Ewing, W. Maurice (William Maurice), 1906-1974

Press, Frank, 1924-

Columbia University Academic Commons

Columbia Slmbersttp ftt ttjc C(t? of ftcto Pot6 LAMONT GEOLOGICAL OBSERVATORY PALISADES, NEW YORK Technical Report on Seismology No. 2 5 Mantle Rayleigh Waves from the Kamchatka Earthquake of 4 November 1952 LAMONT GEOLOGICAL OBSERVATORY (Columbia University) Palisades, New York * j{e * Technical Report No. 25 (CU 31-53-A Fl^( 122)441 GEOL) Mantle Rayleigh Waves from the Kamchatka Earthquake of 4 November 1952 by Maurice Ewing and Frank Press The research reported in this document has been made possible through support and sponsorship extended by the Geophysics Research Division of the Air Force Cam¬ bridge Research Center under Contract AF19( 1 22)441. It is published for technical information only and does not represent recommendations or conclusions of the sponsoring agency. February 1953 1. ABSTRACT Mantle Rayleigh waves from the Kamchatka earthquake of 4 November 1952 are analyzed. The new Palisades long period vertical seismograph recorded orders R6 - R15, the correspond¬ ing paths involving up to 7 complete passages around the earth. The dispersion data for periods below 400 sec are in excellent agreement with earlier results and can be explained in terms of the known increase of shear velocity with depth in the mantle. Data for periods 400-480 sec indicate a tendency for the group velocity curve to level off, suggesting that these long waves are influenced by a low or vanishing shear velocity in the core. Ab¬ sorption studies indicate a value 1/Q=370 x 10“^ for the internal friction in the mantle at periods 250-350 sec. This is a little over half the value reported earlier for periods 140-215 sec. 2. INTRODUCTION In a previous paper^ an investigation of amplitudes and dis¬ persion of mantle Rayleigh waves was presented using data from three earthquakes recorded on the Pasadena linear strain and Benioff seismographs. Rayleigh waves in the period range 1-7 minutes in¬ volving paths with as many as three circuits of the earth (R2 - R7) were described. Orbital motion proper for Rayleigh waves was de¬ monstrated and a group velocity curve of high precision was deduced and accounted for in terms of mantle structure. Analysis of the am¬ plitudes led to values of internal friction in the mantle for periods of 140 and 215 sec. A little over a year ago a special long period vertical seis¬ mograph (To=15 sec, Tg=90 sec) was placed in operation at Palisades for the purpose of recording mantle Rayleigh waves and other long period oscillations. The instrument was quite sensitive, allowing for operation with a highly overdamped pendulum and galvanometer, yet with sufficient magnification to record at the limit imposed by microseisms. Response to long period oscillations was improved in this way. It was found necessary to maintain the instrument at constant temperature as well as to provide buoyancy compensation 1. Maurice Ewing and Frank Press, "An Investigation of Mantle Ray¬ leigh Waves ", Technical Report on Seismology No. 23, also in press Bull. Seism. Soc. Amer. 3. for the pendulum so that spurious oscillations originating in atmo- spheric pressure fluctuations could be removed. A more complete report on the seismograph will be presented separately. On November 4, 1952 a large earthquake occurred on Kam¬ chatka. B.C.I. S. gives for epicentral data 52. 9°N, 160. 1°E, H= 16-58-23 and Pasadena magnitude was 8 1/4. The Palisades in¬ strument recorded mantle Rayleigh waves or orders R6 - R15, the corresponding paths involving as many as 7 complete passages around the earth. Earlier orders were off scale and could not be read. The Pasadena linear strain seismograph wrote an excellent record of RJL, R2 and R4. These long period oscillations, continuing for more than 20 hours after the instant of the shock, could be positively identified with the proper order of mantle Rayleigh wave by use of the dispersion 3 curve derived in the earlier paper. The new data presented in this paper is highly significant in that it extends the observed propagation distances of mantle Rayleigh waves to 288, 370 km, provides additional points for the group velocity curve with a suggestion that the trend of the curve for periods longer than 400 sec is influenced by the core, and supplies an additional 2. Maurice Ewing and Frank Press, Further Study of Atmospheric Pressure Fluctuations Recorded on Seismographs, Trans. Amer. Geophys. Union, in press 1953. 3. See footnote 1. 4. value for the internal friction of mantle rocks for periods 250-350 sec. 5. DATA The original Palisades long period vertical seismogram appears in Figure 1. Various orders of mantle Rayleigh waves are indicated by arrows. The short period crustal Rayleigh waves from the numerous after shocks often occur riding on the longer period mantle oscillations of the main shock, making record reading dif¬ ficult at these times. The method of analysis is similar to that described in detail 4 in the previous paper and only the results will be given here. Table I lists arrival times, travel times and velocities for the different periods observed in each order of mantle Rayleigh waves. The results are presented graphically in Figure 2 where group velo¬ city is plotted as a function of period. Data from our previous paper are included. It can be seen in Figures 1 and 2 that only R6 exhibits both the long period and short period branch representing respectively trains of waves whose periods decrease and increase with time, the two trains merging finally in the Airy phase at the minimum value of group velocity. Although the short period branch of R2 alone is listed, there is a slight but definite indication of a long wave on the Pasadena seismogram. Accurate reading of the 4. See footnote 1. 6. period is made difficult by the insufficient separation of the long waves at this short propagation distance. Similarly R1 appears only as a long period pulse, the propagation distance being too small for any dispersive separation of the component waves. The remain¬ ing orders R7 - R15 appear only as waves corresponding to the long period branch, the propagation distances becoming so large as to lose the shorter period oscillations through their higher absorption. 5 The new data reaffirms the earlier conclusion that a single dispersion curve represents all the orders and that there is no sys¬ tematic departure with increasing length of path. It was also con¬ cluded in the previous paper that the spectra of the various orders are determined primarily by dispersion, since R3, R5 and R7 cover progressively narrower period ranges entering about 225 sec, the period for minimum group velocity. This was in agreement with the well known relation between amplitude and the slope of the group velocity curve. It can be seen from Table I that with increasing order the spectra center on periods from 250-350 sec. Apparently the greater selective absorption of the shorter periods becomes an important factor in determining the spectra of the higher order mantle waves. The relative amplitudes of the higher orders can be used to deduce the internal friction of mantle rocks. If the decay of amp- 5. See footnote 1. 7. litude with distance is given by -va /A i ~ A0 ^71 where Ao is constant for a given period and Y is the absorption co¬ efficient, a plot of In ( Ai *A^ ) should give a straight line whose slope is Y . The average trace amplitudes of R6, R7, R8, R9, RIO and R12 for the period range 250-350 sec are listed in Table II. Average amplitudes are given for this period range rather than amplitudes for specific periods in order to minimize the effect of secondary amplitude variations that occur within a train but show no correlation from order to order. These may originate in inter¬ ferences due to refraction or simultaneous arrival of two orders of mantle waves, both effects becoming more pronounced with increas¬ ing propagation paths of the higher order mantle waves. Reference to Table II shows that the odd order mantle Ray¬ leigh waves are consistently smaller in amplitude than the even order waves. The same phenomenon is apparent on the Pasadena seismogram. Since propagation effects can in no way account for this amplitude variation it must originate in an asymmetric radiation pattern at the source. In Figure 3 a plot of In ( Ai*A^ ) against A is presented for the period range 250-350 sec. Ordinates corresponding to odd order waves have been multiplied by a constant factor to allow for 8. their smaller amplitudes. It is seen that points fall on a reasonably straight line whose slope gives the value X =. 0009 deg~^=. 000008 km From the expression j_ _ y CT Q ■?1 where the phase velocity C = 4. 8 km/sec we derive 1/Q=370 x 10"^ for the internal friction of the mantle in the period range 250-350 sec. The corresponding wavelength is about 1500 km. Earlier re¬ sults^ for periods of 215 and 140 sec (wavelengths of 1000 km and 600 km respectively) gave 1/Q=670 x 10” . These values are within the same order of magnitude. There is a suggestion of a decrease of internal friction with increasing period. Whether this is due to the dependence of internal friction on period or to greater penetra¬ tion of the longer waves to deeper and different mantle layers is difficult to say. In view of the constancy of 1/Q between 140 and 215 sec, the latter alternative is favored. 6. See footnote 1. 9. DISCUSSION For periods under 400 sec the new data discussed in this paper is in excellent agreement with our previous results. With the use of a one layer approximation it was shown that the disper¬ sion was consistent with the known increase of shear velocity with depth in the mantle. Several values of group velocity for periods between 400 and 480 sec are determined for the first time in this study (see Figure 2). Rather than plotting as a continuation of the trend of the group velo¬ city curve for periods under 400 seconds, these points indicate that the group velocity begins to level off for periods greater than 400 sec or wavelengths greater than about 2000 km. Since the depth to the core is 2900 km, the new trend of the group velocity curve sug¬ gests that wavelengths of the mantle Rayleigh waves have become sufficiently long to feel the effect of a low or vanishing shear velo¬ city in the core. In view of the small number of observations for periods great¬ er than 400 sec this conclusion must be regarded as tentative. A new seismograph now in the design stage should provide highly signifi¬ cant data on the dispersion of mantle Rayleigh waves as well as their absorption for the long periods where the core becomes an important factor. Effects of gravity and sphericity of the earth will also become increasingly important for the longer periods. 10. CONCLUSIONS Study of mantle Rayleigh waves from the Kamchatka earth¬ quake of 4 November 1952 has resulted in additional dispersion data from R2, R4, R6, R7, R8, R9, RIO, Rll, R12, R13, R14, and R15. For periods under 400 seconds the group velocities are in ex¬ cellent agreement with our previous observations. A few values of group velocity for periods 400-480 sec have been obtained for the first time and indicate that the dispersion curve begins to level off in this range. It is suggested that this is an effect of penetration to the core. Calculation of the internal friction from absorption of waves with periods 250-350 sec gives the value 1/Q=370 x 10-^. This is a little more than half the value deduced in an earlier study for periods 140-215 sec. 11. s TABLE I Dispersion Data 4 Nov. 1952 R2 (Pasadena) ^2=33,450 km Uncorrected Period Travel time Travel time Velocity time* in sec h-m- s in sec km/ sec 19:28:10 112 02:30:02 9002 3. 72 19:31:00 175 02:32:52 9172 3. 65 19:34:20 225 02:36:12 9372 3. 57 R4(Pas adena) A4=73 i, 450 km 22:22:40 320 05:24:32 19472 3. 77 22:30:20 280 05:32:12 19932 3. 68 22:38:20 220 05:40:12 20412 3. 60 R6 (Palisades) A 6 = 111, 600 km 00:43:00 352 07:44:40 27880 4. 00 00:51:00 340 07:52:40 28360 3. 94 01:01:00 317 08:02:40 28960 3. 85 01:11:00 285 08:12:40 29560 3. 78 01:22:00 270 08:23:40 30220 3. 69 01:33:00 262 08:34.40 3088C 3* 6 r 01:45:00 253 08:46:40 31600 3. 53 01:37:00 245 08:38:40 31120 3. 59 01:34:00 225 08:35:40 30940 3. 61 01:31:00 200 08:32:40 30760 3. 63 01:28:20 184 08:30:00 30600 3. 65 01:26:00 160 08:27:40 30460 3. 66 R7 (Palisades) A 1- 128, 380 km 02:21:20 320 09:23:00 33780 3. 80 02:41:0r0 270 09:42:40 34960 3. 67 12. TABLE I (Cont. ) R8 (Palisades) ^8=151,600 km Uncorrected Period Travel time Travel time Velocity time in sec h-m-s in sec km/sec 03:16:40 400 10:18:20 37100 4. 09 03:28:00 340 10:29:40 37780 4. 01 03:40:00 330 10:41:40 38500 3. 94 03:54:00 320 10:55:40 39340 3. 85 04:10:00 310 11:1i:40 40300 3. 76 04:26:00 290 11:27:40 41260 3. 67 04:41:00 240 11:42:40 42160 3. 60 R9 (Palisades) A 9=168,370 km 04:52:40 335 11:54:20 42860 3. 93 05:15:00 315 12:16:40 44200 3. 81 05:38:00 305 12:39:40 45580 3. 70 05:49:20 260 12:51:00 46260 3. 64 R10 (Palisades) A 10 = 191, 590 km 05:55:00 480 12:56:40 46600 4. 11 06:02:00 420 13:03:40 47020 4. 07 06:12:00 340 13:13:40 47620 4. 02 06:32:20 320 13:34:00 48840 3. 92 07:02:00 315 14:03:40 50620 3. 78 07:28:00 290 14:29:40 52180 3. 67 07:47:20 250 14:49:00 53340 3. 59 Rll (Palisades) A 11 = 208, 370 km 08:06:00 330 15:07:40 54460 3. 83 R12 (Palisades) A 12= 231, 590 km 08:49:00 365 15:50:40 57040 4. 06 09:30:00 320 16:31:40 59500 3. 89 10:30:20 290 17:32:00 63120 3. 67 R13 (Palisades) A 13= = 248, 370 km 11:28:20 300 18:30:00 66600 3. 73 * Clock corrections: Pasadena +15 sec Palisades +3 sec 13. TABLE I (Cont, ) R14 (Palisades) Al4 =271, 590 km Uncorrected time* Period in sec Travel time h-m- s Travel time in sec Velocity km/ sec 12:26:00 330 19:27:40 70060 3. 88 R15 (Palisades) Al5=288,370 km v 13:10:00 355 20:11:40 72700 3. 97 * Clock corrections: Pasadena +15 sec Palisades +3 sec 14. TABLE II Palisades Amplitude Data 4 Nov. 1952 Order A A B y, ln(A b A ) R6 1004° 4. 5mm 4. 96 R8 1364 2. 3 4. 44 RIO 1724 1. 6 4. 20 R12 2084 1 3. 82 R7 1156 2 4. 22 R9 1516 1.4 4. 00 Rll 1876 1. 1 3. 86 Kilometers per Second MANTLE RAYLEIGH WAVE GROUP VELOCITY 4.6t 44- 42- 40- 3.8 3-6- 34 0 4 NOV. 52 R 2 O PA$ R 3 R 4 R 5 R 6 R 7 R 8 R 9 RIO R 11 R 12 R 13 R 14 R 15 BPAS X1PAL A PAL t,PAL APAL cnPAL t£b PAL Opal Cb PAL <bPAL T7 PAL 15 AUG 50 OPAS □.PAS + PAS Apas 2 MARCH 33 ♦ PAS • PAS ■ PAS 8 SEPT 48 ♦ pas 100 200 4 N L — — cz. N THEORETICAL 2= 6.15 km/sec ' /3|r 4 48km/sec ( P / ' * \ / o / * c b N / / IT ; , H1- 516 km P~ 7 r \ L ( HASKFI 1 ) \ \ / / C7 N C P . 7/ / \ / ► X L J □j b \ / \ /A C A 4 \ A / n 3 U \ [7 94 / - \ /l \ > □ dC yt k 0 X / J hr rs > V 4 i • Nl^ \ X t 0 4 L f 1 LAJ 2,1 J - 300 400 46 4 4 42 40 -38 ■36 ■3 4 500 Period in Seconds FIGURE 2. Observed group velocity for mantle Rayleigh waves compared to theoretical curve based on single layer approximation. T= 250-350 sec .1/2 InAzxA for even orders 0 l.l In Az x A for odd orders A a A^ □ A r 1— J r - 20 ~>l Kb L Ry ii RIO ll Rll L R12 il 1000' 1500' 2000‘ 2500° Ain degrees FIGURE 3. Graph of logarithm of product of trace amplitude and dispersion factor to obtain absorption constant. 

Mantle Rayleigh waves from the Kamchatka earthquake of 4 November 1952

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Mantle Rayleigh waves from the Kamchatka earthquake of 4 November 1952

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