PRELIMINARY RESULTS ON UPPER MANTLE STRUCTURE IN CENTRAL FROM SPECTRAL ANALYSIS OF P WAVES

PIERRE MOLJRGUES

ER lS1 du C.N.H.S., Luborufoire de séismofogie ylobule, Insfifuf de physique du globe, 5, rue René-Descartes. 6YOA’d Sfrasboury Cedex. On femporary depufafion from: O.R.S.T.O.M., 24, me Buyard, Y5008 Paris

RÉSUMÉ

RÉSULTATS PRÉLI~~INAIRIzS SUR LA STRUCTURE DU K4wrEAu SUPERIEUR DE L'AFRIQUE CENTRALE D'APRÈS L'ANALYSE SPECTRALE DES ONDES P

Les spectres d’accélération des ondes P, de séismes dont les distames épicentrales sont comprises entre l” et Go, ont été obtenus à partir d’enregistrements sur bandes magnétiques effectués en deux stat.ions : 1’0bservatoire de Bangui, et. la station t,emporaire de Yongossaba. La stabilité des spectres otant constatée pour des séismes de même origine, les variations de la fréquenw du maximum spect.ral sont: considérées - compte tenu de la cohérence des observations - comme une fonction de la distawe épicentrale (fig. 2) qui peut être altérée par des inhomogénéités latérales du Manteau Supérieur. Le fact.eur de qualité Q est déterminé à partir de la pente (négative) des amplitudes spectrales vers les hautes fréquences. Les valeurs expknentales de T/Q sont calculkes en fonction de la distame, T étant le t.emps de propagation, (tabl. II). A la base cl’une couche à faible atténuation (Q élevé) assimilée à la Lithosphère, un niveau à forte atténuation (Q faible) est mis en évidence, à la fois par les deux méthodes d’analyse (maximum spectral et T/Q), et assimilé à l’asthénosphére. L’effet de ce niveau est part.ic.ulièrement net à 14-150 (fig. 2 et 8). Au-delà, entre 200 et 23-250, les différences dans les variations du spectre global (maximum speckral) et de T/Q sont interprétées c.omme résultant des c.onditions particulières de propagation dans un milieu à forte atténLiat.ion qui correspond - en accord avec les travaux de nombreux aut,eurs-- au systkme des rifts de l’Afrique orientale. Ces c-ondit.ions affectent: peu la valeur de T/Q, il est donc possible, indépendamment des variat,ions latérales qui modifient. le spwtre global, de proposer une distribution approximative du facteur de Qualité pour le manteau supérieur c,orrespondant à la structure du bouc.lier africain.

Acçeleration spcctra of P waves are obtained from magnetic tape recordings of trvo seismological stations, Bangui Observatory and Yongossaba a temporary station. Displacements of the frcquetq of spectral maximum are considered as a fonction of the epicentral distance. and ray paths. Prom the &pe

O.R.S.T.O.M., Geophys., 110 17, 1980: 21-36. 21

‘L-1 of spectra tourards high frcquencies, the variation of T/Q is calculated; T being propagalion time and Q being mcan value of Qualify Facior. The existeme of a lorv Q laycr is inferred from bath these types of investigations. Differenccs betuwen global-spectral variations and TlQ suggest that structural particularitim, as discussed by various authors, cari bc associated with rift areas. An approxirnate Q distribution is proposed, from T/Q data, for Upper Manfle structure under the African shield.

Introduction

Bangui seismological observatory is extremely well sit,uated for body wave stuclies bot.11in the t,ime and the frequency domain for various reasons. It,s geographic.al sit.uation, of a wide .aI shield, ensures a very low microseismic noise. Thr Upper mantlc structure under the platform and basins may be considered as unchanging except, for the Rift area. Epicentral distances being alrnost, continuously covered from about, 10, comparison of earthquakes from widelv separatecl (continental and oceanic origin) is possible. The station is equippecl, among other~instruments, with a system of FM recording on magnetic tape by triggering (CHOUDHURY and HOURI, 1973). This syst.em had been operating at Bangui2(40 26.1’ N, 1% 32.8’ E, ait,. 412 m) and Yongossaba, a temporary station (40 53.5’ N, 2X0 57.9’ E, ait.. 600 m). Willmore scismometers, of 2.3 second natural periocl with a damping of 0.8 were used t.o carry out this st.udy on P waves. Earthquakes used in t.his papcr are from various regions in ancl around t,he African c.ontinent. (fig. l), in t.he distanc.e range 1 t.o 45 degrecs. Main interest. is centcred on earthquakes frorn West,ern-

FIG. 1. - 11Zapof epicenters. Events arc indicated ngainst. thr recording station B for Bangui and Y for ‘T70ngossah tcnlporarystation. Cflrfe des épicentres. Ln sfafion d’rnregisiremenf esf indiqu&e pour chaque séisme: 8 pour Bangui, 1’ pour ~‘o!rgos.s~rf),r. African-Rift . Different parameters of events are given in Table 1; t.he recording st,ation is indicated by B for Bangui and Y for Yongossaba. Some earthquakes utilised, for whic.h no USCGS determinat,ion is available, are not reported in Table 1, a number to Bangui c,at.alogue of earthquakes is only given in t.he figure or in the text.

1. Spectral variations with epicentral distances

1.1. PRELIMINARY C:ONDITIONS The observed ground acceleration spectrum A(f) çan be written : A(f)=A~(f)*A.~(f).&(f)*A.i(f) (1) where A,, A.,, A, and Ai are respectively Source func.tion and transfert. function of mantle, trust. and instrument. If we consider the clifferential equation of A corresponding to various earthquakes obtained at the same station with the same instrument: epicentral distance A and eart.hquake energy E are variable ; but. A, and Ai remain constant. We c.an write: [dA=A,.Ai[dii,.A,+clA,.li”]]

Taking the following assumptions: - A, is independant of A for a given seismic zone (which is mainly here African Rift from 80 to 250) where earthquakes lie at a roughly constant depth (ND) - &,, is independant. of energy E -- Ao is dcpendant on E, but neglecting t.his dependancy for a small magnitude range bet,ween 4.3 and 5.5 (most of earthquakes used are from 4.7 to 5.4) we may write: SA dA=Ao.Ac.Ai$dA

Whcn normalized functions A,, A, and Ai are considered to be constant, the variations of normalized spectra A with A cari be directly corrclated t,o the variation of A,,.

1.2 EXPERIMENTAL DATA Acceleration spectra are obtained by band-pass analog flltering with a Mhron-Hite filt,er of 48 db/octave att,enuation. The demodulated signal is successively replaid in the frequency range 0.1 to 8 Hz. Steps of filtering are: 0.1 Hz from 0.1 to 1 Hz, 0.2 Hz from 1 to 2 Hz and 1 IEz from 3 to 8 Hz. Measured amplit.ude of P waves, first arrivai, for every frequency is corrected where necessary for the instrument reponse to obt.ain spec.tra. These spectra are gat.hered after normalisation on a map (fig. 2), where each earthquake is denoted at the corresponding epicentral dist,ance, any spectrum being a c.ross section of thc map along a given ordinate. Sometimes, spect.rum of a second arrivai, of stronger amplitude is given (fig. 3 and 6). This second arriva1 does net start more than 3 sec.onds after the first. The time duration of the window including bot11 t,he arrivals does not exceed 6 seconds. The global map of spectra is given in fig. 2 as a function distance showing coherent variation of two consecutive spectra with respect to distance. The map shows only few of many earthquakes B2 Mar.14.72 14 05 45.79 39.277 N 29.420 E 5.4 36.2 B26 Apre 5.72 w 27 31.36 5.193 N 61.922 E 5.2 43.2 B29 Apr.lO.72 02 06 53.17 21.434 0 52.829 E 6.1 40.4 B31 Apr.11.72 02 21 15.74 0.967 II 28.286 W 6.0 46.9 B34 Apr.18.72 ,5 07 49.05 2.976 s 28.707 E 5.4 12.6 B37 Apr.25.72 00 59 49.76 9.133 a 33.466 E 4.7 20.1 B41 Sep. 2.72 14 53 51.42 31.494 x 16.296 E 5.0 27.1 B69 Jan.13.73 06 05 42.38 16.808 3 28.367 E 5.0 23.4 870 Jan.14.73 13 36 59.78 6.854 s 30.302 E 4.5 16.3 BBO Feb. 2.73 10 15 02.09 7.710 1 59.565 E 4.4 40.9 B88 Fcb. a.73 19 "5 21.90 10.377 a 13.014 w 5.3 34.7 B91 Feb.10.73 14 37 35.08 2.520 B 66.424 E 5.3 47.8 B144 (Y4B) Mûr.28.73 13 35 04. 5 11.673 I 42.930 E 5.0 25.1 B145 (Y4Q Iar.28.73 13 42 06.68 11.739 H 42.746 E 5.3 25.0 8146 (Y4D) l~Lxr.28.73 14 18 52.34 11.703 n 42.927 E 5.4 25.2 B162 May 11.73 13 52 31.70 33.354 ii 57.376 E 5.1 46.3 B176 M?g 24.73 07 15 08. 5 10.341 s 33.764 E 4.3 21.2 Bl94 JU1.18.73 19 39 13.00 II.192 s 34.447 E 4.7 22.2 BZII N0iov.21.73 00 50 30.17 3.607 S 28.186 E 5.1 12.5 B216 Dec. 1.73 16 5, 14. 0 0.647 :J 29.517 E 4.8 11.6 lx250 Pf?b. 1.74 00 0, 02.37 38.552 H 27.015 E 4.9 34.9 B256 FCb.10.74 16 29 26. 4 2.9 s 23.3 E 4.7 8.7 B271 Ilîr.17.74 07 31 25. 7 13.284 PJ 30.761 E 4.2 14.9 B284 (Y141 Apr.25.74 00 03 49.10 0.995 B 30.09, E 5.0 11.9 B314 Jun.20.74 02 44 19.79 3.132 I 31.303 w 5.0 49.7 B316 Jun.22.74 23 30 15.02 41.262 il 23.032 E 5.1 37.0 B325 Jun.27.74 18 46 25.71 1.520 I 30.841 w 5.4 49.4 B32R Jun.M.74 13 26 24. 7 16.0 El 39.6 E 23.7 B330 JUl. 1.74 23 II 14.54 22.642 S 10.674 W 5.6 39.3 3338 JUl. 9.74 02 32 17. 6 36,652 R 28.449 E 5.0 33.5 B356 AU& 1.74 09 36 27. 0 16.7 S 28.0 E 5.1 23.1 B357 AU& 4.74 15 06 17. 1 42.3 N 45.9 E 5.4 45.0 B441 mv.13.74 02 36 25. 5 42.720 14 46.557 E 5.1 45.R I1442 KOV.14.74 13 22 33.06 38.518 H 23.123 E 5.0 34.3 B443 tkw.14.74 14 26 45. 8 38.5 A 23.0 E 5.1 34.3 B444 BOV.14.74 15 29 44. 8 38.5 PJ 23.1 E 5.0 34.3 B452 DQC, 2.74 09 05 44. 2 27.99, 0 55.819 E 5.4 42.5 B47 I 3m. 9.75 ,a 53 43.95 34.756 Ii 24.088 E 4.6 30.7 B47: Jan. 9.75 23 09 46.63 42.889 ï 46.987 E 5.2 45.9 B499 War.26.75 03 40 48.75 5.450 s 30.207 E 5.1 15.2 B5O5 Mar.29.75 09 36 20.97 13.385 14 50.738 E 5.4 32.9 8512 Apr. 4.75 05 16 16.20 38.093 il 21.980 E 5.4 33.8 rd518 Apr. 6.75 04 52 07.63 05.105 s 27.713 E 4.7 13.2 B522 Apr. 7.75 14 3" 10.52 37.521 S 30.881 E 5.1 43.4

B533 Apr.19.75 13 45 50.07 14.461 14 56.515 E 5.3 38.7 B534 Apr.19.75 17 10 54.80 14.514 A 56.510 E 5.0 38.7 B539 Apr.28.75 02 0, 16. 9 33.w5 11 54.825 E 5.3 44.4 YA- (Cf 81441 20.7 YA-4c (cf B145) 20.7 YA-4D (cf B146) 20.9 YA-II 06. 06.73 14 13 54.16 34.431 14 25.250 E 5.1 29.6 YA- 07. 04.73 17 36 42.78 11.690 11 43.02, E 4.7 20.9 YA- 15. 04.73 13 13 33.38 7.183 s 30.274 E 4.7 14.1 Y -14 (cf 8284) B.0 Y -22 29. 04.74 20 04 39.65 30.529 w 31.721 E 4.9 26.9 Y -63 0,. 07.74 23 II 14.54 22,642 S 10.674 w 5.6 42.9 Y -71 09. 07.74 02 32 17.60 36.652 I 28.449 E 5.0 32.5 Y -76 13. 07.74 15 57 25. 2 36.0 N 4.8 E 4.8 35.3 Y 123 17. 09.74 14 30 54.94 8.051 a 32.08, E 15.8 Y 147 23. 10.74 1, 46 55.96 1.009 s 15.968 W 4.9 39.3 Y168 14. 11.74 13 22 33.05 38.510 N 23.123 E 5.0 33.6 Y 169 14. IL.74 14 26 45.d3 38.530 N 22.991 E 5.1 33.6 Y 173 17. 11.74 15 05 47.67 32.805 11 55.073 E 5.2 40.9

u N.D.(noml Depth) - 45 - - 44 - -43-2

- 42 - 3 -41- w 2fIIIVA--k:?--- B 29 +s 2 FI~. 2. -- Global spcctra dickibUtiOIl of P waws - 39 - E as a function of epicentral distance. The legrnd n - 38 - is indicakd by an esample of a cross section. - 37 - Distribution spectrale des ondes P en foncfion de In disfance t;picrntrale. La Ggende est donne’e par - 36- une coupe. I :A------Y 76 - 35 - - 34 - - 33- - 32- - 31 - - 30 -

- 29 - - 28 -

--B4l - 27 - ---Y 22 - 26- ----B 145 - 25- -24-

---B 356 - 23 -

i -p--B 194 - 22 - --B 176 - 21 - --B37 - 20 - - 19 -

- 17 -

1723 - 16 - % - 15 - 18 - 14 - 518 - 13 - 34 284 - 12 - 216 35 - 11 - 68 - 10 - -9- 256 -8- 520 -7- -6- - 5-

603 - 4 - 558 -3- 547 - 2 - 232 45 1 5 10 HZ ,l- 0.2 0.4 0.6 0.8 1.0 2 4 6 10 Frequency I I III I I I I FREQUENCY (Hertz) ( B 271

1.0 -

..S- 14:2 Y 16

.6- c

.4-

.2- B 34 B 211

o- c

KS B 216

0

11 B 35

n

FIG. 3. - Spectra of P waves from 110 io 15”. Specfres des ondes P de 110 à lin. analysed. Among them the following three groups of epicentral distance: 80 to 100 at Yongossaba, 11.70 to 12.50 and 140 to 150 at Bangui show no appreciable variaCon of spectrum morphology confirming the spectral stability of frequençy maximum. It is generaly observed that for P waves in Bangui, for two close distances, two similar spectra are obtainecl even for epicentral zones somet.ime quite different. This is the case of spect,ra of two events at. the epicentral distance of 150 (B499 from Tanganyika lake and 271 from Suclan). Ot.her examples, net, shown, also exist (Y4a from Djibouti and B37 from Malawi at 200, B328 from and B356 from Zambia at. 230). This is net. valid beyond the African domain (more than 300) where P waves from oc.eanic or continental origin give very different spectra. Thus, the stability of spectra, at clistancos under about 300, is uneffected by any variation arising from the choic.e of earthquakes or epiccntral regions. This leads to the following conclusions: first, the upper mantlc and the trust around t.he receiver cari be c.onsidered as widely homogeneous; secondly, spectra of P waves seem, at the first, view, independant of mechanism and magnitude of earthquakes within limits used here as well as source neighbourhood but. clirectly correlated with epic.entral distance as indicated in fig. 2.

O.R.S.T.O.Af.. G&phys., no 17, 1.980: %l-36. In the first. approach, if wc consider thc frequency of the spec.tral maximum which c.orresponcls t,o the vert.ic.ally hat&ed zone of the map, where normalized amplitudes are restrained between 0.8 end 1.0, we sec that from 10 to 90 t,he maxima show a quite good st.abilit,y around 3 Hz. This zone of maxima splits into two branches from 90; one branch on the right-hand side still remains, close to 3 Hz, whereas t.he other draps to less than 0.4 Hz at 150. Around 160, bot.h earthquakes, Y123 and B70, bave a single peak which reaches again high frequenc.ies. At dist.ances bet,ween 170 and 190 there is a 1ac.k of recorded P waves on magnetic t,ape in spite of occurrence of earthquakes at. these clist.ances in , this is probahly bec.ause of too weak P waves to trigger the recorr3ing systttm. The earthquakcs from 200 to 230, perhaps to 250, give cleçreasing frequencies of the spectral maximum from 1 Hz to 0.3 Hz. The three largest epicentral distances for purely African paths c,orrespond t.o Y22 in delta, B41 near t.he toast of Lybia and Y76 in Algeria at distances of 260, 270 and 35O., a11other events in the range 300 to 450 report.ed on figure 2 are located in continental regions (Greece, Turkey, Iran and ) except for B522 from south of Africa. The frequency of spec.t.ral maximum increases regularly between about 230 and 330. From this distance up t.o 450 (the carthquakes are no more in the African domain), t,he frequency of the maximum is unstable. This instahility seems t.o arise from the diversity of epicentral regions which entails import,ant variations in propagation paths and source c.ondit,ions. However, if we take into account. only the gcneral feat.ures, the frequency of maxima seems to remain, on the average, constant.

1.3. THE 150 ANOMALY Keturning back to purely Afric.an paths and earthquakes, we now consider in more det,ails, thc t-listurbed zone of the spect.ral map (11O < a < lG”) where a splitting of spect.ra is conspicuous. Some normalized spec.t.ra from 110 t,o 150 are represented on figure (3). As a c,ontrol, spect,rum of a second arrivai, close to first, onset, is also given for each earthquake. The two waves do not show any significant difference in their spectra. This leads to the conclusion that the nature of the spec.tra is the same in the first P wave and this later arrival. What is striking is the division of maxima into two branches from 11.50 to 150. This splitting evolves with distame: the hi&-frequency branch being attenllated with inc.reasing distances, the low-frequency tendency, on the contrary, becorning preponclerant. The high-frequency branch appears to be a prolongation, up t,o 150, of the type of sp&rum observed at. short distances. It appears clear that this branch is represent.ative of P waves propagating in a low attenuation medium (=high qua1it.y factor): the Lithosphere. The appearence of a sec.ond low frequency maximum up to about 150 may suggest a superposition of t.wo waves having different spectral c.ontents and phase velocit,ies. In the time domain t,he rec.ords show in fac.t, a sec.ond wave group of large amplitude arriving about 3 seconds after the onset.. These two groups have close phase velocit,ies and spectra. At the present stage of analysis, it. is difflcult to say if these waves represent simply t.he source process or if they characterise propagation media. It is nevertheless important. to underline that. around 150 the spec.t.ral characteristics change drastic.ally, t,he maximum appearing at low frequencies and high frequencies sharply attenuated. This shift of maximum towards low frequencies suggest pcnetrat.ion of waves in a low Q medium which is generally considered as asthenosphere. Two remarks may be significant: Firstly, the distance variation from 9 t,o about 130 is obtained from earthquakes loc.ated on the western side of the Rift and propagation path lies away from the Rift domain (fig. 5). Seconclly, a quit.e good similarity of spectra is observed at 150 c.orresponcling t*o earthquakes from vrry different regions suc11 as (B271) and Tanganyika lake (B499). These two points strengthens the hypothesis that the spectral evolution between 90 and 15O is dependant on epicentral distance and not on local anomaly. This Iast assassment is also supported by considering the first arriva1 spectra of two earthquakes whic.1~ have been rec.orcled at the couple of c.ent.rafrican st.ations Bangui and Yongossaba. The first carthqllake (fig. 4-a) c.orresponds to references B284 and Y14 (see Table I), t,he second (fig. 4-b) /

3AN

FIG. 4. - Comparison of spectra for two eartll- qualtes (a) and (23) recordcd at t.wo stations. Comparaison des spectres de 2 séismes (u) et (0) enregistrés en 2 stations.

FIG. 5. - R’fap of propagation pafhs from tht: Rift to Bangui or \Tongossaha. 15 Carte des trajets skismiques du Riff à Bangoi OU Yonyossa bu.

corresponds to B334 and Y68 reported from I.S.C. det.errninat.ion only (Jul. 5, 1974, 05h13mlO.% - 3.64 S, 29.0 E - Mb=4.6 - h=O). Epic.entral distances for Bangui, AB, and for Yongossaba, A,, are respectively for (a) and (b): (a) A,=ll.!+’ h,=8.00, (b) A,=13.20 A,=IW For a given earthquake-A, constant-. the difference between spectra c.an be caraçt,erized by the import,anc.e of low frequency at, larger t1istanc.e. The ressemblance cari be found in thc stability of the high frequençy maximum. This example is in good agrecment with other resu1t.s. However, the phenomenon of the division of spectra into two maxima for the first arriva1 and the c.10~ second one, which is of the same spectral type, suggest an effect, of a limit, or transition zone in terms of attenuation. This cari be regarcled as an experiment,al character of the approach by seismic. P W~V~S of the Lithosphere-Asthenosphere Boundary independently of phase multiplicity. The interpretat.ion of the process of splitting cannot be macle here, but only c.oncurrent,ly with Velocity and Q models.

O.R.S.T.O.M., fXophys., II” 17, 1980: 21-36. 28 268 B 144 i

23.1’ B 356 \

B 134 .6 - 0

.a- B 176 .2-

o- 0 B 37

0

/ 16? Y 123 I

L 0

FIG. 6. - Spect.ra of P waves from 160 to 250.

Spectres des ondes P de 16~ à 250.

1.4. THE 230 ANOMALY In fig. 6 where spectral evolution between 160 and 250 is shown, . we remark that the maximum is unsplitted and moves from about 2 Hz at 160 to about 0.3 Hz at 230. The exist>ence of high frequency at 160 and its weekness around 150 (fig. 3) appears contradictory. Y123 being the only good observation available at this distance, we do not have any straight forward explanation. This is further complicated by the absence of records, as mentioned earlier, between 170 and 190. Leaving aside t.his particular point, we observe that the spectral map and also fig. 6 show a regular drift of maximum frequency from 1 Hz at 200 to 0.3 Hz at 230. The simple shape-without splitting-of these spectra does not allow us to propose an int.erpretation similar to that for distances under 150. At, distances larger than 200, the rays penetrat,e deep enough to traverse any low Q asthenosphere ; the difference in the asthenosphere ray paths for 20 c A c 250 is insufflc.ient to explain the drift observed. This leads us to consider carefully a possible cause of attenuation on both extremities of ray paths.

O.R.S.T.O.M., Géophgs., no 17, 1480: 81-36. 20 For the receiver side: trust an.d lithosphere would be hardly concerned because eart,hquakr!s B37, B176 and B194 which represent the frequency drift., are lying nearly in the same azimuth from Bangui (fig. 5). Besides, the angle of incidence at the station decreases very little for this dist.ance interval. On the othcr side of the ray paths, one cari see that waves are travelling under the Rift, -indic.at.ed by thc contours of lakes-on more than one third of the distance. Considering the Rift as a high attenuation zone (GUMPER, POMEROY, 1970) one may admit that the rlrift of spectral maxima towards low frequency rcsults from a progressively longer ray path in the Rift zone. The case of B69 and B356 is disc,ussed below.

1.5. LATERAL VARIATIONS The above suggestion finds many supports in works published by several authors about. lateral variations of Upper Man& in Africa. A change of velocity and att,enuation between the African Rift and African shield is shown to exist from studies of wave propagations over the c.ontinent (GUMPER, POMEROY, 1970, KNOPOFF, SCHLUE, 1972). More recently impressive result,s were obtainecl from the “Durham Seismic Array in Kaptagat” (LONG, BACKHOU(TE, n'IAMJIRE, SUNDARLINGHAM, 1972). An abnormal trust (MAGUIRE, LONC;, 1976) and a complete 1ac.k of Wlosphere were found under the Gregory Rift (LONG, RACKHOUSE, 1976): The authors bave proposed a morlel-as a map of t,he upper surface of the anomalous low-veloc.itIy zone-deduced from slowness anomalies and relative t,eleseismic delay times. The identification of the shape of this boundary, away from t,he Rift,, emphasizes t.he continuity of the Asthenosphere which is intrusive under the Rift ancl layered at. the base of continental shield. The anomalous character of Upper Mant]e under the Rift. areas is also well c.orrelated with a large wavelength negative Bouguer anomaly ( GIRDLER, FAIRHEAD, SEARLE, SOWERBUTTS, 1969). This anomaly is extendecl to some adjacent regions and interpreted as a thinning of lithosphere (GIRDLER, 1975) like is observed for t.he RifL The connection of this anomaly with teleseismic positive or nul1 delay time (instead of negative for normal shield) is ac.count.ed for by a thinning of lithosphere (FAIRHEAD, REEVES, 1977). Al1 these results enhance the hypothesis of an extension of the anomalous CTpper Mant,le in Zambia and Ethiopia where earthquakes 1669, B356 and B328 at a dist.ance of 230 occur. This would explain that P waves involved in these eart.hquakes are propagated under approximately similar condit‘ions to those travelling in thc Rift domain.

1.6. OCEANIC AND CONTINENTAL EARTHQUAKES As an attempt t.o examine t.he hypothesis of an effect. of lateral variations on P waves, it appears to be interesting to extend our purpose to earthquakes of larger distances. Therefore, it seems signi- ficant t.o consider spectra of earthquakes from cont.inental origin (Greec.e, Algeria, Iran and Caucasus) compared with ones of oceanic origin (, Arabian sea and Carlsberg Ridge) at equivalent epicent,ral dist.ances (fig. 7). For the 330 to 480 dist.ance range, we take advantage of the fa&. that P waves are not much affected by the mantle &ructure. SO, the difference between the two sets of spcctra-low ancl high frequency maximum for oceanic and continent.al origin rcspectively-is assigned to the Upper Mantle properties on the focal side of the ray paths. This properties include here: velocity, attenuation and focal mechanism. The low frequency of P waves of oceanic origin which have propagated a part of their pat,hs under oceanic riclges (similar spee.tra were obtained from c.entral Mid-Atlantic Ridge) cari be considered as bearing some ressemblance between Mid-Oceanic Ridges and the African Rift system. Among,othcr geological and geophysical evidenc.es one may site the weak P veloc.ity (7.1-7.5 km/-)b in the Upper Mantle (RUEGG, 1975, LONG, BACKHOUSE, 1976) and the strong at,tenuation of S, (MOLNAR. OLIVER, 1969) for both type of st,ructures.

O.R.S.T.O.Ai., Gdophys., no 17, 1080: 87-30. 30 n CONTINENT OCEAN

B 441 B 91

45.6 ’ 47.8 o \ C

1~ B 29 B BO 40.4” 40.8 a Q8 \ 0 YUne

0.4

0.2

0 8533 38.4’

B 505 \ 33.0 Q \ . . 0.2 0.5 1.0 2.0 5.0 10.nz

FIG. 7. - Cornparison of spectra froc continental and oceanic origin. Comparaison de spectres de séismes d’origine continenfale et océanique.

2.. Quality factor measurements

%. 1. 1vETHOD IVe consider t,he relation :

A,(f) =G. exp (-II f (3)

Where G is frequency independant geometrical spreading factor, ds an element of ray path, Q(z) and V( z ) are respectively Quality Factor and Veloçity at the depth (z). Talring logarithm of (1) and differenciating with respect to frequency : A’(f) A’O(f) X’,(f) ..=..+A, (4) A (0 Ao (f)

31 and replacing the expression of (3) in (4) : A’(f) A’,(f) XT--=----II log e.Q(A)T PI (5) A (4 Ao (f) where log e is dec.imal logarithm.

BS47 2.5 0.067 (l,l,+) 6-10 B558 3.5 0.100 (I,l,+) 4-9 BS03 4.3 0.058 (1,l.O) 3-5 Y26 8.5 0.064 (l,l,*) 4-a B256 8.7 0.068 c1,1,+1 0.075 V*I,+) 4-a B216 II.6 0.163 (l,I,-1 0.118 (IA+) O.lOO (2,l.O) 1.7-3 B284 11.9 0.042 (1.I.O) 1.8-6 B34 12.6 0.084 (l.l,+) 0.091 (1.2,+) 3-a B21, 12.5 0.150 (l,I,+) 0.136 (IA+) 1.4-S BS18 13.2 0.120 (l,l,-) 0.120 (1,2,-1 5-7 Y18 14.1 0.192 CI,*,-) 0.212 C&I,-) 0.147 CL*,+) 0.6-1.3 md 3-7 B27l 14.9 0.316 (1,1,+) 0.312 u,l.+) 0.218 (.w,O) 0.2-1.5 and 4-7 8499 15.2 0.349 (l,l,-) 0.349 V*I,+) O.I"O (lAO) and (2,2,0) 0.3-1.5 Y123 15.8 0.056 (I,l,O) 0.212 (2, I*+l 0.4-0.7 and 3-6 B70 16.3 0.108 (I.1.0) 0.608 (2,1,W 0.118 (*.*,o) 0.2-0.6 B37 20.1 0.552 c1,1,+1 0.264 (2,1,0) l-2 Y4B 20.9 0.990 (I,l,-1 0.643 (*,l,+) B176 21.2 0.438 cIsI,+) 0.432 (*,l,+) 0.424 (3,1,+) 0.8-Z B194 22.2 0.454 (l,l,+) 0.468 e,l,+) 0.5-2.5 B69 23.4 0.340 cl,],+) 1-2.5 0328 23.7 0.332 (l,l,+) 0.132 (1,2,0) 0.6-l Bl44 25.1 0.346 (I>I,+) 0.270 (l,*,o) 0.280 (*,l,+) 1-3 ri146 25.2 0.346 (l,I,+) l-3 Y22 26.9 0.666 (I.I,-) 0.261 (2.1.0) 0.183 (l,Z,O) md (2,2,o) 0.4-0.7 .md 0.8-4 B41 27.1 0.268 (l,l.+) 0.314 (z.l.+) 0.3-I B471 30.7 0.227 (l,l,+) 1.6-3 x505 32.9 0.749 (I,I,+) 0.29, (1,2.0) 0.505 <2,1,+1 0.4-2 BS12 33.8 0.458 CI,1 ,+) 0.293 GI ,O) 0.8-2 I.2-2 Y168 33.6 0.346 (I,I,+) 0.20, (IA-) 0.9-2 Y169 33.6 0.376 (l,l.+) 0.357 (*,l,+) 1.2-2.6 B442 34.3 0.255 (l,l,+) l-2 B443 34.3 0.333 (l,l,+) 0.333 a],-) 1.2-2 B444 34.3 0.148 (l,*m 1.2-2 B250 34.9 0.364 (l,l,O) 0.442 cl,+) l-2 xl8 34.7 0.616 (1,I.O) 0.952 (2,1,+) U.4-O.6 md 1.6-2.4 B2 36.2 0.272 (l.I,+) 0.3-O. 7 Y76 35.3 0.244 (I.I,+) 1.4-3 B316 37.0 0.339 (l.l,+) 0.349 (l,2,-) 0.238 (*,1,0) 0.5-0.8 and l-2 B533 38.7 0.705 (l.l,+) 0.5-I B29 40.4 0.464 (I,I,+) l-2 B.90 40.9 0.516 (l,I.+) 0.178 (lAO) 0.4-0.8 and 1.5-4 Y173 40.9 0.62, cIpI,+) 0.216 cl,*,+) 0.366 C&l.+) 0.5-3 B26 43.2 0.344 u,I*+) 0.360 (*.I,+) 0.6-1.6 B522 43.4 0.318 (I,I,-) 1.6-2 BS33 44.4 0.563 (l,l,-1 0.324 (l,2,O) 0.3-0.5 snd 1.6-Z B44, 45.8 0.333 cl.],+) 0.239 (*,1.+) 1.4-3 B472 45.9 0.732 <1,1,+1 0.115 c1,2,+> 0.305 e,l,+) 0.8-0.6 and 1.6-2 BP1 47.8 0.572 (I,I,+) 0.2-l

O.R.S.T.O.M., GEophys., no 17, 1980: 21-36. 32 A’(f)/&f) bcing t,he slope of logarithmic amplitude versus frequency curve, T(A) and Q(A) are travel time and mean value of Q at the c1ist.anc.eA. Considering only frequencies greater than about 0.4 Hz, we assume that X’,,(f)/&(f)=O, SO, T(A)/Q(A) cari be deduc,ed from slope of spectra towards high frequencies (CHOLTDHUKY, 1972). Consequently, this met,hod cari be used as another approach of investigation of spectral behaviour independently of it,s maximum. This is done for a11earthquakes and slopes calculated.

2.2. DATA The Table 11 gives experimental values of T/Q; we denote in parenthesis after each value: 10 a numher whic.11 indicates the order of arriva1 of the phase (first or second), 20 a nnmber indicating if it is the first or second observed slope for a given pllase (sec remark), 30 a sign which gives the quality of amplitude distribution to determine the slope: (+) slope ohtained by numerous points in good alignement, (0) slope obtained wit.11 few points or numerous ones disperscd, (-) slope obtained with very dispersed points. For each line, limits of c.onsidered frequencies are given. Remark: The second obserred slope is gencrally the same as that observed at short distances. It cari be attached t,o local high frequcncy noise. Amplitudes of this second slope is a few per cent of the signal amplitude. The general dist~ribntion of experiment,al values of T/& as a function ‘of epic,ent.ral distance appears in Table II above. Results are summarizcd as follows: A <120 : 0.05 T/Q >0.30 250

for a ronstant quality fac,tor distribution- Q=lOOO-applied on P veloc.ity mode1 of HERRIN, the limits of T/Q variations for the same four intervals are: 0.02-0.15, 0.15-0.28, 0.28-0.35, 0.35-0.45. A comparison between t.hese values and those observed demonst,rate that a large discrepency exists in the distance range 12O to 250. The figure (S), where experimental data are reported, shows more abundantly that the phenomenon of a rapide. inçrease of T/Q is particularly evidcnt at 14-150, in agreement. with result,s found from variations of global spectra. It is sufficiently clear from the above result that a low Q layer exists under the lithosphcre of loc.ated more precisely under the. North-Eastern part of Zaïre (fig. 5). It is also clear, in spite of a large scattering, that on the distance int.erval 20-250, T/Q values are decreasing. This sense of variation would imply Q values increasing with depth. Thc result,ing distribution of Q in the Upper Mantle woulcl bc roughly in agreement with three layers:M, (‘Moho-140 km), M, (140-235 km) and M, (235-540 km) as defined to describe the Upper part of the Earth structure (MOHAMMADIOIJN, 1967), where M, only is an absorbent medium. From 25-300 to 450, T/Q values are very muc.11dispersed. Values from oceanic origin (shown by open symbols) being eliminated, t,he dispersion still remains very strong. Some earthquakcs from Greece, Dodecanese, and Turkey are shown to be of a continental type without. knowing their relationship to the east Mediterranean structure. Scarcity of data bet,ween 250 and 350 roughly, does net, allow to draw a definite. c.onc.lusion about the tendency increasing, constant or decreasing of T/Q variation up to 450. More data are needed t.o precise this point.

2.3. DISCLJSSION When variations of T/Q as a func.tion of distanc.e (fig. 8) are compared with global spectral anomalies (fig. 2), the existence of a low Q layer becomes evident. This low Q layer c.an be associated

O.R.S.T.O.M., G&phys., no lY, 1980: 21-36. 33 0” 5‘ 10" 15' 20^ 25'~ 30‘ 351 40- 45' 50"

0 . 1.0 2 A

0.9

0.8

0” 5. lon 15'> 20' 25' 36 35‘ 40 45'

FIG. 8. - T/Q variations with epicent.ral distances. Esperimenlal dat.a are: -caf black circle for flrst. arriva1 of P ami first. slope, little black c.ircle for first. arriva1 and second slope, black triangle for second arriva1 of P and first slope. Open symbols indic.ate owanic origin. For CUPVPS, see Iegend in the test.. Variations de T/Q en fonction de la distance épicentrale. with t,he low ve1ocit.y layer of various models of Upper hfantle (NIA~I, ANDERSON, 1960, .JOHNSON, 1967, JULIAN, ANDERSON, 1968, GREEN, HALES, 1968, HELMBERGER, WIGGINS, 1971). The depth of Che top of t.his low Q layer (i.e. the thicknee of lithosphere) in the African shield should be estimated from local travel time and slowness determinations. At the present state of this work, the depth of the low Q layer as dcmonstrated by our results on T/Q and global spectra behaviour, bas been est.imated, from Herrin’e velocity modcl (HERRIN, 1968), to he more than or equal t,o 160 km. Such a depth was proposed for the Lop of low velocity layer for Western United Si?+t#es(WIGGINS, HELMBERGER, 1973). The relationship between Lithosphere and Asthenosphere being clear enough for short distances (A <150) diffîculty of intcrpretation remains, however, for larger distances. Effectively, the variation of T/Q ancl those of spectral maximum seem to be in c.ontradic.tion from 200 to 250 epicentral dist.ance: the first one involves an increase of mean value of Qua1it.y Factor while the sec.ond suggests a stronger attenuation. If we consider lateral variations in the Upper Mant.le, we cari roughly explain such a discrepenc.y by the hypothesis that the variation of spectral slope resulting from the mean value of Q along t.he ray is not affected by the 10s~of high frequenc.ies near t,he source-so far as the spectral maximum frequency is greater than 0.3, 0.4 Hz-but reflec.ts the variation of mean attenuation properties along the ray through its progressive penetration in high Q media. Under this hypot.hesis, the drift of spec.t.ral maximum towards low frequencies (fig. 2 and 6) would reveal lateral inhomo- geneities as discussed earlier; more precisely, the loss of high frequencies would result from propagation in a low Q rift structure in the source region. It must, howrver, be remembered, that, the above interpretation is based on the assumption that A,, is identical for a11shocks. From these various considerations, it is evident that variations of global spc?c.tra cannot bt: interpreted in details ; conseqllently, we shall limit ourselves to T/Q result which is more representative ,of Upper mantle structure beneath the Afric.an shield. Quality Fac.tor and Ve1ocit.y models were comput,ed in order to check experimental results of T/Q. Curves obtained for three models are repritsent,& on figure (8) as follows: LI) the dotted line rrpresents a constant Q mode1 (Q =lOOO) applied on Herrin’s velocity mode& 6) the crossed line, a variable Q mode1 having a low Q layer st,arting at 185 km dcpth with Herrin’s P model, c) the full line, t.he same Q mode1 as (b) with a modified Herrin’s P mode1 from Moho to 350 km. The main clifference is the existence of a low velocity layer corresponding to low Q layer.

Conclusion As cari be secn in fig. 8, t.he numeric.al models do not satisfy entirely t,he observecl data, we only cari gis-e approximate values of Q: 500 for thc trust, 2000 for the Lithosphere, 200 to 800 for the Asthenosphere and 2500 for t.he Mantle. The main feature of our results is that constant, Q or increasing Q are net. compatible with observations. The struc.ture of the Upper Mant-le cari be perfectible by use of local slowness data, propagation time and multiple arrivals; the hypot.hesis of possible int,erferences on P arrivais (MVICMECH~N, 1976) may bc regarded. The complexity in lateral extension of a high-attenuation medium (Rift structure) as ravealed by various geological and geophysiçal works as well as results present.cd in this paper, clearly demonstrat,es thr: necyssity of a large number of data and underlines the limits of acceptable assumpt,ions.

kKNOWLEDGEMENT

The author is endeht,cd to Dr A. M. CHOUDHURY for providing many suggestions and for his precious advices. Thanks Dr E. PETERSÇHMITT for let,ting me use bis computer programm, to &I&f. DuKHAN and CHAUvIN for their ïkC.hrk!.al klp for I'UIkng the S&t.iOnS and to MM. &14POUKA, OUAYANGUE, TOME for their collaboration.

Manuscrit reca au Service des Pnblicntions de Z’O.R.S.T.0.M. le $6 juillet 1979.

REFERENCES

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O.R.S.T.O.M., Géophys., n” 17, Z9SO: 21-.3iî. 36