The relative amplitude method applied to 19 March 1984 Uzbekistan earthquake and its aftershocks

Physics of the Earth and Planetary Interiors, 47 (1987) 137—149 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands

137

The relative amplitude method applied to 19 March 1984 Uzbekistan earthquake and its aftershocks R.G. Pearce Department of Geology, University College, P.O. Box 78, Cardiff CFI IXL (Gt. Britain) (Received January 3, 1986; revision accepted August 12, 1986)

Pearce, R.G., 1987. The relative amplitude method applied to 19 March 1984 Uzbekistan earthquake and its aftershocks. Phys. Earth Planet. Inter., 47: 137—149. The (presumed double couple) source mechanisms of the 19 March 1984 Uzbekistan earthquake and six teleseismically recorded aftershocks down to mb 4.4 (PDE) are determined using the relative amplitude method. It is found that a well-constrained solution for the main earthquake can be obtained when world-wide long-period P wave first motions are combined with constraints on S wave polarisation angles determined from the relative polarities of three component seismograms. Solutions for the aftershocks are obtarned from between two and four short period medium aperture array observations of P, pP and sP. The main shock and all but one of the aftershocks give thrust type solutions in broad agreement with each other, their fault planes trending northeast—southwest where the available data provide a constraint on the strike. The remaining aftershock gives a vertical strike slip solution though this conclusion is in need of corroborative evidence. Results for the main earthquake are similar to those obtained by other workers, but it appears that the predominant strike directions of the 1984 aftershocks are different from those previously computed for aftershocks of the 1976 earthquakes, which gave a predominance of northwest—southeast striking fault planes.

I. Introduction The 1976 and 1984 earthquake sequences in Uzbekistan were exceptional in that they each included major earthquakes and an extended series of large aftershocks, yet the region could previously claim an extremely low seismicity, with no prior earthquakes reported in the USGS Preliminary Determination of Epicentres (PDE) for the whole of the Uzbekistan region in the period since 1964. A similar conclusion is drawn from data back to 1909 (Medvedev, 1968; Crampin et al., 1976). Considerable damage was caused during the large 1976 earthquakes at Ga.zli, 20 km from the epicentral area (Pletnev et al. 1977; Person, 1978) and again during the large 1984 earthquake. It is not clear why such sequences occurred in this confined area within such a short time; the rele0031-9201/87/$03.50

© 1987 Elsevier Science Publishers B.V.

vance of any local active faulting is made difficult to investigate on account of thick sedimentary cover. Aside from questions relating to their tectonic setting, these earthquakes are of special interest seismologically in that aftershocks yielded high quality ‘clean’ P wave seismograms at teleseismic distances, on which surface reflections are clearly observed at short period medium aperture arrays. It is well-known that short period seismograms from smaller intraplate earthquakes are often simpie, exhibiting discrete arrivals interpretable in terms of standard ray paths, and this is not surprising for earthquakes in continental areas with a uniform crustal structure free from significant anelasticity or other sources of signal degradation or complexity. It is also true that anomalously high stress drop favours the observation of simple P wave seismograms since this

138

implies short rupture duration which will help to keep direct P and surface reflections discrete and within the short period passband. It is likely therefore that these aftershocks do have anomalously high stress drops. The observation of such simple seismograms, and the occurrence of a large number of teleseismically recorded aftershocks decreasing in magnitude through the detection threshold, makes this sequence ideal for testing our ability to constrain small earthquake source mechanisms from long range using the relative amplitude method of Pearce (1977, 1979, 1980). This method is particularly suitable here since the smallest of teleseismically recorded earthquakes are only well-recorded at short period array stations, making it imperative to use all available waveform information; there is insufficient data to attempt direct waveform inversion. The 1976 Uzbekistan earthquake sequence, whose aftershocks were teleseismically analysed by Pearce et al. (1980), yielded presumed double couple source mechanisms for 21 earthquakes using the relative amplitude method. All were shown to be thrust faults, and there was a predominance of NW—SE strikes. Results with some significant differences will be presented for the 1984 sequence; it is clear that there is a different distribution of strike directions, with some evidence of oblique slip. This study concentrates on the application of the relative amplitude method to the six teleseismically recorded aftershocks of the 1984 sequence in Table I, using short-period array data. In addition, global digital seismic data from the main earthquake of 19 March 1984 are used to

attempt the first mechanism for a large shallow earthquake using the relative amplitude approach. It will be seen that the incorporation of S waves into the determination provides sufficient additional data for the method to be applied to such earthquakes. In a future study the 1976 and 1984 sequences will be analysed in more detail with the additional benefit of close seismic observations; discussion of their tectonic environment will be postponed until then.

2. The main earthquake—19 March 1984 mb 6.5 The longer rupture durations of larger earthquakes cause time-domain interference between direct P and its surface reflections, so that it is not possible to measure directly the relative amplitudes of these phases for the purpose of determining a source mechanism. The relative amplitude method can, however, be used to compute what is essentially a P wave first motion solution, by specifying infinite amplitude bounds for stations where an unambiguous P wave first motion polarity can be read. An advantage of this over conventional first motion methods is that the resulting range(s) of compatible orientations represent the true confidence limits on the solution— which for all methods of focal mechanism computation are correctly described only by such an arbitrarily shaped confidence volume in orientation space. Nevertheless, a large number of well spaced stations is always required to produce a well-constrained orientation by first motions, and this imposes limitations on the value of polarity data used alone.

TABLE I Uzbekistan earthquake of 1984 and its six teleseismically recorded aftershocks analysed in this paper (USGS PDE parameters) Event

Main earthquake Aftershock 1 Aftershock 2 Aftershock 3 Aftershock 4 Aftershock 5 Aftershock 6

Date

19 19 19 19 19 19 20

March 1984 March 1984 March 1984 March 1984 March 1984 March 1984 March 1984

Origin time (UT.)

20—28—39.8 20—54—00.0 21—03—46.9 21—21 —20.2 22—03—52.3 23—11—24.4 06—28—40.1

Location

Depth

Lat.°N

Long.°E

(km)

40.29 40.24 40.26 40.37 40.38 40.41 40.25

63.33 63.36 63.35 63.02 63.11 62.91 63.23

15.0 10.0 13.0 10.0 10.0 10.0 12.0

mb

6.5 5.1 5.0 4.9 4.7 4.4 5.4

139

Many authors have augmented P first motion polarities by S wave polarisation directions measured from three-component direct S—but these observations have often done little to improve the solutions owing to the difficulty of assigning appropriate error bounds to such directions, or because of the difficulties in cross-calibrating the three components. In this study three-component S wave polarisation data will be incorporated into the mechanism determination by measurement of the relative amplitude of the three components of direct 5, all observed at the same station, in exactly the same way as is used for P and its surface reflections. Upper and lower bounds are placed on the amplitude of each component within which the true value is considered certain to lie—with a polarity if it is unambiguous. These measurements are converted (after applying any correction for the relative gains of the three components) to a raiige of allowable polarisation angles at the sta-

tion, which is traced back to an equivalent range of angles at the source. It is assumed that there has been no rotation of the polarisation angle along the path—the effects of anisotropy are neglected although they could, in principle, be allowed for. The complexity of observed S waves does not, at least in this example, permit significant restriction to be placed on observed amplitudes, and the existence of S-to-P precursors to direct S often makes it difficult to measure absolute polarities with certainty. However, experience has shown that the observed waveforms often exhibit unambiguous relative polarity—that is, two components may be said to have ‘the same’ or ‘opposite’ polarity with certainty. This constitutes the most elementary measurement possible of a ‘relative amplitude’ and even this can often only be measured on the two horizontal components—the vertical being a poor recorder of teleseismic S on

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Fig. 1. Worldwide digital long-period data from the 19 March 1984 Uzbekistan earthquake, used in the source mechanism computation. Three-component S wave seismograms are shown to the right of vertical component P wave where used. P wave polarities were read as positive, and only the relative polarities of S waves were used as indicated by = for ‘the same as’ and ~ for ‘opposite to’. All seismograms are plotted with arbitrary amplitude scaling. Approximate expected arrival times are shown for the S waves. Station locations are shown on the focal sphere, with a presumed source layer velocity of 6.1 km s~, together with the four seismic arrays EKA, GRA, WRA and YKA used for the aftershock studies.

140

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Fig. 2. Double couple orientations compatible with (a) the P wave first motions and (b) the S wave polarity data given in Fig. 1. ~ihe ranges of orientations compatible with both (a) and (b) are circled in (b) and are represented by the accompanying lower hemisphere equal area projections. They represent a significance of 0.9917 (0.83% of orientations compatible in real space).

account of its near-horizontal particle motion. We might expect such a minor constraint on the 5 waves to be of little value in constraining the source orientation, but it will be shown to provide powerful constraints on source orientation, with the added advantage that even overloaded seismic traces can be used. Such measurements also continue the principle that only data which are unambiguous are incorporated in the computation. The P and S wave seismograms used for this earthquake are shown in Fig. 1; in each case 5, where used, is shown to the right of P. The 17 P wave first motions—all read as positive—impose the constraint on the source orientation shown in Fig. 2a. As expected, there is no constraint on the strike imposed by this set of teleseismic first motions, and a wide range of thrusts and vertical strike slip orientations are allowable—representing 17% of all possible orientations, For the S wave data, only the relative polarities of the two horizontal (and sometimes the vertical) components can be measured with certainty, and these measurements are shown in Fig. 1 as either (for ‘the same as’) or (for ‘opposite to’). No constraints are placed on relative amplitudes in this example. Figure 2b shows those orientations compatible with the 11 5 wave relative polarity observations shown in Fig. 1, and it is seen ‘=‘

‘~‘

that they impose a much greater constraint than 17 P wave first motions. This result may seem surprising since a single S wave relative polarity is equivalent to a single P polarity in that they both eliminate approximately one half of all possible source orientations. However, at any point on the focal sphere, the P polarity can take one of only two values, while the S polarisation direction can lie at any angle. It follows that unless an observation lies close to a P node, P polarities exhibit much more mutual redundancy than S wave relative polarities in the range of orientations which they exclude. It is also noted that since no absolute polarities are specified, the symmetry of the double couple radiation pattern prevents the S wave data from distinguishing between thrust and normal faults with the same orientation, whereas this is precisely the ambiguity that P wave polarities are able to resolve. When the P wave and S wave data are combined, the near-vertical strike slip solutions are almost but not entirely eliminated, the bulk of allowable solutions being almost pure thrusts. The two lower hemisphere stereographic projections included in Fig. 2b show a representative range of the thrusts and vertical strike slip solutions which remain compatible with both the P and S wave data—as circled in the figure. The ‘significance’ of

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Fig. 3. Aftershock 1 of Table I: (a) shows the observed short-period array seismograms with array correlograms indicating the arrival of phased energy (bandpass filtered from 2 to 4 Hz), and the relative amplitude measurements taken. The double couple orientations compatible with these measurements are shown in (b). The degree of constraint imposed by the data is defined by the Significance (Pearce, 1980) as 0.9988 (fraction of orientation space compatible with the data = 0.0012). The lower-hemisphere equal area projection inset shows a representative range of the compatible orientations shown on the vectorplot. (c) Shows synthetic seismograms generated using an orientation near the centre of the compatible range of orientations, as shown in Table II.

142

this dataset (Pearce, 1980) is 0.9917, corresponding to only 0.83% of orientation space being cornpatible. Despite this close constraint on the orientation, the strike itself covers a wide range, clustered mainly around the NE—SW direction. It

should be emphasised that there is no reason to prefer orientations closer to the centroid of an acceptable region since all acceptable orientations have equal probability. It follows that, in principie, the strike slip versus thrust ambiguity remains

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Fig. 4. Aftershock 2 of Table I: seismograms and results presented as in Fig. 3. Significance compatible 0.0032.

0.9969; fraction of space

143

unresolved and the strike of the thrust solutions remains largely undetermined. It is expected that with a better azimuthal coverage of S waves, the remaining near-vertical strike slip solutions would be eliminated. This is only concluded because the strike slip solutions all lie on the periphery of the

region compatible with P waves, and the addition of each successive S wave reading at a different azimuth progressively removes orientations of this type much faster than those of the thrust type. The P wave seismogram at CHTO has a very small first half cycle characteristic of an observab

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Fig. 5. Aftershock 3 of Table I: seismograms and results presented as in Fig. 3. Significance = 0.9926; fraction of space compatible = 0.0074.

144

tion lying close to a nodal plane of direct P—although this information was not included in the focal mechamsm computation. It is reassuring to note that the computation does predict that this station lies close to a P node.

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Earthquakes much below mb 55 usually generate few good quality teleseisms, but each may show discrete P, pP and sP on short-period seismograms. The relative amplitude method of determimng focal mechamsms is then an ideal means of fully exploiting the few observations available. Pearce et al. (1980) used it to determine focal mechanisms for 21 events in the 1976 Uzbekistan sequence using no more than four teleseismic array seismograms. Here the same arrays are used on six 1984 aftershocks, demonstrating the power of a small number of good quality teleseismic relative amplitudes to constrain the fault plane solution. Short-period seismic arrays offer the best teleseismic detection capability and the best quality teleseismic waveform data for smaller earthquakes. Presentation of data from the six largest aftershocks recorded at the four medium aperture arrays EK.A, GBA, WRA and YK.A serve to demonstrate their capabilities for determining the source mechanisms of events in this region. For the other (smaller) aftershocks there were no good quality recordings at the arrays although this was for a number of reasons including non-operation of stations Hence this study does not represent a rigorous determination of the threshold magnitude

.::::

.

for detection or analysis of focal mechanisms for

145

this sequence—this would certainly be lower if additional short-period arrays were available especially if their signal enhancement capability was greater. Figures 3—8a show observed seismograms for the six aftershocks (best-beam array sum) together with a cross-correlogram (product of two semisums bandpass filtered between 2 and 4 Hz) which indicate the arrival time of phased energy and are

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C

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observed synthetic

P: 1.0 to 2.0 1-ye pP: 0.0 to 0.25 1-ye/—ye sP: 0.0 to 0.25 1-ye/—ye

a

______________________________

Fig. 7. Aftershock 5 of Table I: seismograms and results presented as in Fig. 3. Significance = 0.9980; fraction of space compatible = 0.0021.

~

~

YEA

useful for confirming detection of surface reflections. The provisional identifications of surface reflections (in particular pP) are shown, and will be confirmed by the compatibility of phase ampli tudes with the double couple radiation pattern. The amplitude bounds for each phase as used in

____________

-

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this study accompany the seismograms, and the range(s) of compatible double couple orientations are shown in the respective vectorplots of Figs. 3—8b, whose accompanying lower hemisphere stereographic projections give an equivalent representation of these orientations. Each orientation comprises a pair of orthogonal nodal planes whose intersection is identified by a lozenge. Only a small number of representative orientations from the vectorplot are shown on the stereogram to preserve visual clarity. The results for aftershock 1, based on four seismograms, show a well-constrained 45° type thrust mechamsm trending predominantly NE— SW, with little or no strike-slip component. Where threestationsareavailable(aftershocks2and3) the solution is well constrained as a percentage of orientationspacebut, asisalsooftenthecasewith first motion solutions, this small range of compati— ble solutions may contain a surprisingly large range of fault types. The lack of data redundancy for events with only two stations increases the possibility that a single misinterpreted phase or anomalous observation may corrupt the output Where only EKA and YKA are available (aftershocks

146

4—6) both strike slip and thrust solutions tend to be retained since similar relative amplitudes can satisfy both fault types if only a small azimuthal

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Fig. 8. Aftershock 6 of Table I: seismograms and results presented as in Fig. 3. Significance = 0.9317; fraction of space compatible 0.0683.

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range of observation is available.

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.

thrusts until a third station is included. It must, SLIP ANGLE IN FAULT

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however, be remembered that for smaller earthquakes it becomes more likely that the mechanism reflects small-scale stress patterns rather than the broader regional tectonic environment. generated of Synthetic Douglas etseismograms al. (1972) are shown by in the Figs.method 3—8c, using a source centroid of the orientation compatible chosen range from for neareach the aftershock, as shown in Table II together with the assumed velocity structure in the source region In each case a Savage (1966) 1 km radius circular faultmodelwasusedwitharupturevelocityofo.6 times the S wave velocity in the source layer and a stress drop of 100 bars An anelastic attenuation factor t” (= travel time/average Q) of 0.8 s was allowedforexceptforWRA(ls). These synthetic seismograms serve to show the degree to which the seismogram features can be S

147 TABLE II Models used for generating synthetic seismograms in Figs. 3—8 Aftershock

1 2 3 4 5 6 a

Focal depth (km)

Nodal plane 1

(from pP—P time)

Dip

Strike

Slip angle

Dip

Strike

Slip angle

135° 135° 125° 130° ioo° 130°

40° 60° 335° 220° 214° 285°

125° 75° 35° 40° 5° 50°

125° 133° 118° 120° 95° 126°

265° 219° 87° 338° 0 3Q5 52°

60° 105° 139° 132° 170° 127°

112 13.3 8.9 6.3 17.5 a 9.5 Depth not constrained by the data.

Nodal plane 2

The following parameters were common to all models: Source region velocity structure The S wave velocity in each layer is assumed to be 1/Vi of the P wave velocity,

v 1,

Layer

V~,

1 2 3 4

3.0 4.6 6.1 8.2

Density 3) (gcm 2.7 2.7 2.8 3.3

Thickness, (km) 0.5 3.0 29.0 —

Other model parameters are given in the text.

reproduced by the source mechanism computed from relative amplitudes. They demonstrate the accuracy of focal depths recomputed from pP—P times, and show that the assumed anelastic attenuation factors reproduce the shapes and lengths of the pulses. The smaller amplitude onset of pP compared with P is caused by precursive pP-type reflections from near-surface discontinuities—an effect which is reproduced in the models.

4.

Discussion and conclusions

A well constrained, presumed double couple source orientation for the 19 March 1984 Uzbekistan earthquake has been obtained by combining 17 P wave polarities with 11 relative polarities of direct S components. This has demonstrated the feasibility of using such data to determine the mechanism of large shallow earthquakes, where the interference between P and its surface reflections renders P wave relative amplitudes unmea-

surable. Ideally several more S wave readings are required to eliminate a small region of near-vertical oblique faults which are compatible with the data in addition to thrust faults, and to impose a close constraint on the fault strike. Nevertheless the results obtained eliminate over 99% of orientation space, and correctly predict one P wave observation which appears to be close to a nodal line —information which was not included in the determination. For the aftershocks, in cases where at least three stations are available, well-constrained thrust-type solutions are obtained which are broadly similar to that of the main earthquake. Clearly it is undesirable to base fault plane solutions on only two stations, but even for these there is a substantial constraint imposed on the (presumed double couple) source orientation. These results serve to emphasise that for such smaller earthquakes (in this case down to mb 4.4) each teleseismic observation is extremely valuable if it has a good signal-to-noise ratio (i.e., it preferably

148

is an array). A small number of such records is more useful than many poor quality records from which waveform information cannot be extracted. The difficulty in constraining the strike of dip slip sources—a common problem in first motion studies—remains to some extent when relative amplitudes are used: the shape of the compatible region of source orientations is generally much more elongated in the strike dimension. This arises because the near-vertical radiation is azimuthally symmetric in both the upward and downward directions and this is so for any method which uses only near-vertically emerging rays. This point reaffirms the limited value of any technique which fails to define completely the confidence volume in orientation space. Closer observations are generally required to resolve this ambiguity. It is also found that, when there are too few stations, an ambiguity between dip slip and vertical strike slip type sources begins to appear, as observed for aftershocks 4 to 6. This can only occur when there is a large azimuthal range with no station. Results for aftershock 5 indicate a vertical strike slip solution, with no dip slip orientations allowed, This restriction relies entirely on the non-observation of a large pP at YKA and hence may be an artefact of this single anomalous and uncorroborated observation. It may alternatively be the correct orientation—bearing in mind the tendency for the mechanisms of smaller sources to be more related to local stress release than to the overall regional stress pattern. It is concluded that at least three, and preferably four, good quality seismograms well-distributed in azimuth are sufficient to provide a fault plane solution with narrow confidence limits, but two stations—even when only 45° apart in azimuth as in the case of EKA and YKA—can eliminate typically 90% of orientations, providing a good estimate of the fault type or principal stress directions, It is instructive to compare these results with solutions obtained by other workers for these earthquakes, and with solutions obtained for the 1976 sequence. For the main earthquake of 19 March 1984, Ekström et al, (1987) calculate, using the centroid-moment tensor method of Dziewonski et al. (1981), a thrust oriented NE—SW as in

the present study, but with a slightly steeper north-facing nodal plane. From the above discussion the absence of a confidence volume must call into question the strike of their solution. The best fit solution of Ekström et al. also incorporates a small non-double couple component. Non-double couple sources can be identified using the relative amplitude moment tensor program (see Rogers and Pearce, 1987); here a double couple is assumed. The solutions of the NEIS, and Eyidogan et al. (1985) are similar, but all the results cover a 90° range of strikes, and Shebalin (1985), who incorporates Soviet observations, favours a NW—SE strike. No other determinations of aftershock focal mechanisms were available for the 1984 sequence. Mechanisms for the aftershocks of the two large 1976 events, calculated by Pearce et al. (1980), show a strong predominance of NW—SE trending pure thrust faults, although some events either differ somewhat or were poorly constrained. Mechanisms of the two large 1976 events were computed by Sykes and Burdick (1979), and yielded for the 8 April earthquake an east—west trending 45° pure thrust, and for the 17 May earthquake an oblique fault striking either north—south or NE—SW. Both of these solutions are based on P wave first motions alone. In Phedorov (1984, p. 70) mechanisms for these two earthquakes are presented with the aid of many Soviet observations at regional and local distances, including both P and S waves, and both yielding oblique faults with strikes close to the NE—SW direction. Other results for the 8 April 1976 earthquake include those of Kristy et al. (1980) (roughly east—west striking thrust) and Aptekman et al. (1978) who give an oblique east—west thrust. For 17 May 1976 event Aptekman et al. (1978) give an east—west thrust while Hartzell (1980) gives an oblique fault with NE—SW strike. From this range of solutions there i~clearly a difficulty in constraining the strike—typical of dip slip faults—and careful analysis is required if this parameter is to be reliably determined. There is no doubt that the studies of the two aftershock sequences by this author indicate a predominance of northwest—southeast strikes for the 1976 sequence,

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and northeast—southwest strikes for the 1984 Sequence. The observation of thrusts along two distinct strike directions raises the possibility that thrusting of a ramp-like structure is responsible for these earthquakes, as discussed by Shebalin (1986). It has also recently been suggested that these earthquakes might have been induced by the redistribution of stress resulting from working the gas field in this area (Simpson and Leith, 1985). In a further study all the teleseismically recorded earthquakes, including those between 1977 and 1983 not previously examined, will be analysed with the aid of local and regional observations where possible—for example, those of several authors reported by Phedorov (1984). The relative amplitude approach is uniquely suited to such a study on account of its ability to provide teleseismically determined solutions for the aftershocks as well as for the large events, and the use of additional data will help to resolve the difficulties associated with poor resolution of strike, and will allow for an interpretation of these earthquakes in terms of the local active faulting.

Acknowledgements I thank Dr. E.R. Engdahl and colleagues at the USGS for providing global digital data for the main earthquake and Dr. N.y. Shebalin for useful discussion, I also thank Dr. E. Chesnokov for providing a copy of the reference edited by Phedorov. This work was partly supported under NERC Research Grant No. GR3/5190.

References Aptekman, Zh.Ya., Graizer, V.M., Pletnev, KG., Rustanovich, D.N., Shebalin, NV. and Shteinberg, V.V., 1978. Some data on processes in the epicentral zone of the 1976 Gash earthquakes. In: Epicentral Zones of Earthquakes, Questions in Engineering Seismology, Nauka, Moscow, 19: 405—418 (in Russian). Crampin, S., Fife, C.J., Bickmore, D.P. and Linton, R.H.W., 1976. Atlas of seismic activity 1909 to 1968. Inst. Geol. Sci., Seismol. Bull. No. 5. HMSO. Douglas, A., Hudson, J.A. and Blamey, C., 1972. A quantitative evaluation of seismic signals at teleseismic distances III

—Computed P and Rayleigh wave seismograms. Geophys. J.R. Astron. Soc., 28: 385—410. Dziewonski, AM., Chou, T.-A. and Woodhouse, J.H., 1981. Determination of earthquake source parameters from waveform data for studies of global and regional seismicity. J. Geophys. Res., 86: 2825—2852. Ekström, 0., Dziewonski, A.M., Franzen, J.E., Giardini, D. and Woodhouse, J.H., 1987. Source parameters for the 51 IASPEI selected earthquakes, 1980—1984. Phys. Earth Planet. Inter., 47: 62—66. Eyidogan, H., Nabelek, J. and Toksöz, M.N., 1985. The Gazli, U.S.S.R., March 19, 1984 earthquake: the mechanism and tectonic implications. Bull. Seismol. Soc. Am., 75: 661—675. Hartzell, S., 1980. Faulting process of the May 17, 1976 Gazli, U.S.S.R., earthquake. Bull. Seismol. Soc. Am., 70: 1715— 1736. Kristy, M.J., Burdick, Li. and Simpson, D.W., 1980. The focal mechanisms of the Gazli, U.S.S.R., earthquakes. Bull. Seismol. Soc. Am., 70: 1737—1750. Medvedev, S.V., 1968. The international scale of seismic intensity. In: S.V. Medvedev (Editor), Seismic Zoning of the U.S.S.R. Nauka, Moscow (in Russian). Pearce, R.G., 1977. Fault plane solutions using relative amplitudes of P and pP. Geophys. J. R. Astron. Soc., 50: 381—394. Pearce, R.G., 1979. Earthquake focal mechanisms from relative amplitudes of P, pP and sP: method and computer program. AWRE Report 0 41/79, HMSO. Pearce, R.G., 1980. Fault plane solutions using relative amplitudes of P and surface reflections: further studies. Geophys. J. R. Astron. Soc., 60: 459—488. Pearce, R.G., Bainbridge, H., Young, J.B. and Key, P.F., 1980. The 1976 earthquake sequence in Uzbekistan: focal mechanisms determined using the relative amplitude method. AWRE Report No. 026/80, HMSO. Person, W.J. (Editor), 1978. Seismological Notes. Bull. Seismol. Soc. Am., 68: 256—262. Phedorov, S.A. (Editor), 1984. Gazli earthquakes 1976. Geological and Geophysical Nature of Focal Regions. Nauka Publishing House, Moscow 200 pp. (in Russian). Pletnev, K.G., Shebalin, NV., Shteinberg, V.V., Graizer, V.M. and Alexin, P.A., 1977. Seismic engineering Program Report. U.S. Geol. Sur. Circ. 762-A, 28 pp. (Jan.—Apr. 1977). Rogers, R.M. and Pearce, R.G., 1987. Application of the relative amplitude moment-tensor program to three intermediate-depth IASPEI earthquakes. Phys. Earth Planet. Inter., 47: 93—106. Savage, J.C., 1966. Radiation from a realistic model of faulting. Bull. Seismol. Soc. Am., 56: 577—592. Shebalin, N.Y., 1985. Kum-Dagh, Turkmenia, March 14 1983 and the third Gazli, Kyzilkum, March 19 1984 Earthquakes. Contribution S12-5, IASPEI Assembly, Tokyo. Simpson, D.W. and Leith, W., 1985. The 1976 and 1984 Gazli USSR earthquakes—were they induced? Bull. Seismol. Soc. Am., 75: 1465—1468. Sykes, L.R. and Burdick, L., 1979. Velocity and Q structure. Annual Technical Report Part 2. Lamont-Doherty Geological Observatory, Columbia University.