Journal of Seismology1: 383–394, 1998. 383c 1998Kluwer Academic Publishers. Printed in Belgium.

Fault mechanisms and tectonic implication of the 1985–1987 earthquakesequence in south-western Ethiopia

Atalay Ayele1;2 & Ronald Arvidsson11 Seismological Department, Uppsala University, Box 2101, S-750 02, Uppsala, Sweden2 Geophysical Observatory, Science Faculty, Addis Ababa University, Box. 1176, Addis Ababa, Ethiopia

Received 22 November 1996; accepted in revised form 6 November 1997

Key words:fault mechanisms, Main Ethiopian Rift, waveform inversion

Abstract

Integrated inversions of short-period P, broadband P, and long-period P & S waves are done for fault mechanisms,focal depths, seismic moments, and source-time functions from the largest four earthquakes of the 1985 and 1987earthquake sequence in south-western Ethiopia. These earthquakes had similar normal-faulting mechanisms. Thegeneral trends of the fault planes follow the Main Ethiopian Rift which is in agreement with foreshock-aftershockdistribution, surface breaks and geology. Despite the morphological discontinuity of the Main Ethiopian Rift at itssouthern tip, the mode of deformation of the continental crust under study shows its extension southward. There areno significant strike-slip components trending NW–SE in all the mechanisms which would have been associatedwith the Aswa Fault Zone in southern Sudan or Anza Rift in northern Kenya. We also infer that the relativelybroad fracture zone at the southern extreme of the Main Ethiopian Rift demonstrates the early stage of the break-upbetween the Nubia and Somalia plates in comparison with the Main Ethiopian Rift proper and the Afar Depression.The main shock of the sequence (Mw = 6:3) ruptured at a depth of 6.8 km, shallower than expected since thedepth of earthquakes generally increase southward from the Afar Depression. The shallow depth of earthquakeoccurrence is supported by surface deformations with an overall trend in the direction of the Main Ethiopian Rift.

Introduction

The rift structure (Figures 1, 2) of the Main EthiopianRift (MER) is neither a continuation of the Gregory Riftin Kenya (eastern branch) nor of the western branchof the East African Rift system. The rift morpholo-gy simply fades away around the Turkana Depression(Moore and Davidson, 1978; Ebinger et al., 1989). Theidea, which of the two branches has a clear structuralconnection with MER, has intrigued earth scientistsconsiderably. There are no practical geodynamic mod-els yet which try to explain the relationship among thethree rift arms. Most of the geological and geophysi-cal investigations in the East Africa Rift have focusedeither on the Afar Depression of Ethiopia or GregoryRift (GR) in Kenya. The objective of this paper is not tosolve this problem but to reassess the seismotectonicsof the southern part of MER from recently availableearthquake data and discuss its implication.

The Main Ethiopian Rift, which trends SSW fromthe Afar Depression (Figures 1, 2), is one of the well-developed continental rift segments in East Africa.Rifting is evident from topographic expression, geolo-gy, volcanism, seismicity, and gravity (Mohr, 1967;Moore and Davidson, 1978; Gouin, 1979; David-son and Rex, 1980; Ebinger et al., 1993; Ayele andKulhanek, 1997; Figures 1, 2). The narrow rift valleytopography of MER is primarily caused by subsidenceof fault-bounded sedimentary basins and uplifts of theadjacent rift flanks. A comparison of rift structuresand topographic relief in MER reveals that Tertiaryvolcanoes and flood basalts also contribute to the highamplitude and short wavelength aspects of the topogra-phy, particularly in the vicinity of the rift system. Theeffective elastic plate thickness found beneath MER(21 km) is less than that found beneath either the west-ern rift (31–36 km) or GR (27–31 km) (Ebinger et al.,1989) owing to longer period of active rifting along

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Figure 1. The topographic relief map of Ethiopia and Eritrea. This topographic relief is characterised by uplifted plateaus and narrow (MER)and broad (Afar) rift valleys. The gross highly dissected rugged terrain and multiple rifts of south-western Ethiopia in the white rectangle is thearea of discontinuity between the southern terminus of MER and the northward projection of GR (Gregory Rift) indicated by the white arrow.

MER. Although detailed kinematic data are not yetavailable, the overall topographic relief of central andsouthern Ethiopia argues for overall normal extensionof the rift (Bosworth, 1992, Figure 1). Topography andseismicity reveal that MER is the only less complicat-ed and well-developed boundary between the Nubiaand Somalia plates. This is not common in most ofthe other rift segments in the region which are usuallycharacterized by diffuse seismicity.

The southern extreme of MER (Figure 1), charac-terized by highly dissected rugged terrain and multiplerifts, marks the offset between its trend and the north-ward projection of GR into south-western Ethiopia.This area is known to be seismically active (shadedregion in Figure 2). It is dominated by NNE trend-ing multiple rifts but are distributed in the NW-SEdirection. Macroseismic and teleseismic earthquakesdata have been observed and reported in this regionstarting from the turn of the 20th century. Accord-ing to nearby villagers’ reports, some of them havecaused roaring and rumbling noise as well as consid-

erable damages associated with surface deformations(Gouin, 1979). This clearly hints to shallow sourcedepth and probably to a rather thin lithosphere too.None of these earlier earthquakes have been analyzedfor fault mechanisms or other source parameters dueto a lack of reliable data. But one earthquake in 1985and three others in 1987 which occurred in this regionare amenable to teleseismic body wave analysis. All ofthem caused considerable damages in their epicentralarea (Asfaw, 1990). Estimates of fault mechanismsare listed in the Harvard (HRVD) Centroid MomentTensor (CMT) catalogue. They indicate predominantlynormal-faulting mechanisms. Focal depth is, howev-er, not well resolved by the HRVD CMT inversion forsuch shallow crustal earthquakes. From first motionpolarities of P-waves, Asfaw (1990) studied the faultmechanism of the main event, the 25 October 1987earthquake of sizeMw = 6.3. The result indicated apredominantly normal faulting and strike-slip compo-nent striking NW-SE which is believed to be supportedby seismicity starting from the turn of the 20th centu-

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ry. Asfaw’s (1990) conclusion was that the tectonicfeature at this juncture is influenced by a prototype oftransform fault in an extensional continental crust.

In the earlier days of event monitoring before1960s, the accuracy of earthquake locations was onlyabout 100 km in Africa (Sykes and Landisman, 1964).Consequently, the geographical distribution of the thendetermined earthquake epicenters is not suitable to elu-cidate the details of small scale tectonics. But thegeographical distribution of 10 well-located eventsbetween 4�N and 6 �N for the period 1900–1974,from a good correlation between macroseismic andteleseismic data (Gouin, 1979), shows a clear trendalong the MER with a small dispersion in a NW–SEdirection around the shaded region of Figure 2.

Earthquake occurrence below depths of about20 km (Nyblade and Langston,1995; Shudofsky, 1985;Wagner and Langston, 1988) and strike-slip mecha-nisms in an extensional continental crust (Asfaw, 1990;Doser and Yarwood, 1991; Gaulon et al., 1992; Lepineand Hirn, 1992) are two newly emerging concepts inthe East African Rift development. Care has to be tak-en, therefore, in estimating the earthquake source para-meters of interest and interpreting the results accord-ingly. Focal depths given by global seismic data centersare generally not very reliable for crustal earthquakes.Besides this, there are commonly large discrepanciesin focal depths determined and reported by the U.S.Geological Survey (USGS) or the International Seis-mological Center (ISC) on the one hand and depthderived from the HRVD CMT analysis on the oth-er hand. The latter have, for crustal earthquakes, alower depth resolution due to the use of very long-period seismic waves. The earthquake source mecha-nism depends, besides the state of the stress, also on thefocal depth and crustal structure of the source region.The aim of our study is to asses the fault mechanismand other source parameters of the 1985–87 south-western Ethiopia earthquake sequence from momenttensor inversions of teleseismic body waves using acrustal model pertinent to the region.

Data

Only four major earthquakes with available digitaldata, out of many events in the shaded zone of Fig-ure 2, are considered here (Table 2). Long-period Pand S, short-periodand broadbandP waveform data areused from GDSN (Global Digital Seismographic Net-work) stations at teleseismic distances. Two vertical-

component long-period seismograms from stationsUPP and QUE are digitized and included for the mainshock of the October 1987 sequence. The digital datafor all long-period P and S waveforms are equally sam-pled with a sampling frequency of 1 Hz.The broadbandand short-period P waveforms are digitized with either10 or 20 Hz digitizing frequency. The P and S waves inthe appropriate distance range of stations, 30� to 90�,were selected from seismograms with fairly good sig-nal to noise ratio. For the SH waves, only long-periodNS and EW seismograms were used. In the case of Pwaves, short-period, long-period, and broadband datawere used. A fairly reasonable azimuthal coverage offocal sphere was obtained for both P and SH wavesof all the events studied. The number of stations usedwith clear SH waves for the 25/10/1987 event wasrather low because of the contamination of most of theS-wave arrivals by an earthquake in the New Guinearegion about 9 minutes later.

Method and data analysis

The earthquake source is modeled as a point double-couple with a time-dependent source-time functionparameterized by strike, dip, and rake angles, and thecentroid depth. The source-time function is represent-ed by a set of overlapping isosceles triangles (Nabelek,1984, 1985). The rupture time history is broadband innature (Zhang and Lay, 1989; Velasco et al., 1995).Using a wide spectrum of the seismic signal such asshort-period, long-period, and broadband will result inan improved modeling of the shape and duration of thesource-time function.The least squares inversion of theseismograms for retrieval of fault mechanisms, seismicmoment, focal depth, and rupture duration is done byminimizing the misfit between the observed and syn-thetic seismograms. The theoretical background of thewave propagation is based on the classical ray theoret-ical method (Helmberger, 1974; Langston and Helm-berger, 1975; Nabelek, 1984).

The synthetic seismograms comprise a combina-tion of elementary seismograms that are summed anddelayed by the appropriate amount. The elementaryseismogram,E, is the complete seismogram producedby a particular phase leaving the source.E is expressed(McCaffrey et al., 1991) as

E = Gs� I �Q � �, (1)

whereGs is impulse response of the source structure,I is instrument response,Q is the attenuation operator,

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Figure 2. Major tectonic elements and the corresponding seismicity of the Horn of Africa for the period from 1960 to 1993 as taken from thecatalogue of Ayele (1995). In this study, special attention is paid to the shaded region. Thin lines are political boundaries.

and� is triangular source-time element. The completesynthetic seismograms are produce by summing theelementary seismograms for the number of subeventsNs, the number of source-time function elementsNt,and the number of elementary seismogramsNp withthe appropriate total delay timet0. The synthetic wave-form, s(t), can be expressed as

s(t) = GMGR

NsXk=1

Mk

NtXm=1

Amk

NpXj=1

Ej(t0)Ujk, (2)

whereGM is geometrical spreading,GR is receiverresponse,Mk is seismic moment of thekth subevent,Amk is amplitude of the time function element, andUjk is the radiation pattern. Once the synthetic seis-mograms are calculated, residuals will be determinedand the inverse problem can be solved. The misfit func-tion,f , to be minimized is

f =

sXi

(oi � si)2, (3)

Table 1. Crustal velocity model in the area under studyfrom Gumper and Pomeroy (1970)

Layer thickness Vp (km/s) Vs (km/s) Density

(km) (g/cm3)

7.0 5.90 3.35 2.70

11.0 6.15 3.55 2.80

18.0 6.60 3.72 2.85

Half-space 8.05 4.63 3.30

whereoi andsi are the observed and calculated seismo-gram amplitudes, respectively. The summations withrespect toi are made over the data points within theinversion window for a given seismogram. Equation3 takes care of the amplitude variation between theobserved and synthetic seismograms.

Though inversion of teleseismic data for earthquakesource parameters is not much sensitive to the accu-racy of the source structure, a reasonable workingsource model pertinent to the region can contribute

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Table 2. Summary of hypocentral parameters and fault mechanisms

Event date 1985/08/20 1987/10/07 1987/10/25 1987/10/28

Latitude (�N) 5.49 6.30 5.44 5.75

Longitude (�E) 35.93 37.82 36.80 36.74

magnitude (mb) 5.3 5.2 5.6 5.4

Depth (km) 15:5� 0:0 7:4� 0:03 6:8� 0:0 12:8� 0:4

Strike (�) 227� 6 202� 9 196� 1:3 218� 3

Dip (�) 35� 1 64� 3 40� 0:4 49� 0:5

Rake (�) 292� 3 317� 3 268� 1:3 289� 1

Mo (1016 Nm) 9:3� 0:56 6:1� 0:72 310:0� 0:07 93:0� 0:29

Duration (s) 4:7� 0:4 3:3� 0:40 6:8� 0:2 5:5� 0:2

to the stability of the inversion results and special-ly a reasonable depth estimate. The program allowsonly one rock layer overlaying a half-space that con-tains the source for continental region. The receiverstructures are all assumed to be a half space. Thereis no up-to-date and detailed crustal structure invert-ed from multidisciplinary aspect in the source regionunder study. The two top most layers from the avail-able models (Gumper and Pomeroy, 1970; Muellerand Bonjer, 1973; Prodehl and Mechie, 1991) werefirst tried independently for the 25/10/87 event sincethis earthquake has a better data coverage. The inver-sion for focal depth resulted in 21 km for Mueller andBonjer (1973), 12 km for Prodehl and Mechie (1991),and 7 km for Gumper and Pomeroy (1970) model. Abetter fit between the observed and synthetic seismo-grams and minimum variance reduction were observedfor the Gumper and Pomeroy (1970) model than forthe others. These models were also compared for gen-erating the body-wave part of synthetic seismogramsat a regional distance using the discrete wavenumberapproach (Bouchon and Aki, 1981). This was donefor an earthquake of magnitude 5.0mb, 210 km southof Addis Ababa, which occurred in MER on 20 Jan-uary 1995 and compared with observed seismogramat the AAE station. The synthetics from Gumper andPomeroy (1970) model fairly resembles the observedwaveform (than the others) though the crustal struc-ture still needs further refinement for this type of data.The two top most layers from Gumper and Pomeroy(1970) model (Table 1) are hence used as one rock lay-er over a half-space in the entire inversion for the rest ofthe studied earthquakes. The source model is adjustediteratively to determine the least squares best-fit of thesynthetic seismograms to the observed waveforms.

If the inversion starts with the short-period dataincluded, convergence will be unlikely and it will be

locked into a local minimum since these seismogramsare sensitive to the finer details of the rupture process.Short-period data are, therefore, avoided at the begin-ning and taken into account only later to improve thedetails of the inversion. It is a common problem inAfrica to obtain a good azimuthal coverage of stationsabout the earthquake source of interest. Due to thisproblem, it is not easy to observe directivity and sourcefiniteness effects for these moderate-size earthquakes.A guaranteed convergence of inversion, starting fromanywhere in the focal mechanism parameter space,was possible only for the largest event of the wholesequence which occurred on 25/10/87. This event hasa relatively good coverage of stations. The focal mech-anism parameters of this earthquake were used as ini-tial solutions for the nearby foreshock of 07/10/87 (seeTable 2 and Figure 3). CMT estimates were used as ini-tial parameters for the remaining two events studied. Inthe final solution, synthetic seismograms were alignedwith observed records by cross-correlation. Weightswere also assigned for each station to compensate forthe bias that could be introduced by the uneven cov-erage of the focal sphere and to balance the amplitudedifference between P and SH waves. The effect ofattenuation was calculated by using a constant t� of1.0 and 4.0 seconds for P and SH waves, respectively(Lay and Wallace, 1995). For the detailed explanationof all the procedures used in our analysis, one canconsult McCaffrey et al. (1991).

Results and discussion

The fault mechanisms and other source parametersare listed in Table 2. The epicenters reported by ISCare adopted and kept fixed throughout the inversion.The mechanisms are similar, with minor variations.

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Figure 3. Epicentral distribution of well-located earthquakes, at a teleseismic distance, from 1985 to 1987 in MER is shown in the inset. Thisdistribution is, generally, the foreshock-aftershock series of the studied sequence. The attached photo is one of the many samples of surfacefractures taken by L. M. Asfaw at a field observation after the 29 December 1987 earthquake swarm. This swam occurred about the epicentralarea of the main shock, around the tail of the black arrow.

Although the general result is consistent with the modeof deformation in an extensional continental crust, thepeculiarities of each event will be discussed below inchronological order.

The 20/08/85 event

This event (Mw = 5:3) occurred around the south-ern extreme of MER (Figure 3). There is no detailed

macroseismic information available. A reasonable fitis obtained between the observed and synthetic seis-mograms from the body wave analysis (Figure 4).From varying width of the source-time functions withazimuth, rupture directivity, i.e., the rupture directionand the temporal finiteness of the source can be estimat-ed, given a good azimuthal and ray parameter coverageof the source-time function variations (Velasco et al.,1995). In the given case, however, there is no resolv-

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Figure 4. Focal mechanism (lower hemisphere projection) for the1985, August 20, earthquake in south-western Ethiopia. Seismogramamplitudes are normalised to their maximum amplitude. Synthet-ics (dashed traces) are superimposed on the observed seismograms(solid traces) to compare the quality of fit. The time scale for thebroadband data is the same as for the long-period data. In the focalsphere, solid circles are P axes while the open circles with an Xsymbol are T axes. Small open circles in the focal sphere representstation distribution. Small letters represent the phase data type: w=long-period P and SH (WWSSN), d = long-period P and SH (GDSN),x = short-period P (GDSN), and b = broadband P (GDSN).

able source directivity owing to the uneven azimuthalcoverage of the available stations with sufficiently goodrecordings. The entire source rupture process was mod-eled by two major pulses of total duration 4.7 seconds,with the smaller fraction of the total moment beingreleased during the first 2.7 seconds and the dominantrupture in the last two seconds. The determination ofsource depth from seismic waves requires an accuratedescription of the source-time function and vice versa.

This affects the mechanism accordingly. Our inver-sion resulted in a low-angle normal fault at a depth of15.5 km, in close agreement with the ISC report (14 kmobtained frompP-Pdata).

The 07/10/87 event

It is the smallest (Mw = 5:1) event relative to theother studied earthquakes. The location is the one fur-thest to north along the MER (Figure 3) out of all fourstudied events. Buildings were damaged with cracksand masonry houses collapsed in the nearby regiondue to the earthquake. No casualties were reported.The focal mechanism parameters of the main event(25/10/87) obtained in this study were used as startingparameters in the inversion for this event. The inver-sion resulted in a predominantly normal-faulting witha significant strike-slip component of probably localsignificance (Figure 5). The depth resulting from ourinversion (7.4 km) is less than that given by ISC andCMT (10 and 15 km, respectively). As compared withthe 20/08/85 event, the rupture started with a largermoment release followed by a smaller one of the sameduration (almost 1.8 s each).

The 25/10/87 event

It is the largest (Ww = 6:3) earthquake in this sequence.The intensity information collected about the epicen-tral area indicates that it caused considerable damages,landslides and thunder-like noise. It was felt and report-ed over a wide area. There was no surface deforma-tion observed during the occurrence of this earthquake.High-quality digital data at teleseismic distances fromGDSN stations and two digitized, vertical-component,seismograms from analog stations (UPP and QUE)were used with a reasonable focal sphere coverage.This enabled us to start the inversion from anywherein the parameter space and end up with an accept-able result. Our depth estimate is significantly small-er (6.8 km) than that given by ISC and HRVD CMT(15.0 km). But there is a high probability to resolvethe depth to 6.8 km from short-period and broadbanddata which we have used in this study. The mechanismis purely low-angle normal-faulting. The rupture dura-tion is 6.8 seconds. The rupture history was modeledwith a relatively simple source-time function consist-ing of a single pulse of moment release with a triangularshape on average (see Figure 6).

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Figure 5. Focal mechanism (lower hemisphere projection) for the 1987, October 7, earthquake. Symbols and labels are described in the captionof Figure 4. The time scale for the broadband data is the same as for the long-period one.

The 28/10/87 event

This event (Mw = 5:9) occurred close to (about 30 kmaway) and northwest of the main shock. It was clearlyfelt but not as strongly and widely as the two eventsdiscussed earlier. If its epicenter is so close to themain shock, it should have produced lots of macro-seismic information in the area where the main shockwas felt. Since this was not the case, it may be anotherindication for the occurrence of this event at a relative-ly larger depth (Table 2). This earthquake is the onlystrong aftershock usable for body-wave analysis. It hasa similar rupture history as the main shock but shorterduration. This is supported by waveform similarities inthe records of the stations used in the inversion for bothevents (e.g. KEV, NWAO, LEM for long-period P inFigures 6 and 7). A comparison of waveform similarityfor a longer time sample (Figure 8) of these two eventsis made for the WMQ station.

The focal mechanism and shape of the source time -function are also similar to those of the main shock, butwith insignificant strike-slip component for the after-shock.

Discussion

On 29 December 1987, two months after the Octobersequence, a swarm of earthquakes with maximum mag-nitude 4.0 (ML) ruptured around the epicentral area ofthe main shock. The rupture process was accompaniedby multiple fractures trending NNE-SSW but distrib-uted in the NW-SE direction for about a distance of30 km (Figure 3) which is also roughly the distancebetween the epicenters of the 25/10/87 main shock andthe 28/10/87 largest aftershock. These surface breakshave never been observed during the main sequenceevents in October, 1987. A network of mobile sta-tions was deployed in the region in order to monitor

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Figure 6. Focal mechanism (lower hemisphere projection) for the 1987, October 25, earthquake. Symbols and labels are described in the captionof Figure 4.

aftershocks following the events of October 1987. Thisnetwork recorded aftershocks which were too small tobe observed by the permanent stations of the region andlocated by Asfaw (1990). Even though the locations ofthese aftershocks were affected by sparse station distri-bution, poorly known crustal structure, and sometimestime-stamping at individual stations, the distributionepicenters of trends NW–SE. This distribution is con-centrated along the line connecting the epicenters ofthe 25/10/87 and 28/10/87 events. From first motion P-wave polarity study,Asfaw (1990) showed that the faultmechanism of the main shock (the 25/10/87 event) ispredominantly normal faulting with a strike-slip com-ponent. From the strike-slip component of focal mech-anism of the main shock, aftershock locations and dis-

tribution of fractures, Asfaw (1990) concluded thatMER and GR are connected by an active shear zonecorresponding to an intercontinental transform fault.This hasn’t been well justified, however, from compre-hensive focal mechanism studies for the possibility ofstrike-slip faulting in the extensional continental crustof the East African Rift (Gaulon et al., 1992; Lepineand Hirn, 1992; Nyblade and Langston, 1995).

At the point of offset between MER and GR, therifting between the Nubia and Somalia plates seem tobe at early stage at least morphologically.The extensivestress regime both in MER and GR may contribute forstress accumulation at the offset which could be a like-ly candidate for a prototype of transform fault whichAsfaw (1990) suggested. Our normal faulting mecha-

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Figure 7. Focal mechanism (lower hemisphere projection) for the 1987, October 28, earthquake. Symbols and labels are described in the captionof Figure 4.

nisms determined from earthquakes at the southern endof MER contradict, however, the existence of a trans-form fault at this point. The rupture of the 25/10/87earthquake at about 7 km depth triggered the largestaftershock on 28 October 1987 at about 13 km depth.This whole failure of the stress might have influencedthe 29 December earthquake swarm. The two earth-quakes are almost identical normal faulting and possessimilar time history of the rupture process (Figures 6,7, 8).

Conclusions

The topography in south-western Ethiopia is interme-diate between the Afar rift proper and the Ethiopianplateaus but highly dissected with multiple rifts andactive seismogenic zones probably throughout the con-tinental crust. Despite the fact that strike-slip mecha-

nism is a likely candidate in some of the rift segmentsof the extensional East African Rift system, the inte-grated waveform inversion for several recent strongevents in the Main Ethiopian Rift indicate that thesouthern part of MER is mainly dominated by NNEtrending normal-faults. In other words, though the pos-sibility of a strike-slip mechanism at a local scale forsmaller magnitude earthquakes cannot be ruled out, theextensional nature of the largest four earthquakes stud-ied here, in our opinion, is related to the commonlyobserved large-scale geodynamic process in the EastAfrican Rift. Digital seismograph stations’ distributionin the region is improving to date. This will enable tostudy source characteristics of regional earthquakes sit-uated in between the Main Ethiopian Rift and GregoryRift in more detail. Seismotectonics at a regional scaleas such together with other geophysical disciplines willresolve the issue if there is any transform fault in thisparticular area.

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Figure 8. Vertical component seismogram plots at station WMQ for the main shock and aftershock of the 25 October 1987 and 28 October 1987events, respectively. The general similarity between the waveforms reflects the similarity in path and source mechanism for the two events.

Acknowledgements

We thank Madeleine Zirbes for providing us part of theGDSN data. We are grateful to Laike M. Asfaw for hisdetailed comments and offering us field reports. Ourgratitude goes to Peter Bormann and Ota Kulhanek forcritically reading the manuscript. We also would like toacknowledge Robert McCaffrey, Geoffrey Abers, andPeter Zwick for making the teleseismic body wavesinversion program (SYN4) available to the IASPEISoftware Library. Atalay Ayele was supported bySAREC (Swedish Agency for Research and Educa-tional Cooperation). Ronald Arvidsson was partly sup-ported by the Swedish Foundation for InternationalCooperation in Research and Higher Education underSwedish Natural Science Foundation contract Dnr 72022/95 and by NFR contract G-AA/GU 03164-318.

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