert method. Values of velocity obtained by this method are shown by asterisks in Fig.4. We attempted to use another method of the V and h determination to consider those parts of the curve (I) where the monotonous changing of the travel time curve derivative was destroyed. For this purpose we conventionally assume that the longitudinal wave velocity increases with depth in accordance with the linear law, e.g: v=
v/8(1 +6h)
where V is the wave velocity at depth h from the MohoroviEiC discontinuity. The mean value of V, was determined as 7.8 km/set for the south Kuriles (Fedotov et al., 1961). It is necessary to note that all the calculations here were made for the subcrustal rays. The coordinates of the velocity section, corresponding to the epicentral distances il< 1,000 km (the flat earth), were determined with the aid of the formula (Gamburzev, !959):
V?--L-=_ dt/dn
d 1~ dt
where: dt
iL =
P
AUC
h =-.
2
(l- PV,)
(4)
l- v*2p2
where Auc is the epicentral distance, corresponding to the subcrustal part of the ray. When 071,000 km, as in the Gerlotz-Wichert method, the velocity at the top of the ray is determined by the formula: v=pR
R-h
Here R is the earth’s radius (without the crust).
Fig.4. The comparison between the velocity sections of the upper mantle for longitudinal waves. (1) The average linear velocity section, obtained on the base of empirical travel-time curves; (2) according to Jeffreys and Bullen (1940); (3) according to Gutenberg (1959); (4) according to Wadati and Oki (1933); (5) the supposed velocity section corresponding to the anomalous variation of the travel-time curve derivative; (6) the points of the velocity section calculated according to Gerlotz-Wichert method by the monotonous variation of the travel-time curve derivative (Fig.SA-1); (7-9) the errors of the determination of longitudinal wave velocity: (7) 6~ = + (0.054.1) km/set;(8) 6~ = l (0.11-0.2) km/set;(9) 6~ 0.2 km/set. Tectonophysics,
2 (2/3) (1965) 22’7-237
VELOCITIES
OF P WAVES
IN THE UPPER
MANTLE
Using the above method, we determined the coordinates V and /z for 59 points within the upper 300 km of the mantle. The average linear velocity section, shown as a thick line in Fig.4, was constructed from these data by the least square method. As can be seen from Fig.4, the average velocity section constructed with the help of our method practically coincides with the dependence V(h) determined by the Gerlotz-Wichert method. The average linear section, as given in Fig.4, is compared with the relations V(h) according to Gutenberg (1959), Jeffreys and Bullen (1940), and Wadati and Oki (1933). The supposedly more exact velocity section is shown by the dashed line. It is constructed by taking the above pointed anomalous variation of the travel time curve derivative into consideration. On the basis of this supposed section, L.S. Oskorbin (Sakhalin Complex Scientific Research Institute) calculated the theoretical travel-time curves, which were then differentiated. The obtained theoretical curve (I) has the same sinusoidal nature as the curves in Fig.3A, B. The parts with the anomalous variation of the derivative on this curve nearly coincide with those given in Fig.JA, B, Taking this fact into consideration, the velocity section in the considered region may be adapted as consisting approximately of the following parts with different average velocity gradients: (?.) 30-80 km; (2) 80-125 km; (3) more than 125 km. The most interesting depths are within 80-125 km. The limits of this depth interval probably may be represented both as boundary surfaces of the first type (as is shown by the line of dashes in Fig.4) or of the second type. Within these limits, a rather increased velocity gradient, as compared with the average one, is noted. Some other data also give evidence of peculiarities in the upper mantle structure of the depths under consideration. The anomalously high absorption of transverse waves, at depths of 60-110 km, is noted for the south Kuriles. Maximum absorption occurs at a depth of 80-90 km; for Japan, maximum absorption is noted at a depth of about 100 km (Fedotov, et al., 1963). The upper limit of the asthenosphere in the region of Kamchatka and Japan, according to Shebalin (1961), is a depth of 80 km. On the base of the available average velocity section, we calculated the travel time curves of longltudi& waves (0 d H (150 km, 0 CA
CONCLUSIONS
As a result of our work, we can make the following conclusions: (1) The upper mantle in the zone under consideration 1s characterized by the higher velocity of the longitudinal waves, as compared with the velocities obtained by Jeffreys, Gutenberg and Wadati. (2) The systematic deviations from the average velocity section found are due to the layered structure of the upper mantle, which is particularly distinguishable at depths of 80-125 km. (3) It is possible that the decrease in the velocity gradient of the lon-
Tectonophysics, 2 (2/3) (1965) 227-237
UPPER MANTLE VELOCITIES IN ASIA-PACIFIC OCEAN ZONE
237
gitudinal waves within depths of 125-250 km is due to the low velocity layer present at these depths. To make more definite conclusions concerning the upper mantle structure, the accuracy of earthquake parameter determinations must necessarily be increased. Along with the kinetic characteristics of the waves, the dynamic ones must be used. REFERENCES Bugayevsky, G.N., 1957. The method of parabolic approximation for the determination of the empiric function derivative. Tr. Irkutsk. Gosuniversiteta, 164-173. Fedotov, S.A., Averyanova, V.N., Bagdasarova, A.M., Kuzin, I.P. and Tarakanov, R,Z. 1961. Some results of the detailed study of the south Kurile Islands seismicity. Ann. Geofis. (Rome), 14(Z): 119-136. Gamburzev, A.A., 1959. The principles of the seimic prospection. Gostoptekhizdat, Moscow, 26 l-265. Gutenberg, B., 1959. Wave velocities below the Mohorovi&c discontinuity. Geophys. J., 2(4): 348-352. Jeffreys, H. and Bullen, K., 1940. Seismological Tables. Office of the British Assoc., Burlington House, London, 48 pp. Savarensky, E .F. and Kirnos, D.P., 1955. The Elements of Seismology and Seismometry. Gos. Izd. Tekhn. Teor. Lit., Moscow. Shebalin, N.V., 1961. Intensity, magnitude and focal depth of the earthquakes. In: Zemletryaseniya v S.S.S.R. Izd. Akad. Nauk. S.S.S.R., Moscow. Wadati, K. and Oki, S., 1933. On the travel time of earthquake waves. Geophys. NW., 7: 87-153.
Tectonophysics,.2
(2/3) (1965) 227-237
THE VELOCITY SECTION OF THE UPPER MANTLE IN THE TRANSITION ZONE FROM ASIA TO THE PACIFIC OCEAN
R. Z . TARAKANOV
Sakhalin Complex Scientific Research Institute, Academy of Sciences of the U.S.S.R., Novoaleksandrovsk, Sakhalin (U.S.S.R.) (Received
May 29, 1964)
SUMMARY Velocities of P waves are measured in the upper mantle of the earth to a depth of 300 km along the profile of the middle Kuriles-central Japan. Three layers - at the depths of 30-80 km, 80-125 km and more than 125 km - are distinguished within the mantle. These layers are characterized by different average velocity graaients. The presence of a wave guide with a sharp drop of 0.2-0.3 km/set in the velocity of the longitudinal waves is quite possible at the depth of more than 80 km and more than 125 km in this profile.
INTRODUCTION
During the treatment of data concerning earthquakes in the Kuriles and northern Japan, the unusually short travel times of longitudinal waves were marked in comparison with the travel-time curve of Wadati (Wadati and Oki, 1933). The average empirical travel-time curve constructed by the author, and that of Wadati for focal depths of 30 and 80 km are compared in Fig. 1. The differences in travel times increase with the epicentral distances, and reach 4-6 set for the longitudinal waves, and more than 10 set for the fictitious S-P waves, at A = 1,500-2,000 km. These differences cannot be explained only by the differences in the structure of the earth’s crust. It is naturally supposed, therefore, that the upper mantle under the Kuriles and northern Japan is characterized by higher velocities of longitudinal and transverse waves than the velocities obtained by Wadati. In this paper the dependence of velocity on depth, only for the longitudinal waves, is considered.
NEW AVERAGE
EMPIRICAL
TRAVEL-TIME
CURVES
The upper mantle velocity section was constructed on the basis of average empirical travel-time curves for focal depths of 0, 30, 60, 80 and 120 km. For the construction of average travel-time curves the most qualitative observations of 69 earthquakes for the period of 1953-1961 representing about 2,000 experimental means of travel time, were used. Tectonophysics,
2 (2/3) (1965) 227-237
228
Tectonophysics,
2 (2/3) (1965) 227-237
I
200
400
600
800
1000
I3 4.
2
@
----
no0
-A&m)
S-P
Fig.1. Comparison between empiric travel-time curves and that of Wadati for the focal depths: A. 30 km. B. 80 km. (1) The empiric travel-time curves of the P and S-P waves; (2) the travel-time curve by Wadati; (3,4) travel-time curve points based on the station data: (3) the stations of general type, (4) the stations of regional type.
0 B
H=80 km
P
230
i
Fig,2 (Legend see p.2 Tectonophysics, 2 (Z/3) (1965) 227437
UPPER
MANTLE
VELOCITIES
IN ASIA-PACIFIC
OCEAN
ZONE
231
The epicenters of the earthquakes, and the seismological stations, whose observations were used for the construction of travel-time curves, are located approximately on the south Kamchatka-central Japan profile (Fig.2). The main earthquake parameters were determined by methods independent of the adopted travel times. The mean errors in the determination of individual parameters are: for the epicenter ca. lo-20 km, for the focal depth ca. 5-10 km, and for the time in foci ca. 0.5-l sec. The use of observational data from the regional seismological stations on the south Kuriles increased the accuracy of the earthquake parameter determinations. According to their data, the arrival times of waves are determined with an accuracv of ca. O-O.2 sec. (Fedotov et al., 1561.) The location of the stations and epicenters provides for the nearly uniform distribution of observations up to distances of l,OOO-1,200 km. The standard error of experimental travel-time curves is ca. 1.5 sec.
METHODS
OF VELOCITIES
CALCULATION
The construction of the depth-velocity distribution, as it is known, is reduced to the differentiation of the empirical travel-time curve, and the estimation of Vand h corresponding to the top of the ray. The results of the determination of the derivative of the travel-time curve by two methods are shown in Fig.SA, B. In the first case the traveltime curve is represented by overlapping linear sections, and in the second by a system of overlapping parabolas (&gayevsky, 195’7). The accuracy of the determination is shown by vertical segments in Fig.SB. In both cases the derivatives are determined by the least square method. It can be seen from both figures that a general decrease of the derivatives is accompanied by their relative increase within distances of 800-950 and 1,200-1,400 km. These anomalous changes in the behaviour of the derivative are probably ‘connected with the peculiarities of the upper mantle structure in the considered region. For the determination of the dependence of velocity on depth, based on travel-time derivatives, the Gerglotz-Wichert method is generally used. (Savarensky and Kirnos, 1955.) However, it cannot be used for those parts of the curve: dt,‘dh
=-f(A)
(I)
where
the derivative increases with distance. To determine the dependence method, we were forced to change the complex empirical curve by monotonously decreasing the curve (Fig.SA). Consequently, it is imposV(h) by this
Fig.2. The earthquake epicenters and seismological stations, data of which were used for the velocity-section construction. (l-3) Classifications of the earthquakes according to magnitude: (1) 6f
2
(2/3)
(1965)
227-237
232
N
6
Tectonophysics,
2 (2/3) ( 965) 227-237
UPPER MANTLE VELOCITIES
IN ASIA-PACIFIC
-.__I_ ____
-
-,
-.-
I_ ~.-
-.____ -.-.-
Tectonophysies, 2 (Z/3) (1965) 227-237
OCEAN ZONE
233
234
h (km) 30
50
IP
IS
I
ot
I
I
1
I
2OS
-i-
I
---.-b-
2so
-1 -2 -.-.
3
.. . . .. . . , soo-
--_
3
t: @
8
0
0
1 ac
Tectonophysics,
2 (2/3) (1965) 227-237
9.0
a!5
Fig.4
(Legend
see p.235).
UPPER
MANTLE
VELOCITIES
IN ASIA-PACIFIC
OCEAN
ZONE
235
sible to find all the peculiarities of the velocity section by the Gerglotz-Wichert method. Values of velocity obtained by this method are shown by asterisks in Fig.4. We attempted to use another method of the V and h determination to consider those parts of the curve (I) where the monotonous changing of the travel time curve derivative was destroyed. For this purpose we conventionally assume that the longitudinal wave velocity increases with depth in accordance with the linear law, e.g: v=
v/8(1 +6h)
where V is the wave velocity at depth h from the MohoroviEiC discontinuity. The mean value of V, was determined as 7.8 km/set for the south Kuriles (Fedotov et al., 1961). It is necessary to note that all the calculations here were made for the subcrustal rays. The coordinates of the velocity section, corresponding to the epicentral distances il< 1,000 km (the flat earth), were determined with the aid of the formula (Gamburzev, !959):
V?--L-=_ dt/dn
d 1~ dt
where: dt
iL =
P
AUC
h =-.
2
(l- PV,)
(4)
l- v*2p2
where Auc is the epicentral distance, corresponding to the subcrustal part of the ray. When 071,000 km, as in the Gerlotz-Wichert method, the velocity at the top of the ray is determined by the formula: v=pR
R-h
Here R is the earth’s radius (without the crust).
Fig.4. The comparison between the velocity sections of the upper mantle for longitudinal waves. (1) The average linear velocity section, obtained on the base of empirical travel-time curves; (2) according to Jeffreys and Bullen (1940); (3) according to Gutenberg (1959); (4) according to Wadati and Oki (1933); (5) the supposed velocity section corresponding to the anomalous variation of the travel-time curve derivative; (6) the points of the velocity section calculated according to Gerlotz-Wichert method by the monotonous variation of the travel-time curve derivative (Fig.SA-1); (7-9) the errors of the determination of longitudinal wave velocity: (7) 6~ = + (0.054.1) km/set;(8) 6~ = l (0.11-0.2) km/set;(9) 6~ 0.2 km/set. Tectonophysics,
2 (2/3) (1965) 22’7-237
VELOCITIES
OF P WAVES
IN THE UPPER
MANTLE
Using the above method, we determined the coordinates V and /z for 59 points within the upper 300 km of the mantle. The average linear velocity section, shown as a thick line in Fig.4, was constructed from these data by the least square method. As can be seen from Fig.4, the average velocity section constructed with the help of our method practically coincides with the dependence V(h) determined by the Gerlotz-Wichert method. The average linear section, as given in Fig.4, is compared with the relations V(h) according to Gutenberg (1959), Jeffreys and Bullen (1940), and Wadati and Oki (1933). The supposedly more exact velocity section is shown by the dashed line. It is constructed by taking the above pointed anomalous variation of the travel time curve derivative into consideration. On the basis of this supposed section, L.S. Oskorbin (Sakhalin Complex Scientific Research Institute) calculated the theoretical travel-time curves, which were then differentiated. The obtained theoretical curve (I) has the same sinusoidal nature as the curves in Fig.3A, B. The parts with the anomalous variation of the derivative on this curve nearly coincide with those given in Fig.JA, B, Taking this fact into consideration, the velocity section in the considered region may be adapted as consisting approximately of the following parts with different average velocity gradients: (?.) 30-80 km; (2) 80-125 km; (3) more than 125 km. The most interesting depths are within 80-125 km. The limits of this depth interval probably may be represented both as boundary surfaces of the first type (as is shown by the line of dashes in Fig.4) or of the second type. Within these limits, a rather increased velocity gradient, as compared with the average one, is noted. Some other data also give evidence of peculiarities in the upper mantle structure of the depths under consideration. The anomalously high absorption of transverse waves, at depths of 60-110 km, is noted for the south Kuriles. Maximum absorption occurs at a depth of 80-90 km; for Japan, maximum absorption is noted at a depth of about 100 km (Fedotov, et al., 1963). The upper limit of the asthenosphere in the region of Kamchatka and Japan, according to Shebalin (1961), is a depth of 80 km. On the base of the available average velocity section, we calculated the travel time curves of longltudi& waves (0 d H (150 km, 0 CA
CONCLUSIONS
As a result of our work, we can make the following conclusions: (1) The upper mantle in the zone under consideration 1s characterized by the higher velocity of the longitudinal waves, as compared with the velocities obtained by Jeffreys, Gutenberg and Wadati. (2) The systematic deviations from the average velocity section found are due to the layered structure of the upper mantle, which is particularly distinguishable at depths of 80-125 km. (3) It is possible that the decrease in the velocity gradient of the lon-
Tectonophysics, 2 (2/3) (1965) 227-237
UPPER MANTLE VELOCITIES IN ASIA-PACIFIC OCEAN ZONE
237
gitudinal waves within depths of 125-250 km is due to the low velocity layer present at these depths. To make more definite conclusions concerning the upper mantle structure, the accuracy of earthquake parameter determinations must necessarily be increased. Along with the kinetic characteristics of the waves, the dynamic ones must be used. REFERENCES Bugayevsky, G.N., 1957. The method of parabolic approximation for the determination of the empiric function derivative. Tr. Irkutsk. Gosuniversiteta, 164-173. Fedotov, S.A., Averyanova, V.N., Bagdasarova, A.M., Kuzin, I.P. and Tarakanov, R,Z. 1961. Some results of the detailed study of the south Kurile Islands seismicity. Ann. Geofis. (Rome), 14(Z): 119-136. Gamburzev, A.A., 1959. The principles of the seimic prospection. Gostoptekhizdat, Moscow, 26 l-265. Gutenberg, B., 1959. Wave velocities below the Mohorovi&c discontinuity. Geophys. J., 2(4): 348-352. Jeffreys, H. and Bullen, K., 1940. Seismological Tables. Office of the British Assoc., Burlington House, London, 48 pp. Savarensky, E .F. and Kirnos, D.P., 1955. The Elements of Seismology and Seismometry. Gos. Izd. Tekhn. Teor. Lit., Moscow. Shebalin, N.V., 1961. Intensity, magnitude and focal depth of the earthquakes. In: Zemletryaseniya v S.S.S.R. Izd. Akad. Nauk. S.S.S.R., Moscow. Wadati, K. and Oki, S., 1933. On the travel time of earthquake waves. Geophys. NW., 7: 87-153.
Tectonophysics,.2
(2/3) (1965) 227-237